Monday, July 30, 2012

Violent outbursts: The unruly youth of the universe

As far as galaxies are concerned, things just aren't what they used to be. When we look around in the local universe, both the Milky Way and its neighbours seem quite quiescent (quiet). If they were people, they would be in a happy relaxed middle-aged phase. However, like any person they started small and had to grow up. Unfortunately, we don't have a complete record of how this happened, but we can look back to a time when the universe was still in its lively and somewhat unruly youth.

Galaxies have grown from small fluctuations in the early Universe to the massive groups of stars we see today, either by the gradual assembly of stars from dust and gas, or through merging with other galaxies. As far as we know, the vast majority of galaxies form stars at a roughly fixed rate relative to their mass, known as the specific star-formation rate. Put simply, this means the more massive a galaxy is, the more rapidly it will be forming stars. Over the vast stretches of cosmic time, this gradual accumulation of mass is enough to build up galaxies that are 100 billion times more massive than our Sun. However, things are rarely as simple as they seem, and interestingly, some galaxies appear to have taken this to an extreme. The rate at which some of the biggest galaxies in the universe form their stars has not been constant, but was actually higher in the past. It seems that, like people, some galaxies did most of their growing in their youth and have since stayed at about the same mass, forming only a few more stars.

Over the past few decades astronomers have opened up a new window on the universe, through the advent of submillimeter astronomy. These detectors are sensitive to light with wavelengths that are a 1,000 times longer than we can see with our eyes and is typically emitted by dust. As stars form they emit ultraviolet light, and as stars form inside clouds of gas and dust, much of that ultraviolet light is absorbed by this cosmic dust which is then heated to a few tens of degrees above absolute zero (-273 degrees Celsius). The "warmed" (but still rather "cool") dust then re-emits the absorbed energy at far-infrared wavelengths, which is then further redshifted to longer sub-millimeter wavelengths en-route to the Earth by the expansion of the Universe, where it can be collected and measured.

Submillimeter astronomy truly entered the academic scene in the 1990s with the introduction of the Submillimetre Common-User Bolometer Array (SCUBA), so important it can now be seen on display at the national museum of Scotland. While our understanding has developed over time with additional ground-based surveys (using, e.g., AzTEC and LABOCA), the study of sub-millimeter sources is poised to undergo a revolution stemming from a new generation of instruments. Starting with the Balloon-borne Large-Aperture Sub-millimeter Telescope (BLAST), and now with the Herschel Space Observatory, the quality and volume of sub-millimeter data has increased at a tremendous rate. These key projects will soon be joined by the Atacama Large Millimeter Array (ALMA -- see recent post), which will for the first time provide submillimeter imaging at a similar resolution to that of the famous Hubble Space Telescope. However, many mysteries remain regarding the nature of the currently known sub-millimeter sources, and many more are sure to be uncovered with the new generation of instruments and telescopes.

Example of two-dimensional modelling from Targett et al. (2012) for a single component clumpy disk (top), multi-component disk-dominated system (middle), and apparently irregular system (bottom). The first left-hand panels show false-color HST I, J, H-band images. The center left-hand panels show the H-band CANDELS postage-stamp images centered on the sub-millimeter galaxies. The center panels show the best-fitting two-dimensional models. The right-hand panels show the residual images after subtraction of the models from the data.
One of the most interesting and important discoveries in this new field was that some galaxies weren't simply forming more stars in the past, but that they were forming stars at a rate up to 1000 times more rapidly than expected. As these sources were detected at submillimeter wavelengths, they are rather originally named submillimeter galaxies (SMGs). Clearly, understanding the nature of these violently star-forming galaxies is important given how abnormal their growth seems when compared to that of 'normal' galaxies. Specifically, many scientists are interested in whether sub-millimeter sources are large disk-like galaxies (such as our Milky Way) or merging systems (where two or more galaxies have collided and coalesced). Knowing this would help to determine which of the theoretical models currently used to explain them are correct, and therefore enhance our understanding of what role they play in galaxy evolution. Using galaxy modelling software we were able to measure the sizes and morphologies (shape) of submillimeter galaxies in a field known as GOODS-South. The exquisite depth and resolution of the CANDELS data available in this field has provided some of the most detailed morphological analysis of galaxies to date (see above picture). As with many results in science the picture is not always clear, and many of our sources show evidence of merging or interaction, but overall these submillimeter galaxies were well-described by either a single component exponential disk, or a multiple component system in which the dominant constituent is disk-like. Taken together with other results, these data seem consistent with the view that most submillimeter-selected galaxies are simply the most extreme examples of normal star-forming galaxies at that era, and could therefor be critical to the formation and evolution of the most massive galaxies.

Friday, July 27, 2012

The Curious History of Luminous Infrared Galaxies

A selection of luminous infrared galaxies from the Great Observatories All-sky
LIRG Suvey (GOALS). Credit: 
NASA/STScI/NRAO/A.Evans et al.
Our understanding of galaxy evolution changed dramatically after the launch of IRAS (the Infrared All-Sky Satellite) in 1983 when the first all-sky infrared survey was conducted. Upon analyzing this new data, taken in a portion of the electromagnetic spectrum that up until then had been little studied, a new class of galaxies was discovered. These galaxies contained more energy in the infrared portion of the spectrum, beyond what the human eye could see, then they did at all other wavelengths combined. The total amount of energy emitted by these galaxies is comparable to quasars, which had previously been known as the most energetic objects in the universe.

These unique objects were given a name, LIRGs (an acronym of course), which stands for Luminous Infrared Galaxies, precisely because these objects are so luminous in the infrared. These galaxies have infrared luminosities of over 100 billion times that of the sun. Even more luminous classes were named in a similar fashion, ULIRGs (Ultraluminous -- more than a trillion times more luminous than the sun), and HyLIRGs (Hyperluminous -- greater than 10 trillion solar luminosities). These objects are very rare in the nearby universe, but a number of them were identified because the IRAS survey covered the entire sky.

What makes these galaxies so bright in the infrared? Most of the light that we see from galaxies comes from the stars, but some galaxies also contain large amounts of gas and dust (we call these galaxies 'gas-rich'). This material absorbs a lot of the light from stars and re-emits it in the infrared. So this light still comes from stars but it is processed by the dust in the galaxy first. Most of the light we see from stars is dominated by young, very massive stars. For these galaxies to have so much energy emitted in the infrared means that they are made up of a lot of young stars. In fact, they are forming stars at incredible rates, up to 1000 times the rate that the Milky Way forms stars. Not all of the light in these galaxies comes from star formation - some comes from AGN activity. In fact, a large fraction of galaxies with these extreme luminosities are known to host an active black hole.

After luminous infrared galaxies were first discovered, astronomers turned to optical telescopes to find out what these galaxies looked like. The answer was stunning. Almost all of the objects looked at turned out to be merging or interacting galaxies (such as those shown in the image above). Such a large fraction, that it had to be more than a coincidence. From this, they deduced that the reason these galaxies were so luminous was that the merger of two gas rich spiral galaxies had induced huge bursts of star formation (explaining the very high star formation rates observed) and in many cases, drove gas toward the central black hole. This was held up by the realization that the more luminous systems tended to be at the most advanced merger stages.

