The Winding Road of Becoming an Astronomer

When did I decide to become an astronomer? That's an interesting question, since I don't remember actually deciding, it just sort of happened.  I've always liked space; I remember watching Space Shuttle launches on TV as a kid, and I vividly remember where I was when I heard about the Challenger disaster. But I went through a lot of phases, at times also thinking I wanted to be a vulcanologist and a meteorologist (though really I just wanted to be a tornado chaser, inspired in no small part by Twister). So science was always high on my list, but I think it was my senior year in high school when I really got attached to astronomy, mostly due to my awesome experience in my physics class (thanks Mr. Haff!). I remember when we were learning about gravity, I went home and, just for fun, did all of the homework problems in that chapter of the book (yep, I was a nerd). So, when it came time to choose a major, I didn't agonize too hard before choosing astronomy (though I nearly chose Music Education; I played trombone throughout high school and college).

Keely and I near Anchorage, doing some sightseeing during the June 2012 AAS meeting.

Things got a little bumpy in college at the University of Washington; due to the aforementioned trombone playing, I didn't have a lot of time for classwork, since I spent my nights practicing marching band formations in Husky Stadium (explaining my lifelong love of the Huskies to anyone who knows me; and yes, they were good back then!). In fact, I remember a key point at the beginning of my junior year when I nearly quit the major altogether. I was taking an upper division electromagnetism (E&M for those in the know) course at the same time as an upper division math course that was listed as a prerequisite for the E&M course, but they allowed me to take it concurrently. I remember being utterly lost for the first few weeks of the E&M course, since the professor was using math that I hadn't learned yet (multivariable calculus for those scientists out there). I have a vivid memory of standing outside the physics building in the rain (Seattle, so it was raining of course), about ready to quit. But, in hindsight what was probably one of the major turning points in my life, I didn't quit. A few months later, I decided I wanted to go to grad school, and I busted my tail off for the next year to get my grades up, and prepare myself for the next step.

Our son Kieran, super excited about his
octopus face painting.

I attended Arizona State University for graduate school. At the time, the department was on the small side (though it has grown rapidly), and did not represent a high pressure environment. But that environment is exactly what caused myself and my classmates to thrive (of the five of us who earned our Ph.D's in 2008, three already have permanent positions, and two more are CANDELS postdocs). A huge advantage of being at ASU was access to the Steward Observatory telescopes, which I took full advantage of, observing at the MMT, Magellan, and the Steward Bok telescopes (as well as spending time at both the NOAO Kitt Peak and Cerro Tololo 4m telescopes). Grad school is a tough time for everyone, but we had a great group of friends that were committed to their work but also to having fun, which made it not *too* bad.

Another major event happened for me in Arizona -- I met my wife, Keely. We had actually both gone to UW for undergrad, but we were a year apart so we never met. We started dating towards the end of my first year, and we got married in August of 2008 (right in between our thesis defenses, talk about a crazy time!). After carefully weighing our options we decided to accept postdoctoral positions at Texas A&M University. While College Station is not exactly a cultural mecca, we enjoyed working in a growing department with several young faculty, who were great at promoting our careers. It was also great for our family life, as halfway through our stay, we welcomed our son Kieran to the world.

The space shuttle flying over Austin
on its way to LA.

After three years in College Station, I was fortunate to win a Hubble Fellowship (on my fourth try nonetheless), which I took 100 miles west to the University of Texas at Austin. UT happened to be searching for a new faculty member in my area (high redshift galaxies), so during my first year in Austin I spent much of my time applying and interviewing for that job, which I was eventually offered. So, now after a little over four years in Texas, my family has settled in for the long haul (and those of you who have been to Austin know that its not a bad deal!).

So, that's my life story (or at least my astronomy life story). What can be learned from it? First, being an astronomer/scientist/academic is hard. While I'm sure some people know their whole life who and what they want to be, and never waver, that was not my experience. I nearly left astronomy at least three times throughout the years, and it was only a few key choices (that were well thought out, though also some were the result of being in the right place at the right time) that got me to where I want to be. Astronomy is just a job, but we each only have one life, and I've tried to remember that when I've been confronted with choices throughout my career.