All of these pieces lead to the evolutionary scenario, first suggested in 1988 by David Sanders, currently at the University of Hawaii. In this scenario, the merger of two gas rich spiral systems forms a luminous infrared galaxy that gets more luminous as the merger progresses. At the same time, the central black hole is being fed by infalling gas and obscured by the surrounding dust. Over time, the fuel gets used up and possibly expelled from the system at which point the infrared luminosity decreases and the central active nucleus is exposed. We observe such a system as a quasar. Eventually, the black hole runs out of fuel and is no longer active, and the merger remnant relaxes into an elliptical galaxy.

Diagram illustrating the merger scenario, where the merger of two gas rich spirals forms a luminous infrared galaxy, which evolves into a ULIRG, then a Quasar, and eventually an elliptical galaxy. All images are from Hubble.
This picture of galaxy evolution appears to explain many observables in the local universe, but the question of whether this was also the case for galaxies in the early universe remained a mystery. Samples of infrared galaxies at higher redshifts were found through various other surveys, but the pace really began to pick up in 2003 after the launch of the Spitzer Space Telescope (and accelerated with the launch of Herschel in 2009). All of these facilities look at light in the infrared, and each has been more sensitive and had better resolution than its predecessor. What was realized early on, and confirmed by initial results from Spitzer, was that luminous infrared galaxies are much more common in the distant universe. In fact, even though they are rare oddities locally, at one point in time they were the norm. If luminous infrared galaxies were so common in the universe's history, clearly they played an important role in galaxy evolution and in shaping the universe that we observe nearby. 
The immediate question that comes to mind is whether or not these distant luminous infrared galaxies have the same properties as the nearby ones. Are they all the result of galaxy interactions, as has been shown locally, or are their incredible luminosities caused by something else? Do they also harbor obscured accreting black holes? There are certainly reasons to believe that different processes may be at work in the early universe. Many initial studies have found a mix of galaxy shapes and properties using deep Hubble images - some of them are clearly mergers while others are not. Other observations have shown that galaxies in the early universe may have had a lot more gas than is present in galaxies today, providing a much larger reservoir from which to form stars - with or without a merger. Certainly, the picture is a lot more complicated!

Deep imaging in the near-infrared from CANDELS provides the ideal window into the properties of these systems because at high redshift, the observed light was originally emitted in the optical, just like we observe in nearby galaxies. The combination of rest-frame optical light and the exquisite resolution of images from Hubble allow us to study galaxy morphologies at these distances in greater detail than has ever been possible. In a future post, I will discuss the results of our recent paper analyzing a sample of these dramatic systems in the distant universe.

Wednesday, July 25, 2012

CANDELS and the Extragalactic Background Light

Is the night sky dark?

Depending on your current location and what instrument you’re using, the answer can vary wildly.

The zodiacal light on the left and the Milky Way on the right. 
(Image credit: Daniel Lopez)
In the first place, any non-city dweller can instantly report that a large area of the night sky is not completely dark: the band of the Milky Way proves this. Our own Milky Way galaxy, seen edge on, is the most well-known “diffuse” structure in the night sky – diffuse, in this context, means any astronomical source of light that is spread out from our perspective and not limited to a single object. Thus individual stars, galaxies, planets, the Sun, and the Moon are all referred to as “sources,” but everything else is referred to as diffuse emission.

Another diffuse structure in the night sky is the Zodiacal light, which is mainly visible for observers with exceptionally good nighttime seeing. The zodiacal light, like the Milky Way, looks like a faint, glowing band in the sky. Unlike the Milky Way, the zodiacal light is located in the ecliptic plane along with the planets, the Sun, and the Moon. Its brightness is concentrated near the sun, and for this reason can be seen only shortly after sunset or shortly before sunrise. For this reason, it has sometimes been called “the false dawn”. Frequently Venus will reside within the zodiacal light from our perspective. (For this reason, native Australian peoples referred to the zodiacal light as the rope that held Venus to the earth.)

The diffuse zodiacal sky brightness is produced by the solar system’s dust cloud, which reflects sunlight back to us from the ecliptic plane as a hazy glow. Similar dust disks also reside around other stars. In fact, if an alien civilization were to image our solar system from their own solar system, the zodiacal dust cloud would probably be the most prominent feature in the image, apart from the Sun itself!

Both the Milky Way and the zodiacal light appear to the naked eye as “bands” on the sky. But take the Hubble Space Telescope and point it at any apparently dark area of the sky. The first surprise you would see, as you might guess if you’ve read some of the previous posts on this blog, is that you’ll see thousands of stars and galaxies (each with hundreds of millions of stars in them). Even then, between the stars and galaxies in a deep Hubble image, there’s a faint glow present. Primarily, though not exclusively, the faint glow in the image would be zodiacal and Galactic (Milky Way) diffuse emission. Thus, while the “bands” we see are the brightest concentrations of the zodiacal and Galactic diffuse emission, the entire sky, even when looking outside of the ecliptic and the Galactic planes, is aglow with faint emission. In other words, not a single part of the night sky is truly dark if your eyes are powerful enough! 

But the story doesn’t end here. One can imagine measuring the total brightness from the Galactic and zodiacal diffuse emission and subtracting them, seeing if then, finally, the night sky would truly be dark. The details are how to do this are complicated, and lead to some uncertainty. Nevertheless, when astronomers have subtracted out the sky intensity from the Galaxy and the solar system, there is a glow remaining. This glow appears to be coming from every direction in the sky with about the same brightness. Because this residual brightness therefore probably comes from extreme distances far beyond the Milky Way or other nearby galaxies, we call this remaining sky brightness the Extragalactic Background Light, or the EBL.

Because of the uncertainties, not everyone agrees on the EBL intensity, or indeed whether it has actually been detected. If it is there, what could be producing it? Certainly, to some degree, it consists of unresolved faint galaxies that we haven’t observed individually yet, but which are so numerous that they become smeared together as a “glow” from our perspective. For example, most of the galaxies visible in the deep fields of CANDELS would have looked like an “extragalactic background light” a generation ago before they were individually resolvable by Hubble. As part of my PhD thesis, I took the deepest galaxy counts from Hubble and extrapolated them down to fainter magnitudes. Reasonable extrapolations fall short of explaining the EBL.

The edge-on galaxy NGC 5907 with its faint tidal streams. 
(Image credit: D.Martínez-Delgado et al.)
The EBL can also consist of fainter components of galaxies we have already resolved. Take, for example, some images of local galaxies which have extremely faint components. The image to the left shows a long tidal trail from a smaller galaxy being ripped apart. The interesting thing about this image is that the faint parts weren’t known about until better cameras imaged this bright edge-on galaxy (NGC 5907, which had already been well-known). So if one imagines that all galaxies have some faint structures like these, the currently unseen parts of known faint galaxies such as those in CANDELS might sum together to appear as a significant part of the EBL. Also as part of my own recent PhD work (which used some of the CANDELS data) I showed that a significant fraction of the EBL at some wavelengths can be explained by the summation of faint wings of galaxies in deep Hubble images.

Thus, even after we extrapolate the faint galaxy counts and account for the missing wings of galaxies, we still haven’t accounted for the measured EBL. So we have a puzzle. There are two solutions: (1) the subtraction of the foreground light is incorrect, or (2) there are some unexpected very faint sources producing this light. We need to look for clues in the EBL itself.
The all-sky infrared background light, from the DIRBE instrument on board the COBE satellite. (Image Credit: Michael Hauser, the COBE/DIRBE Science Team, and NASA.)

One possibility is that the EBL is coming from very distant galaxies forming their first stars. These galaxies are individually very faint, but if they formed enough stars, collectively they might have produced enough light to dominate the EBL. Using CANDELS data, we are looking to test whether the EBL comes from very distant galaxies in two ways. First, if the galaxies are very distant, they will have high enough redshifts that their light would only contribute to the long-wavelength CANDELS images. Second, the theory of structure formation makes a pretty solid prediction that the background won’t be entirely uniform, but will fluctuate. The largest fluctuations should appear on scales of about ten to twenty arcminutes. The telltale signature that the EBL is coming from very early galaxies would be to find the predicted fluctuations in the long-wavelength CANDELS images, but not find it in the shorter-wavelength images. 

This is hard. We need to be certain to remove all of the scattered light from Earthshine and nearby stars. We have to make sure that we have accurately calibrated the sensitivity of the WFC3 camera across its full field of view. Fortunately, for some of our fields, we are taking many images at different times with different shifts and rotations. So we have a lot of cross checks. But we also have a lot of work to do to get this right.

The measurement is difficult enough that observations are continuing on a number of other fronts (independent of CANDELS). One example of another project is CIBER (Cosmic Infrared Background ExpeRiment, see image to the right), which is a rocket mission that has launched from Wallops Island in Virgina as well as the White Sands missile range in New Mexico. It contains a camera specialized for diffuse, faint emission as well as a near-infrared spectrometer. Another possibility currently being explored is to mount an EBL-specialized camera on a spacecraft traveling to the outer planets, to Jupiter or beyond, where the zodiacal light is drastically reduced when one looks out of the solar system.

The Black Brant IX sounding rocket, which carries CIBER.
(Image credit: NASA)
Returning to our initial question: the night sky is not dark! With the eyes of the Hubble Space Telescope, the sky-spanning glows in the zodiacal light and the Milky Way’s emission are only the tip of the iceberg – a faint glow, with contributions from the solar system, the Galaxy, distant galaxies, and possibly the first galaxies in cosmic history, lurks everywhere you look. There is a famous thought experiment called Olbers' Paradox (really going back to the time of Kepler in the sixteenth century) which asked the question: if the universe were infinitely old and infinitely vast, why is the night sky then dark at all? Shouldn’t every dark area be a window to infinitely more distant stars which, while faint, would be so numerous that they must add up to a diffuse glow as bright as the Sun itself? There are many reasons why an absurdly high brightness is the wrong prediction, but the correct prediction is still not zero. And so the most important part of the answer is the night sky only appears dark to us because our eyes aren’t powerful enough see the trillions of galaxies within every pinhole in the blackness. Fortunately, we have the Hubble Space Telescope to make up for it.

Monday, July 23, 2012

A Week at the LBT

One of the most rewarding aspects of a career in astronomy is the chance to go observing at a professional observatory. I recently spent a week at the Large Binocular Telescope (LBT) in Arizona, observing for the Max Planck Institute. With me were two other observers, Hugh and Mario, members of other research teams at the Institute. In total, we had 6 full nights with which to pursue our various science programs, as well as collect astronomical data for other scientists in our research consortium.

The LBT is a marvel of engineering. It consists of two enormous primary mirrors, each 8 meters (26 ft) in diameter, mounted on the same huge superstructure, a squat but airy truss of red-painted steel, carbon fibre, cabling and glass. The mirrors are built to look up at the same region of sky and the LBT can track an object through its pair of binocular 'eyes' with incredible sub-arcsecond accuracy. The two telescopes will eventually be combined into an Interferometer - an advanced technology instrument that behaves a bit like a single telescope 12 meters in size.

Standing below one of the LBT mirrors.
Click for an album of images from my trip
A large telescope such as the LBT is used at the forefront of astronomical research. Take for example the kinds of science that we were pursuing for those 6 nights. I was using an instrument called LUCI that is sensitive to light in the near-infrared. With LUCI, I obtained spectra of galaxies that are tens of billions of light years away, when stars were forming throughout the Universe at a furious pace. Hugh was using LUCI to precisely measure the apparent sizes of nearby stars, allowing him to look for the presence of disks of dust around them, solar systems in their youth. Intense star-forming regions in our Milky Way galaxy interested Mario: he wanted to train LUCI on some of their most massive stars.

The 2 hour drive from Tucson went up a windy mountain road rising from the desert to the pine and fir crested peak that hosts the Mount Graham International Observatory. In a large trolley, we carried up a week's worth our of groceries up to the Observer's dining area, which sported large windows overlooking the Pinaleño mountains, a row of big refrigerators, a full kitchen, comfy couches and - joy! - a pool table. This, and the attached sleeping cubicles, were to be our home for the duration of our run. We chatted with the last shift of observers, talked about science and telescope issues, settled into our rooms and got accustomed to the thin air at 11,000 ft.

The next day we started our run. Things didn't go according to plan.

In the heady days of tall ships, being caught in the Equatorial becalming zones called the Doldrums came to be a bane of sailors. The word was adopted into modern English to mean times of ennui and lack of activity. Bizarrely, at the LBT, the Doldrums happen when the wind is high, because the telescope dome cannot be opened if it's blowing so hard that the telescope structure could suffer serious stress. We waited and watched unsuccessfully for the telescope to open while howling gale-force winds buffeted the top of Mount Graham. An unusual weather system, which started before we arrived at the peak, kept up a steady barrage of dusty winds for four whole days and nights (taking a night away from the observers before us). As a result, we never actually looked at sky for half of our run, though, if one had stood on the balcony of the telescope dome and look up without being blown off, one would have seen a lovely Milky Way over the horizon and bright stars burning against a velvet black sky. This was a painful reminder of one of the misfortunes of astronomical observing - Weather. Observatories are built in far off and inaccessible places for a reason: those very places, usually deserts or high mountain tops, have the best observing conditions. Time on a big telescope comes at a premium, both monetary and in man hours, so they are designed for a maximum return.

In contrast, the next three nights were fantastic! As the winds died away, the front of calm weather beyond brought superb, twinkle-less skies (heavy twinkling is a sign of turbulent air, something we call "bad seeing") and a thin young moon. With the help of our fantastic Support Astronomer, Olga, who helped deal with a niggle that kept putting the telescope an arcsecond away from where it should have been, I managed to get 7 hours of great data with which we mapped the movement of star-forming regions in twelve galaxies at a redshift of 1.5, seeing them as they were in a 5 billion year old Universe. Hugh, with a twinkle in his eye, showed us the subtle blooming of his stellar spectra that revealed a thin disk of young planetesimals in a star light years away. Mario fussed about excitedly with his spectra, judging quality and applying numerous calibrations. Things proceeded without a hitch and we even had time, while the telescope was kept busy running automated scripts on a long exposure, to play the odd game of pool in the observer's dining room.