Our dogs Jasmine (left) and Bella (right).

Meet Christina Williams

My name is Christina Williams, and I am a grad student in Astronomy at the University of Massachusetts in Amherst. I study galaxy evolution with CANDELS data as part of my PhD thesis. I'm working with Mauro Giavalisco as my thesis adviser, studying compact, massive, elliptical galaxies and their evolution. How and why I arrived at an astronomy department for work is in some ways similar to other astronomers, and in some ways different. Like many, I had an early fascination with the world around me, how nature works, and in particular the night sky, which is what led me here! I grew up in Washington, DC, a city full of culture and people from all over the world, but not so much in the way of nature and dark skies. I was lucky to have extremely interesting and inspiring science teachers in elementary and middle school, who showed us all sorts of fascinating gadgets in the lab and taught us about things like volcanoes and tornadoes, which you don't find in Washington. But I was especially lucky to have science teachers and advisers throughout my life, who took a special interest in my ambition to learn science, and made sure to foster it. By high school, I was convinced Astronomy was my route in life, giving in to my hunger to learn more and more fundamental aspects of science. The universe, it seemed to me, was about as fundamental as it gets!

I went to college to study Physics, not too far from home at Johns Hopkins University (JHU) in Baltimore, MD, which had a very research oriented department of Physics and Astronomy. With its location across the street from Space Telescope Science Institute, it seemed like the perfect place to be introduced to the world of astronomical research. I immediately joined a research group focused on low-mass stars and brown dwarfs, with whom I worked for all my four years there. Since JHU is a part of the Sloan Digital Sky Survey, we had loads of telescope time at its host observatory, Apache Point, in New Mexico. I traveled there several times for observing runs and also observed remotely (through a computer) from Baltimore.

Aurora Borealis over my cabin in Fairbanks, AK.
Photo credit: Christina Williams

The summer before my senior year, I decided to try something new for one summer, and received an internship studying the polar ice caps of Mars at the Geophysical Institute at the University of Alaska in Fairbanks (UAF), as part of the Research Experience for Undergraduates program (REU; see this recent blog post by a CANDELS REU student). This was an important scientific, and life changing, experience. It was there that I first learned how to make physical models with a computer, which is what many astronomers spend much of their time doing. And it was then that I fell in love with Alaska, with its big mountains, glaciers, and vast wilderness to explore. Not ready to leave Alaska, but desiring still to continue with scientific research, I took a brief hiatus from astronomy and enrolled in a masters program in Geophysics at UAF. While there, I wrote a thesis on the formation of the Arctic Ocean and its tectonic history, (something totally new for me), which is a part of the Earth that is still very poorly understood because the sea ice in the Arctic makes studying the ocean floor difficult. Living year-round in Alaska offers many exotic experiences for an exploratory spirit. Like many other Fairbanksans, I lived in a cabin without running water. There were Northern Lights to photograph, hot springs to ski to in the winter, and remote and wild rivers to float in the summer.

Fieldwork on the sea ice near Barrow, AK

I finished my masters degree mid-(academic) year, so before returning to the world of Astronomy and starting my PhD the next fall, I accepted a job with the arctic sea ice research group at the Geophysical Institute at UAF. They hired me to work with the marine radars they had set up on the Arctic coast, as part of an ice observatory, which monitors the real-time motion and other changes in the coastal sea ice. I also went to Barrow, Alaska, for fieldwork out on the ice. To get out there, we rode snow-machines on landfast sea ice (sea ice which is grounded off the coast after winter), testing the electric conductivity and albedo of the ice, and taking core samples to learn about how the ice composition changes over time. One of many goals of this constant monitoring is to learn about how the ice patterns have been changing in recent decades. This is increasingly important in part because Inupiat Eskimo communities, who have relied on knowledge of seasonal landfast ice patterns for subsistence hunting for centuries, are now faced with the need to adapt to changes in the arctic.