For a century, since the rise of large institutional observatories, astronomers have packed their cases - once full of photographic plates, chemicals, and cold weather gear; now with laptops, manuals, and comfortable shoes - and climbed mountains to expand our view of the Universe. Observatories, whether on the ground or up in space, continue to enrich our knowledge and serve as test-beds for cutting edge technology, and we hope that we, as astronomers and as humans, will always be able to find that lone narrow and windy road that takes us up to the stars.

Friday, July 20, 2012

ALMA Opens its Eyes to the Sky

High in the Atacama Desert of northern Chile a new astronomical instrument is under construction. It is the Atacama Large Millimeter/Submillimeter (mm/submm) Array, aka ALMA. With its large number of antennas, its location in one of the driest places on the Earth, and at an altitude of 5000m (16,500 feet), ALMA represents a huge step forward for millimeter/submm astronomy, with promises of fantastic science to come. But why build such an expensive observatory (ALMA is the most expensive ground based instrument ever built) at such an inhospitable place (high altitude and extremely dry), and, how is it important for CANDELS?

ALMA antennas at the Chajnantor Plateau. Photo credit: Babak Tafreshi

First a few definitions: astronomers use the term 'radio' when dealing with electromagnetic radiation with wavelengths from hundreds of meters to less than a millimeter (submm). It has nothing to do with actually listening, like we listen to a normal radio, but with detecting this electromagnetic radiation using sensitive receivers. In this blog I will use the terms 'radio', 'mm' and 'submm' to refer to this type of emission - and the instruments used to detect it. Wavelengths are given in millimeters (mm), sometimes referred to as 'submm' when it's slightly less than 1mm, and in microns (a millionth of a meter).

If you could view the sky with eyes that were sensitive to millimeter and submillimeter wavelengths, it would look very different from what we see with the eyes we have. Instead of warm and hot celestial objects, such as stars and ionized gases, you would see dark and cold interstellar clouds, dusty disks around young stars, faint cirrus clouds permeating our Galaxy, and the occasional point-like quasar. The interstellar dust grains are heated by stellar light that is re-radiated at far-infrared and submm wavelengths. The characteristic dust temperature is in the range 20-40K and the molecular gas that permeates interstellar space can be even colder than that. Your ‘submm eyes’ would pick up the light from thousands of spectral lines, mainly coming from rotational transitions of simple – and some not quite so simple – molecules. Most of the light would be diffuse and spread out over the sky, but a few point-like sources would also be seen. These are synchrotron radiating Active Galactic Nuclei (AGNs) - the main part of the emission is, however, thermal in character.

As a matter of fact, your eyes wouldn’t actually see much of anything, even if they were sensitive to these wavelengths. The reason for this is two-fold; First, there isn’t as much energy in the mm/submm light as there is at optical wavelengths. A photon with a wavelength of 1mm has about 2,200 times less energy than one at 0.45 microns (visible light). Second, the angular resolution provided by the eye would be very limited indeed. The small diameter of the eye’s pupil would make the sky look blurry, providing an angular resolution roughly 1,000 times worse than what they do at optical wavelengths. You would have a hard time to pick out even the Moon or the Sun.

These degrading effects associated with observing the universe at longer wavelengths translate to more conventional telescopes as well; a radio telescope has to be bigger, in fact a lot bigger, than an optical telescope in order to achieve a similar performance. From an engineering point of view it is easier to build a large radio telescope than to build a corresponding optical telescope, but gravity and money limit how big they can really be. To circumvent these limitations, radio astronomers have long used the technique of interferometry, where the light from several telescopes is combined to improve angular resolution, and, in most cases, improve sensitivity. The use of several radio telescopes, or 'antennas', as an interferometer is referred to as aperture synthesis. Rather than go into details here, I recommend wikipedia's general overview of astronomical interferometers.

In fact, the first radio interferometer observations were done back in 1946, using a single antenna that happened to look out over the ocean. With this setup, the rising sun reflecting off the ocean surface creates interference fringes with the light directly reaching the receiver. The results showed that solar radiation at radio wavelengths during a flare is co-located with the corresponding sunspot and active area. These sea-cliff interferometers could create baselines up to ~200m.

As an aside, interference from single antennas looking towards the horizon were first identified during WWII when trying to detect approaching aircraft along the British coast and was correctly interpreted as an ‘interferometric’ effect.

ALMA antennas being tested at the Operations Support Facility. Photo: T. Wiklind

The latest radio interferometer is currently being built high in the Chilean Andes, quiet far from any ocean. This is of course the Atacama Large Millimeter/submillimeter Array, or ALMA; a project with a history going back more than 25 years. ALMA is a collaborative effort of three partners: North America, Europe, and East Asia. ALMA represents a huge step forward in terms of both sensitivity and angular resolution at mm and submm wavelengths, being about 10-1000 times more sensitive than existing instruments (depending on the wavelength). When completed, ALMA will consist of 66 high precision antennas located on the Chajnantor Plateau at an altitude of 5000m (16,500 feet). The antennas are divided into a main array, made up of fifty 12m telescopes, and the ALMA Compact Array (ACA) with twelve 7m and four 12m antennas. The total telescope area will correspond to a single dish with a diameter of 91m.

ALMA Operations Support Facility. Photo: T. Wiklind
ALMA produces both images and spectroscopic data with a wavelength coverage currently ranging from 3mm to 430 microns. When all planned receivers are installed, the wavelength coverage will range from 10mm to 320 microns. The antennas will be spread out over the Chajnantor Plateau with the longest baselines reaching 15km. This will allow an angular resolution of 5 milli-arcseconds for the most extended configuration and the shortest wavelength. This resolution would make it possible for ALMA to view NASA's left-over Lunar Rovers on the Moon (at least in principle – the Rovers don't radiate well at mm/submm wavelengths).

The Salar de Atacama, looking up towards the Chajnantor plateau.
Although the construction of ALMA will not be over until late 2013, scientific observations have already started. The deadline for the second call for Early Science Proposals was July 12, 2012 and resulted in more than 1,000 proposals. During the Early Science phase, ALMA will operate with fewer antennas than it will have when completed, but it already represents the best sensitivity and the highest angular resolution available at these wavelengths – which explains the intense interest among astronomers in using ALMA. Cycle 1 observations (ALMA science started with Cycle 0 in 2011) will be done with a minimum of 32 12m antennas, allow the use of a limited ALMA Compact Array (ACA), but do not include all the receiver bands that will eventually be installed. The introduction of the ACA in this cycle represents a big step forward. ALMA is extraordinarily sensitive and has an enormous angular resolution, but the penalty is that it loses sensitivity to structures more extended than a few arceconds (again, wavelength dependent). The reason is that the 12m antennas cannot be placed close together, lest they start shadowing each other. This means that the ‘inner’ part of the synthesized telescope is missing. By using the compact array, with the smaller 7m antennas, it is possible to fill the hole and allow ALMA to image large- as well as small-scale structure.
Sunset at the Operations Support Facility. Photo: T. Wiklind

So what can ALMA do for CANDELS aficionados? The science specification for ALMA states that it shall be able to detect a galaxy like the Milky Way at a redshift z=3 in the lines of carbon monoxide (CO is the second most common molecule in the Universe – the most common is molecular hydrogen but it does not radiate effectively since it is close to perfectly symmetric) or [CII] (singly ionized carbon atom) in less than 24 hours – remember that ALMA was conceived at a time when very little was known about the high redshift universe. The full ALMA will certainly be able to do this, but only with a few hours to spare. Fortunately, galaxies at high redshift are now believed to be considerably more gas-rich than their present day counterparts making them even easier to detect at these wavelengths, and it will in fact be possible to observe a lot more than a single galaxy per day.

An example of a high-redshift observation, done as part of the ALMA Science Verification program, is the far-infrared luminous quasar BR1202-0725. This quasar has a redshift z=4.7, meaning that the photons we observe from this object were emitted 12.4 billion years ago, just 1.3 billion years after the Big Bang. BR1202-0725 was observed with seventeen 12m antennas for a total on-source time of 25 minutes. The receivers were tuned to the atomic fine-structure line [CII] with a rest-frame wavelength of 158 microns, here redshifted to 900 microns. Two sources are detected in the [CII] line, the quasar and a nearby galaxy just 3.8” to the NW of the quasar. A third component 2.5” to the SW is seen in the continuum emission. The third component is not seen in the [CII] line and was previously unknown. It is presently not clear that the SW source is associated with the two other sources – but the proximity suggests it is. We will know as soon as we get additional data on this new object.
BR1202-0725, Top: ALMA continuum (green contours) and HST images in 3 different bands. The bright source seen in the HST images is the quasar. The submm sources seen to the NW and SW of the quasar have weak optical emission associated with them. Bottom: Spectra of the [CII] line from the quasar and the NW source. Notice that the NW source has a very broad line profile. From Wagg et al. 2012, ApJ 752, L30.
While ALMA provides unprecedented sensitivity for line observations, it really excels when it comes to continuum emission. The state-of-the-art receivers, broad bandwidth, and the high-and-dry location all conspire to provide a high sensitivity. On top of that, the Universe is very kind to submm observers; at a fixed observed wavelength, the distance dimming is offset by the increase in restframe flux. This leads to a sensitivity that is almost independent of distance in the redshift range z=1–10 (a redshift z=10 corresponds to a time only 480 million years after the Big Bang). This statement is true when observing at a relatively long wavelength (850 microns) and for dust grains characterized by a temperature 40K or lower. Already in Cycle 1 (the second call for proposals), it is possible to detect far-infrared luminosities of about 1011 Lo (that is 100 billion times the Sun's luminosity, a rather modest luminosity for star forming galaxies) at redshifts up to z~10 in less than 10 minutes of integration time.

The observed flux at two fixed wavelengths (850 and 450 microns) as a function of redshift. The total far-infrared luminosity is set to 1012 Lo and the flux is shown for several different dust temperatures. Figure ripped from one of my powerpoint presentations.

One thing that ALMA is not, however, is a survey instrument. The field of view (FoV) is given by the field of view of an individual antenna. For a 12m antenna, the FoV - aka as the ‘primary beam’ – ranges from ~62” at a wavelength of 3mm (100GHz) to 8.5” at 420 microns (720GHz), that corresponds to 0.8 – 0.016 square arcminutes, respectively. As a comparison, seen from the Earth, the Moon covers more than 2800 square arcmintes). ALMA therefore needs targets to point at and this is where the CANDELS fields become an invaluable source for ALMA targets. The GOODS-S, UDS, and COSMOS fields can be observed with ALMA, while the EGS and GOODS-N is outside the reach of ALMA.

To summarize, ALMA is already an existing scientific facility, providing the highest sensitivity and angular resolution at millimeter and submm wavelengths. The sensitivity is such that sub-L* galaxies (fainter than the most common galaxies) can be detected with a modest investment of telescope time. ALMA produces a data set that contains both spatial and spectral information. That is, is gives information on the velocity of the emission as well as an image of its distribution. ALMA has a very limited field of view and therefore needs targets to point at. This is where the synergy between ALMA and projects like CANDELS shine.

Wednesday, July 18, 2012

A tour of the five CANDELS fields. Last stop: EGS (Extended Groth Strip)

This post is the last in a series of posts that tour the five CANDELS fields. Our previous posts discussed the GOODS-North and -South fields, COSMOS, and UDS.

I hope it is true that the best is saved for last! But what does "best" mean? If you want the deepest data from X-ray to radio energies, the two GOODS fields win hands down! But each of the fields are small, so if, instead, you want the largest field, COSMOS is a no-brainer. On the other hand, since the near-infrared is where the action is to reach farthest back in time, UDS , with its panoply of some of the deepest near-infrared pictures ever taken from the ground, is also a contender for "best". But for a balance of area against depth from X-ray to radio and for sheer richness of spectral and imaging data, the Extended Groth Strip (EGS) is hard to beat. Prior to CANDELS, EGS was already well endowed and organized under an umbrella project known as the All-wavelength Extended Groth strip International Survey (AEGIS) -- check it out --it is a lovely site and has plenty of details on why the EGS is such a cool area for astronomers to explore! EGS was a natural to be one of the CANDELS fields.
Here is an example of what you will see with Google Sky
for the AEGIS field when choosing the Spitzer Space Telescope 
mid-infrared images. Note the markings of interesting objects 
such as a gravitational lens and a supermassive black hole. All 
the objects with circles can be clicked to reveal a wealth of data 
such as the redshift measured from spectra taken with the
Keck Telescope and even links to more information at

AEGIS is also a great area for non-professionals to explore. It is especially appealing to the eye with its relatively large region (twice the area of the two GOODS fields), all in color (using two filters). Not only can you enjoy "trekking through the field" comprised of 63 tiles of the HST Advanced Camera for Surveys (ACS), but also you can explore the region as if you had eyes sensitive from the X-ray through ultra-violet and all the way to the sub-mm energies. AEGIS was selected to be the first guinea pig for GOOGLE SKY to implement a feature that enables visualizing, simultaneously, a region with many, over-lapping, multi-wavelength surveys. The figure on the right is a glimpse of what you can do and see with this feature -- check here for details of how to implement this tool.

An interesting aspect of EGS is its origins. While the last in this series for CANDELS, its birth is actually the oldest and thus first of the CANDELS fields. The story began 18 years ago in 1994 when Professor Ed Groth of Princeton University, the namesake of this CANDELS field, led a new survey with the recently repaired Hubble Space Telescope (HST). HST had been refurbished with a new "set of glasses", the Wide Field Planetary Camera (WFPC2), that helped it see clearly and sharply, despite the spherical aberration problem in its primary mirror -- check out Groth's website under "Some Goodies" to read more about the vast improvement in the quality of images, as shown below:

This image illustrates a comparison of a ground based image, an HST image
before the spherical aberration was fixed, and an image after the fix -
all on the same star field at the same scale. You can see more and fainter stars
with the fixed HST for two reasons: first, the star images are smaller, so there's less
overlap; second, the smaller images can be detected against a smaller patch of
background light. Image credit: Ed Groth
This survey was a single long chain of 28 pointings, totaling about 140 square arc minutes in area, and known as the Groth Strip or Groth Strip Survey (GSS). Each pointing, except one, took roughly two hours of exposure time with the new camera and reached depths nearly 1,000,000,000 (1 billion) times fainter than seen with the naked eye. The exception was an "ultra-deep" pointing that had exposures nearly 7 times longer and thus reached about 2.5 times yet fainter. Groth designed the combination of area and depth of the survey to enable astronomers to answer our most profound questions in cosmology (the shape, size, and age of the universe) and about the birth, assemblage, and evolution of galaxies through the clustering distributions, counts, colors, sizes, and morphologies of the faintest, most distant galaxies.