ASTE site, in the Atacama Desert, Chile

The next fall I started my PhD in Astronomy at UMass. The astronomy program here is structured such that you have two different research projects before choosing a thesis topic, and in this way get exposed to a variety of sub-fields within astronomy. In my first few years here, I studied sub-millimeter galaxies (SMGs), and their clustering (see this recent blog post, and also this one). These are galaxies which are so dust-obscured, they are often only observable at long wavelengths. The dust blocks and absorbs the starlight which heats the dust, and the dust re-radiates in the far-infrared part of the spectrum. By the time the light reaches us, it is red-shifted to the sub-millimeter part of the spectrum. The wavelength we observed in is 1.1 mm, and is extremely sensitive to the amount of water vapor in the atmosphere. This means observations need to be taken from extremely dry regions of the Earth. We were using a Japanese telescope located in the Atacama Desert in Chile, called the Atacama Submillimeter Telescope Experiment (ASTE), which at the time was using a sub-millimeter detector called AzTEC (acronym translation available here) that was developed here at UMass. (This detector is now in Mexico being tested on the new Large Millimeter Telescope (LMT), which will be the biggest single dish sub-millimeter telescope on Earth!) To balance all the time I end up sitting in front of a computer, I went down to Chile for observing and general observatory maintenance at ASTE for a month!

Sub-millimeter astronomy is intriguing because very little is known about the galaxies that produce this kind of light because detectors and observatories in this wavelength regime are relatively new advances in technology. So there are many unanswered questions, which hopefully other new observatories such as the Atacama Large Millimeter Array (ALMA) in the Atacama desert and the LMT in Mexico will help us understand. Part of my goal in studying clustering of SMGs was to understand if they have an evolutionary connection to massive elliptical galaxies. But the unfortunate thing about current sub-millimeter observatories is that they have very low resolution imaging, which means you rarely get to see the shape and morphology of what you're looking at in detail. Perhaps that's one of the reasons that led me to Hubble Space Telescope (HST) and working with CANDELS data for the rest of my PhD thesis. I definitely love looking at the beautiful high-resolution images from HST that show the morphologies of galaxies!

Climbing Pigeon Spire, Bugaboo Provincial Park,
British Columbia

But, I might be a pretty boring scientist if science was all I was interested in. Getting the mind off work periodically is really important, not only because we are human, but also because it allows the brain some perspective for solving problems and can result in small epiphanies. What better way to gain a little different perspective than to climb hundreds of feet off the ground? Probably my biggest passion outside of science is climbing, which I started in college, and it has taken me to remote corners of the world. But even exploring the climbing here in New England has been a wonderful opportunity, because Amherst is centrally located between many world-class climbing areas (which are much more accessible than much of the climbing in Alaska!).

Where in the world I go from here is anyone's guess! I hope to graduate soon, and find a good postdoctoral position where i can continue exploring unanswered questions about high-redshift galaxies. Some serious hurdles currently face junior astronomers on the job market. The number of PhDs in astronomy exceeds the number of available permanent positions in astronomy, so competition for jobs is fierce and the prospects can be quite daunting. Astronomers typically do two postdoctoral positions before finding a permanent position. These are things I will face soon enough. But its important to keep in mind that often a big hurdle to success is lack of confidence in ones own abilities. Among all the science I've learned in graduate school, one of the most important lessons I've learned thus far is that believing in yourself is not just a cliche phrase, but has some serious truth in it. None of us arrived where we are by doubting ourselves. And this lesson I'll take with me no matter where I go.

Meet Steve Rodney

From time to time we'll bring you a biographical post introducing one of the astronomers writing for the CANDELS blog.  This week, we introduce Steve Rodney.  You can find his earlier posts here.

View of Hanauma Bay on Oahu.

Currently I'm a postdoctoral researcher and a Hubble fellow at the Johns Hopkins University (JHU) in Baltimore, Maryland. Before landing here on the shores of the Chesapeake, I was a graduate student at the University of Hawaii Institute for Astronomy. Several of my classmates from Hawaii are also part of the CANDELS team: Dale Kocevski, Liz McGrath, and Jeyhan Kartaltepe were all Hawaii grad students while I was there. We're now scattered across the country, but team meetings give us the occasional opportunity to reminisce about shave ice, moonlight surfing, and Andy's sandwich shop. 

Me and the kids in Baltimore.