GSS is noteworthy for the use of two filters in separate images. Such pairs of images provided a color in the optical, not only making it visually interesting, but also endowing the image with a vast potential of new information for astronomers to glean such information as the approximate distances of the galaxies, the youth of the galaxies, and even the masses in stars of the galaxies. Another noteworthy aspect was the intensive follow-up surveys for years to come, not only from the ground via images through additional filters or via spectroscopy using the world's largest optical telescopes (check out the Deep Extragalactic Evolutionary Probe or DEEP survey that used the 10-meter Keck Telescopes) but also from space, e.g., with 5 pointings of 200,000 seconds and 3 pointings of 800,000 seconds with an X-ray camera aboard the Chandra Space Telescope .

This is a map of the Extended Groth Strip (EGS) region of the 
sky, showing the sky coverage for several of the AEGIS data 
sets and the complexity of their shapes, sizes, and relative 
orientations. For reference, the full moon is shown to scale. 
Shown next to the moon at the 4:30 position is the size of 
the original Hubble Deep Field (pink shape). The Hubble 
Space Telescope (HST) images taken as part of AEGIS using 
the Advanced Camera for Surveys (ACS) are shown in grey in 
the center of the EGS. The new CANDELS WFC3 images and 
ACS images are imbedded within the central region of the original 
HST images. (Image credit: Christopher Willmer & Dale Kocevski)
Thirteen years later in 2007, after the next (3rd) generation instrument, the Advanced Camera for Surveys (ACS) became available, the original strip was extended in length and area. These Extended Groth Strip (EGS) images were acquired and served as the core of the AEGIS survey. Like its predecessor, two filters in the optical were used, but the area of sky covered was expanded by over a factor of 4 to about 600 square arc minutes, the largest, deep, contiguous, color mosaic image in the optical with HST of the distant universe. CANDELS, with its addition of more ACS optical images and exciting, new, WFC3 near-infrared images, is the natural next generation.

As with all the other CANDELS fields, AEGIS was a fertile hunting ground for astronomers worldwide to make new science discoveries, some resulting in "paradigm shifts" from an older, commonly-accepted view or paradigm to a new one. One example was the common view of the dominance of "major mergers", i.e., cosmic collisions of two hefty galaxies, and their resultant strong bursts of star formation to explain the rapid increase of star formation in galaxies back in time. But new data from AEGIS showed otherwise. Exploiting the rich, multi-wavelength data to estimate the star formation activity and amount of stars ("stellar masses"), Kai Noeske and his colleagues discovered that galaxies had a "single-track mind" during most of their lives while forming stars. They would lie on a narrow path in a plot of the amount of their star formation activity versus their mass in stars, meaning that few took large detours. Yet, such excursions should have been frequently seen if bursts of star formation activity, induced by major mergers, commonly dominated their lives. More recently, this view was reinforced by the exquisite HST images from four of the five CANDELS fields that were used to find close pairs or highly disturbed galaxies that were "major mergers" as well as "minor mergers" between dwarfs and hefty galaxies by Jennifer Lotz and her colleagues. They discovered that major mergers were less frequent while the minor mergers were dominant at earlier times.

As another example of a "paradigm shift", Kirpal Nandra and his colleagues discovered that distant galaxies which hosted active supermassive black holes (active galactic nuclei or AGN) were not, as commonly expected, blue. Such blue colors are the expected consequences of active births of new stars that should concurrently accompany the infusion of gas needed to fuel an AGN. These galaxies hosting AGN, chosen by being bright in X-rays, were, instead, surprisingly more passive and redder. This finding supports the alternative view that, while AGN may, through still-uncertain physical processes, help galaxies transition from their blue, active stage to their redder, quiescent stage, the actively growing stage of the supermassive black hole may last for an extended period, many 100's of millions of years, long after the galaxy has been "quenched".

In conclusion, while each of the CANDELS fields have their pros and cons, and proponents of each field may legitimately argue why their field may be "best" for solving this or that science problem, we can all clink our beer mugs together in agreement that the full set of five of the truly outstanding regions of the sky form a cohesive whole that is greater than any combination of its parts. The science originally proposed by Groth are as relevant today as when he envisioned them for his far humbler survey two HST generations ago. We all have no doubt that CANDELS will continue to serve as the premier real estate for the deepest, richest astronomical surveys for new generations of instruments and telescopes to come.

Monday, July 16, 2012

Midnight in the Garden of GOODS and AEGIS

I just could not resist the pun on John Berendt's best-seller as a title for the following tale of recent CANDELS behind-the-scenes adventures with Hubble Space Telescope ("HST") scheduling.  Much of my CANDELS effort, thus far, has been devoted to leading our Observations Planning and Scheduling Working Group.  In the metaphorical garden-of-science that will be the CANDELS observations, I fancy myself as both landscape architect and master gardener.  I do hope the kind reader will abide, as the explication of the title's strained pun will prove useful to the narrative.

CANDELS burning low...

At the time of writing, the HST observations that comprise CANDELS are roughly two-thirds finished. What still remains? Two target regions of our five total, the two northern-most in the sky:
In short: GOODS and AEGIS (see title, above).  I hasten to emphasize that absolutely no moral judgement whatsoever is to be inferred by this pun — both surveys have produced good science by good people! Indeed, large elements of both survey teams (including yours truly, from GOODS) merged HST proposals to form the CANDELS collaboration.

Anchorage in the the Spring

In mid-June, I had the pleasure of attending the American Astronomical Society ("AAS") semi-annual conference.  This time around, the AAS convened way up north, in Anchorage, Alaska. Along with a good bit of sight-seeing beforehand/afterward, I and some other team members showcased early CANDELS science results (see recent blog post).  The northern portion of Alaska, well north of Anchorage, resides above the Arctic Circle in the so-called "Land of the Midnight Sun". So called, because anywhere north of the Arctic Circle, for a portion of the year, the Sun will never set.  Translated into astronomy jargon that will re-appear later: the Sun will never enter occultation by the Earth. 

For most inhabitants of this rotating orb, the always-daily occurrence that the Sun enters occultation by the Earth (and also rises) is unremarkable, the natural order of things.
However, this is a conspiracy of triple-coincidence! Namely:
  1. The Sun has not yet tidally locked the Earth, which is rotating at 0.000696 RPM.  (slightly faster than once per 24 hours!)
  2. The axis of that rotation is only moderately tilted, by 23.4 degrees of arc, with respect to our orbit around the Sun.  Thus the Sun's coordinates in the sky are always oscillating within 23.4 degrees of the Celestial Equator.  Still enough wandering to drive our seasons, though!
  3. Virtually all people (and virtually all else) are living more than 23.4 degrees of latitude away from the North and South Poles.
The sky behaves very differently at the Poles (both North and South) — nothing sets daily: the Moon sets monthly; the Sun sets yearly.  All of the stars in the sky at the Poles never set: they wheel around the horizon at their same elevations, perpetually in view, and visible continuously throughout the months-long intervals when the Sun is far enough below the horizon for a dark sky.  During winter at the Poles, one may say that the entire sky is a "continuous viewing zone", where stars never set and are never hidden from view by the sunlit sky. 