JHU is a great place to be, and I am very lucky to be working here with some fantastic people… but unfortunately living in Baltimore is a source of some significant environmental stress for me. Not because of the drugs and crime of Baltimore (excellent dramatizations notwithstanding, Baltimore is actually a great city to live in). No, I'm referring to the deep emotional trauma that comes from growing up a fan of the hapless Cleveland Browns, and now watching my 4-year old daughter wearing her Ravens jersey to pre-school. (If you're not an NFL fan, then think of the Ravens vs Browns as Manchester United vs Liverpool, but imagine Liverpool is hopelessly inept over many decades. If you're not a soccer fan either, then... well, nevermind). I can only hope that my daughter will eventually outgrow this phase of moral depravity.

One of the conversations that will come up for most any astronomer from time to time is the discussion of "Why astronomy?" Sometimes this is about personal choices: "Why did you choose to become an astronomer instead of a chemist or a doctor?" For myself, astronomy has always been a study that I was drawn to because of the stories. As a kid, I read the legends of Greek mythology, and was fascinated to find them echoed on the sky in the constellations. I loved to trace the stories of ancient heroes, gods and monsters that were painted across the heavens. Later, the study of physics opened up for me the fascinating stories within the stars, from the birth of a star incubated in a dusty envelope of gas, to the fragile beauty of stellar death throes. Studying astronomy takes these wonderful images and unfolds them to reveal the complex puzzles and deep mysteries of the universe. For me personally, this was the hook that drew me in. 

Cherry blossoms and the Jefferson
Memorial in Washington, D.C.

Sometimes the "why astronomy" question is framed more broadly: "OK, maybe astronomy is great for you, but what practical use does it serve in our society?" A glance at recent Nobel Prizes can illustrate this very legitimate question. Astronomers won a Nobel prize in 2011 for discovering Dark Energy. Exotic and fascinating, but also intangible and wholly disconnected from everyday human lives. In contrast, the recently announced 2012 Nobel prize in chemistry recognizes advances in the understanding of G-protein-coupled receptors (GCPRs). This work is fundamentally connected to drugs used in the treatment of a wide range of ailments, from common allergies and high blood pressure to breast cancer and schizophrenia. One might look at these two fields and legitimately question whether we as a society should be investing so much (both in dollars and in human capital) in the study of distant stars, when there are real problems with real people that can be addressed with other avenues of scientific exploration. 

I think this is an important question, and a conversation that we astronomers and fans of astronomy should have more often and more publicly. Especially in a time of tight budgets for research funding and significant skepticism about the value of science in general, we all should have a coherent argument for why basic research is important. For me, the answer is that the pursuit of understanding is a fundamental quality of humanity. What separates us from other species on this planet is our ability to consider the universe, to seek a deeper understanding of how it works and what is our place within the grand cosmos. In a way, astronomy is a bit like poetry, art and music. It is a discipline that provides its own reward by enriching our lives. We astronomers should never forget that it is a special privilege to be able to devote ourselves to this task, and that we have a responsibility to share what we learn with the world around us. 

Sunrise over Maui, viewed from Makapu'u point on Oahu.

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

 NASA Extragalactic Database.(Image credit; Google Earth)

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.

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:

• the second of two narrow-area, deep-exposure regions, both within the Great Observatories Origins Deep Survey ("GOODS"); (see recent blog post)
• the third of three wide-area, shallow-exposure regions, this one located within the All-wavelength Extended Groth Strip International Survey ("AEGIS").

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.

UDS: The Ultra Deep Survey

This post is the third in a series of posts describing the five fields being targeted by CANDELS and is focused on the Ultra Deep Survey. Our previous posts described the GOODS-North and -South fields and COSMOS.

It's probably fair to say that the Ultra Deep Survey (UDS) is the least well known of the five survey fields that are now being targeted by CANDELS. However, even before the start of the CANDELS program, the UDS featured the deepest near-infrared (near-IR) imaging of any wide-area survey and was a premier field for studying the evolution of massive galaxies in the distant Universe. In fact, the UDS field has a rich history, extending back more than a decade and, as is often the case in astronomy, tends to be known by different names, depending on what wavelength of light you happen to be working with. When we refer to the field as the UDS, we are specifically referring to a near-IR imaging program (Principal Investigator: Omar Almaini, University of Nottingham) which has been pursued by the United Kingdom Infrared Telescope (UKIRT) in Hawaii for the last seven years. Given that CANDELS itself is fundamentally a near-IR imaging survey, before discussing the UDS in detail, it is perhaps worthwhile considering why observing galaxies in the near-IR part of the electromagnetic spectrum is such a powerful technique.

Left: The UK Infrared Telescope (UKIRT) on Mauna Kea, Hawaii. Right: The Wide-Field Camera (WFCAM) which is being used to observe the Ultra Deep Survey (along with four other major surveys). Image credit: Joint Astronomy Center, Hawaii

One of the key goals of extra-galactic astronomy is to try to obtain a full understanding of the formation and evolution of the most massive galaxies in the Universe. This goal is fundamental because most stars in the Universe eventually end up in massive galaxies and the evolution of the most massive galaxies has traditionally been the most difficult to accurately describe within the framework of theoretical models. Consequently, the optical properties of massive galaxies in the relatively nearby Universe have been the subject of intense study for many decades.

However, in order to gain a better understanding of galaxy evolution it is clearly necessary to investigate how the galaxies evolve as a function of cosmic time. It is within this context that observations in the near-IR (wavelengths of 1.0-2.5 microns, just long-ward of where the human eye loses sensitivity at about 0.7 microns) are key. In fact, observations in the near-IR have numerous advantages (and disadvantages), but two strengths of near-IR imaging are particularly relevant here.

The first key advantage of near-IR observations is that they allow us to study the optical properties of very distant galaxies.  As we observe galaxies at greater and greater distances, the light emitted by these distant galaxies is stretched, or "redshifted" in the jargon, during its journey towards us due to the continued expansion of the Universe. In fact, because redshift (denoted by the letter "z")  is directly related to distance, astronomers invariably refer to galaxies as being "at redshift z=1", rather than at a distance of so many billion light years. Due to the redshifting effect, the ultra-violet (UV) and optical light emitted by distant galaxies is shifted towards the near-IR part of the electromagnetic spectrum. Consequently, by observing distant galaxies in the near-IR we can actually study their optical properties, and therefore compare them on an equal basis with galaxies observed in the local Universe. The second key advantage of near-IR observations is that they allow astronomers to get a much more accurate measurement of the stellar mass of a galaxy, because near-IR luminosity is a much better tracer of stellar mass than optical/UV light.  The straightforward reason for this is that near-IR light is dominated by the low-mass, long-lived, stars that actually account for the bulk of a galaxy's mass. In contrast, if a galaxy is undergoing star-formation, optical (and particularly UV) light can be dominated by very massive/luminous short-lived stars which, although spectacular, only account for a small percentage of the total stellar mass.

These two key properties of near-IR observations were the primary drivers behind the first generation of near-IR surveys which were undertaken about a decade ago. Perhaps the key result from these early near-IR surveys was that, unexpectedly at the time, at a redshift of z=1 (at which point the Universe was basically half its current age) a large fraction of the most massive galaxies (galaxies containing the same stellar mass as 100 billion stars like our Sun) where already in existence. However, if you push your observations further back in time, out to perhaps a redshift of z=3 (when the Universe was only 15% of its current age), you find that only a small fraction of the massive galaxies we observe locally already existed at this earlier epoch. Consequently, it became clear that the 3.5 billion years of history contained within the redshift interval 1<z<3 constitutes the "epoch of massive galaxy assembly", during which the vast majority of the massive galaxies we observe in the local Universe were formed. The primary motivation for the Ultra Deep Survey was to study this key epoch in the evolution of the Universe.

A cluster of massive galaxies discovered using a combination 
of the X-ray, optical, near-IR and mid-IR data in the UDS. The 
galaxy cluster (concentration of red objects in the center of the 
image) is at redshift z=1.6, an epoch when the Universe was only 
30% of its present age. Image credit: NASA/JPL-Caltech/
C. Papovich (Texas A&M University).