The AAS conference ended just a week short of the Northern solstice (or "summer solstice", for us northerners), when the Sun reaches its farthest point north of the Celestial Equator, and daytime is at its longest in the Northern Hemisphere.  In Anchorage, 19 hours and 20 minutes of daytime, to be exact. I can report firsthand that the five-ish sunless hours of mid-June Anchorage only darkened the sky to a twilight before the sunrise.  Alas for us astronomers, all those non-setting stars wheeled around while hidden by the ever-too-bright sky during the several-day conference. 

Midnight in the Garden...

In June, midnight in the (Alaska Botanical) Garden is almost
this bright.  [Credit : Barbara Miller]
What is bad for astronomers in Alaska (nighttime never dark) is probably good for plants in Alaska.  I did some nature sightseeing around Anchorage, but unfortunately was unable to visit the city's lovely Alaska Botanical Garden.  I gather from their website that, unlike most of the rest of Anchorage, the various Garden exhibits remain open all through the "night". 

Had bright-night insomnia (and/or jet lag) been a problem for me, I could thus have spent many a midnight in the Garden, perhaps reading a best-selling novel, perhaps contemplating GOODS and AEGIS overhead, continuously viewable but for the twilight sky.

Panning upward from this pastoral scene, consider now the view from the Hubble Space Telescope some 550 kilometers above, racing along a full circuit of its low-Earth orbit every 96 minutes.  This remote-controlled satellite-telescope must keep pointed well away from the Sun (greater than 50 degrees) at all times, to avoid overheating its sensitive ultra-cooled internals. For the same reason, it must also keep pointed out away from the limb of the Earth, by at least 20 degrees if daytime there, or by at least 6 degrees if nighttime. (Not to worry, as HST apparently has multiple close-cousins with no such restrictions against peering downward.) Despite all these profound differences in perspective, between HST on high, and that garden up north, there are surprising parallels to be drawn.  Particularly as regards our current CANDELS observations of the GOODS-North field. 

Continuous viewing, Hubble-style…

It turns out that HST has its own Continuous Viewing Zone ("CVZ"): specific regions of the sky where, for specific intervals during the year, the Earth never gets in the way, orbit after orbit.  Moreover, the Sun's position in HST's sky is slowly oscillating throughout the year, just like its yearly-oscillating height above and below the polar horizon.  Even better, HST's view of the firmament does not wheel around once per day, like it does for observatories down here on terra firma.  One might suspect that observing targets in Hubble's CVZ would make the most efficient use of HST's precious time, never needing to pause picture-taking because the Earth has blocked the shot.  That pause can be lengthy: typically 40 percent of every orbit suffers from occultation. 

The CVZ efficiency was a prime motivation for deciding the coordinates of the original Hubble Deep Field ("HDF") — 342 separate HST exposures taken during 10 consecutive days in 1995.  The HDF proved to be such a smashing success that the intervening 17 years have witnessed many deep-sky surveys, from a wondrous variety of telescopes on the ground and in space, blossoming all around the HDF.  These include the HST surveys GOODS and now CANDELS. 
The CANDELS GOODS-North region is located within Hubble's
northern Continuous Viewing Zone, as shown by this diagram. 
[Credit : NASA]
One of the primary objectives for both GOODS and CANDELS science has been the discovery of supernovae at extreme distances. We find these supernovae by comparing sensitive HST images of the same spot of sky, separated in time by several weeks. Because CANDELS is using a newer, infrared ("IR") light-sensitive camera on Hubble, we can hunt supernovae even farther away than had been possible with GOODS.  The brightening and fading of these cosmic fireworks appear slower with increasing distance, so the cadence of the CANDELS time-lapse photography is longer: 7.5 week intervals, versus 6.5 week intervals for GOODS. 

By happy coincidence, the opportunities for Hubble CVZ observations at the location of CANDELS GOODS-North also recur every 7.5 weeks! Lest you be overcome with irrational exuberance, I will spend the remainder of this post elaborating upon several complexities of our Hubble CVZ observations.  Some are generic to any CVZ-desiring program; some are exceptional headaches for CANDELS.  Surmounting these difficulties is an ongoing challenge, to be sure, but hopefully will prove worthwhile in added science returns from the venture.

Earth never gets in the way, but it is never far from view...

As can be seen from the CVZ orbit diagram above, the geometry is such that Hubble always points along a grazing incidence to the Earth.  In fact, the CVZ orbits flirt with the forbidden zone below 20-degree separation from the daytime Earth limb.  And fully half of every CVZ orbit will be looking past the edge of daytime Earth, with that annoyingly bright sky.  CANDELS is searching for exceedingly faint galaxies and supernovae by stacking multiple long-exposure HST images.  When those images are pointing near to the sunlit Earth, it is akin to midnight stargazing in mid-June Anchorage — an exercise in frustration.  So how does one make effective use of that bonus time in CVZ orbits, then?

Here on the ground, when confronted with an excessively bright sky, people resort to sunglasses that near-completely block the ultraviolet ("UV") portion of the scattered sunlight.  Up on Hubble, the CANDELS team has adopted a nearly opposite solution: during the bright-Earth exposures of our CVZ orbits, we use light-blocking filters that transmit only UV through to the camera.  The intensity of scattered UV light from the daytime Earth is low enough to avoid polluting our long-exposure images of the UV cosmos.  Rather than putting the 40 percent CVZ bonus time into glared-out optical or IR exposures, we obtain a sensitive UV survey complementing our optical/IR survey at no extra cost! 

Continuous Viewing Zone opportunities are not so plentiful...and not so continuous...

When CANDELS was first proposed, our intention was to exploit the matching cadences of the GOODS-North CVZ and the supernovae-searching by conducting ten successive search epochs of 15–16 HST orbits apiece.  The HST Time Allocation Committee approved this strategy, but the implementation quickly ran afoul of the realities of HST scheduling.  In the immortal words paraphrased from Moltke the Elder, "No battle plan survives contact with the enemy." Due to the vagaries of the HST orbit, the windows of opportunity for GOODS-North CVZ have been very narrow in our first few epochs.  Three to four days, tops; sometimes as narrow as two days. Doing the orbits-per-day math with 96 minute orbits (= 14–15 orbits per day), you might conclude that even two days is more than ample for our 15–16 orbit epochs.

Alas, there are several devils-in-the-detail that prevent CANDELS from observing during more than five-ish orbits on a given day, CVZ or otherwise. Foremost among these complications is the forebodingly-named South Atlantic Anomaly, or "SAA".  Unsure why, but this term always reminds me of the paranormal Bermuda Triangle.  Nothing paranormal about the SAA, though, which is much larger, and well to the south.  It is simply the point of closest approach to Earth of the irregularly-shaped Van Allen radiation belts girding our planet.

As you might guess, HST battens down its proverbial hatches every time its orbit passes through, or even near to, this quasi-stationary radiation storm.  The SAA is so large that 7–8 consecutive HST orbits per day are buffeted as the Earth (including the South Atlantic and its Anomaly) rotates underneath.  These SAA-impacted orbits are no good to us: CANDELS has way too much picture-taking jam-packed into every orbit to pause for a radiation storm. 

As for the six-ish remaining orbits per day that are clear sailing, CANDELS must share these together with all other HST observing programs, and HST station-keeping duties.  Amazingly enough, CANDELS is not the only HST observing program with stringent calendar constraints for the HST schedulers to juggle.  The end result is that CANDELS cannot expect all its "CVZ-length" orbits to fit within the available windows — sometimes up to half the orbits spill out.  The silver lining is that GOODS-North CVZ windows are usually lined by a day or two of "almost-CVZ" orbits.  These are still much longer than normal, and perfectly fine for CANDELS.

Snake in the garden…

Despite wanting entirely non-occulted orbits for the fullest amount of UV picture-taking, CANDELS never actually has HST stare at the same spot for more than one orbit. Every new orbit is targeting a new, nearby location, in order to map out an area of sky 15–16 times larger than the HST camera's (small) field of view.  Sliding Hubble's boresight and taking aim at a new target are actions that require several minutes of down-time between picture-taking.  With careful advance planning, we can arrange for this down-time to coincide with occultation, so as not to lose any picture-taking opportunity.  At worst, we need to jettison one of our two nice, long UV exposures to fit these sub-CVZ orbits. 

One noteworthy complication of using close-but-not-quite CVZ orbits is that we need to plan the orbit differently, depending on whether the HST schedulers have slotted us before or after the true-CVZ window.  For CANDELS purposes, all occulted HST orbits are defined to "start" immediately after the prior occultation ends.  For GOODS-North, the orbits immediately preceeding the CVZ window are starting with the bright Earth below, and transitioning over to dark Earth during the orbit.  The exact opposite is the case for orbits immediately following the CVZ window — the orbit starts over dark Earth and ends over bright Earth.

As you may guess, some care is needed with these near-CVZ orbits to ensure the optical/IR exposures and the UV exposures are commanded in the proper order! Hopefully the following diagram may clarify the situation better than another thousand words.  First, a shout-out to CANDELS post-doctoral fellow Marc Rafelski, for creating this highly useful graphical representation of CANDELS CVZ scheduling.  The three panels march along through the hours of one single day: in this case, 22 July 2012.  The undulating ribbon through the middle of the panels is the so-called "limb angle" of the Earth with respect to HST when pointing at our CANDELS GOODS-North region.  Each undulation represents one full orbit of HST around the Earth (taking 96 minutes).  The limb angle cannot dip below 20 degrees, or the target is considered to be occulted by the Earth.  The play of colors along the ribbon may remind you of a poisonous snake in the garden, but is intended to denote bright Earth limb (yellow), dark Earth limb (black), and twilight Earth limb (purple).  You can see that the snake is hacked into many pieces by the Earth occultations (light gray bars) and the SAA passages (salmon-colored bars).  Only the first half of the day is true-CVZ (no occultations), and one of those orbits is spoiled by an SAA passage.  The goal is to precisely align our CANDELS observations with the largely or completely uninterrupted orbits, placing down UV exposures during "day" and optical/IR exposures during "twilight/night".  Toward the end of this particular day, you can see that the occultation zones have grown quite large.  They will remain large until the next GOODS-North CVZ window approaches, in another 7 weeks. 

A day in the life of HST, while pointing at the CANDELS GOODS-North field.  The predicted angle between the Earth limb and HST is plotted over the course of a particular upcoming day in July.  During the first half of this day, there are several opportunities for HST Continuous Viewing Zone observations.  See text, for detailed description.  [Credit : Marc Rafelski]

Timing is everything...and is unpredictable...and is hard to pin down...

As mentioned above, our goal in each CANDELS CVZ orbit is to obtain UV exposures when HST is pointing out across the sunlit Earth, and optical/IR exposures when pointing out across the nighttime Earth.  If we get this wrong, we risk harm to some or all of our bread-and-butter optical/IR exposures by drowning them in glare from the bright Earth.  Minutes count, here. 

Problem 1: the timing of bright-Earth/dark-Earth transitions requires knowing exactly where HST will be, which turns out to be an impossible feat.  You might think that 550 kilometers overhead would qualify as more than "low Earth orbit", but there is actually enough tenuous atmosphere that far up to drag HST very slowly downward.  The drag on HST varies unpredictably based on the wispiness of that thin air, and HST's orientation as it plows on through.  This translates to inherent unpredictability of HST's exact location into the future.  No coincidence that the accuracy is akin to weather forecasting, according to the HST Primer: "For example, the predicted position of the telescope made two days in advance can be off by as much as 30 km from its actual position.  An estimated position 44 days in the future may be off by 4000 km (95% confidence level)." This seems like a vast uncertainty, until one considers that HST merrily rolls along its orbit at 7.5 kilometers per second.  Meaning that our timetable of day/night boundaries may be off by 9 minutes.

For CANDELS CVZ observations, a 9 minute miscalculation is bad news. One, if not two, of our five dark-sky exposures in every orbit will be "scorched" by either starting too early or ending too late.  To best avoid this unpleasantness, with each successive epoch of CANDELS GOODS-North observations we calculate exposure start-times using the most recent HST ephemeris prediction possible before those orbits' exposures must be placed on Hubble's next to-do list.  So far, this labor-intensive, real-time supply chain scheme has worked acceptably well. 

Problem 2: The HST orbit-scheduling juggernaut, which must fit together the jigsaw puzzle of thousands of orbits allocated each year to hundreds of deserving programs, needs to be fitting that puzzle together weeks ahead of actually commanding the telescope to carry out the observations.  This juggernaut is not designed to allow the end-user to lock down orbits' start-times to the minute, particularly when that desired minute is ill-defined in advance.  Fortunately for CANDELS, we have overcome this hurdle by virtue of even-deeper-behind-the-scenes legerdemain from our Program Coordinator, assigned by the Space Telescope Science Institute to ferry our program (and many others) from the drawing board to the telescope commanding. 

I refer curious readers to the Hubble Space Telescope Primer for a more thorough description of these complexities, and for an excellent overall description of HST and best practices for its use.  To conclude this rambling tale, I first cite the dire warning in the HST Primer to those who dare contemplate HST observations in the CVZ:
"There have been cases in the past (e.g. the Hubble Deep Field observations) where optical imaging has been interleaved with other kinds of observations.  However such observations are difficult to schedule and require strong science justification. ... CVZ observations are also generally incompatible with special timing requirements (e.g., timing links, special spacecraft orientations, or targets of opportunity...)."
Naturally, CANDELS is fiddling with all three of these special timing requirements in and around our CVZ observations.  So far, with two out of our ten CVZ epochs completed and the third nearing execution, we have been largely successful in scheduling these complex observations as desired.  Here's hoping that 18 months from now, when all these CANDELS CVZ epochs are completed and the images all stacked together, you will be reading blog posts about the wonderful resulting science and will have long forgotten these behind-the-scenes tribulations.