The Ultra-Deep Survey (UDS) is actually just one of five surveys being pursued by the United Kingdom Infrared Telescope (UKIRT), using a dedicated, wide-field, near-IR survey camera (WFCAM).  As is nearly always the case these days (and is certainly true for CANDELS), the near-IR surveys conducted by UKIRT form a so-called "wedding cake" structure, which describes the arrangement of increasing sky coverage, but decreasing sensitivity, as you proceed from top-to-bottom of the wedding cake.  Covering a sky area of 0.8 square degrees, the UDS forms the top layer of the wedding cake, covering the smallest area of sky, but with the necessary sensitivity to detect and study galaxies in the very distant Universe.

As a result of the huge investment of observing time necessary to build-up any large-scale multi-wavelength survey field, the observations are always obtained over a number of years. Consequently, the same area of sky tends to end-up being referred to by a confusing number of different names and acronyms. This is certainly the case for the UDS.  The original dataset which initiated everything else consists of deep X-ray observations by the XMM-Newton X-ray satellite, taken as part of the so-called XMM Large Scale Survey (XMM-LSS). Subsequently, very sensitive optical imaging was taken within the same area by the 8m-class Subaru telescope in Hawaii which, together with the existing X-ray imaging, formed the Subaru XMM Deep Survey (SXDS). The near-IR observations with WFCAM that constitute the UDS were then located in the middle of the SXDS in order to take full advantage of the existing optical and X-ray imaging.

In addition to the existing data, another crucial aspect of the location of the UDS is the fact that it is an equatorial survey field. This simply means that it is located close to the celestial equator (the projection of the Earth's equator onto the celestial sphere) and as such, is within the overlap region which can be readily observed by telescopes situated in both the northern and southern hemispheres. This is a key advantage because it means that the world's most powerful telescopes, which are mostly either located on Hawaii in the north or in Chile in the south, can all contribute to building-up the necessary database of multi-wavelength imaging and spectroscopy.

Indeed, over the last decade the archive of multi-wavelength data in the UDS has become enormous, spanning wavelengths from high energy X-rays at one extreme, to ultra-sensitive radio data at the other (more details of multi-wavelength data available in the UDS can be found on the UDS website). In addition to the imaging and spectroscopy contributed by some of the world's most powerful ground-based telescopes (i.e., Subaru, VLT, UKIRT), many of the key datasets in the UDS have been delivered by space-based observatories. In addition to the original X-ray data from the XMM-Newton satellite, the UDS has been imaged in the UV with the GALEX satellite, in the mid-IR with the Spitzer satellite and now, with the advent of CANDELS, at high-spatial resolution in the near-IR with the Hubble Space Telescope (HST).

A true-colour image of the Ultra Deep Survey field. The zoom-in shows the location of a galaxy at redshift z=6 (red arrow), identified at an epoch when the Universe was only 5% of its present age. Image credit: Omar Almaini, Nottingham University (P.I. of UDS)

Armed with this impressive archive of multi-wavelength data, many astronomy groups around the world are now exploiting the UDS to tackle the outstanding issues of galaxy evolution. In the past few years, much work has been done to address the key science goals which originally motivated the UDS, exploring how the number densities, luminosities, stellar masses, sizes and morphologies of massive galaxies evolve as a function of cosmic time. Moreover, the UDS dataset has been used extensively to study how massive galaxies cluster together, both on small and large scales, and how their clustering is linked to the properties of the dark matter halos in which they reside. Interestingly, out-with the original science goals, the UDS dataset has also been extensively used (including by this author) to push the study of massive galaxies back into the early history of the Universe at redshifts of z>6, when the Universe was only 5% of its current age.

In this context, the availability of HST imaging from CANDELS offers the prospect of hugely enhancing the power of the UDS dataset for pursuing  detailed studies of galaxy evolution at high redshift. The unrivalled sensitivity and sharpness of the near-IR imaging provided by HST will allow the properties of galaxies to be studied in detail at much larger distances than was previously possible. As a consequence, in combination with the vast array of data already assembled, the HST imaging provided by CANDELS promises to secure the position of the UDS as a leading resource for the study of galaxy evolution for many years to come.