Spy Satellites to Look Up?

The National Reconnaissance Office has given NASA two telescopes with mirrors the size of Hubble's. This is potentially a game changer.

The CANDELS survey, which has the largest time allocation ever granted on Hubble -- 902 orbits, or about 3-4 months -- could be accomplished in just a few hours on such a telescope if it were to be outfitted with a camera with a wide field of view.

The Hubble Space Telescope, seen from the space shuttle
Credit: NASA

Hubble's main advantages over ground-based telescopes are:

• It is unaffected by turbulence and clouds in the earth's atmosphere.
• It can detect ultraviolet radiation, which does not penetrate the earth's atmosphere.
• It can observe efficiently at near-infrared wavelengths because the sky is darker from orbit and the view is unaffected by water vapor and other interference from earth's atmosphere.

Hubble's big disadvantages are:

• It has a relatively small primary mirror by today's standards.  That means it has less light gathering power. The Keck telescope in Hawaii, with its 10-meter mirror (compared to Hubble's 2.5 meter mirror) captures 16 times more photons per second.
• It is warm. About room temperature. This means that the mirror glows at infrared wavelengths, which limits its observations to wavelength shorter than about 2 microns.
• It has a small field of view. Hubble's Advanced Camera for Surveys has a field of view of about 11 square arc minutes. That's about the size of the period at the end of this sentence viewed from normal reading distance (unless you have enlarged your font). Pretty small.  If Hubble exposed on each spot for an hour, it would take it 1500 years to cover the whole sky. -- Correction. About 3000 years, since the earth gets in the way half the time.

Artist's conception of the James Webb Space Telescope
slated for launch in 2018.
Credit: NASA

Okay, so how do we improve on Hubble? To address the first two disadvantages, we would like to build a bigger, colder telescope. That's the James Webb Space Telescope (JWST). It has a 6.5 meter mirror, giving it about 7 times Hubble's light gathering power. And it will be orbiting a million miles beyond the moon, with a giant sunshade that will let the telescope cool to -405 degrees Fahrenheit (30 degrees Kelvin). It will be able to observe infrared radiation out to a wavelength of 28 microns.  It will be spectactular for observing very faint, distant galaxies, probing the inner depths of star-forming regions in our own galaxy, and studying planets around nearby bright stars.

But the James Webb Space Telescope's field of view is not much larger than Hubble's.  It is designed for detailed observations of small areas of sky, not surveys of large swaths of sky.

Use the right camera for taking a panorama

There are important projects that require a large field of view. For example:

1. Finding thousands of distant supernovae to better constrain the behavior of Dark Energy.
2. Measuring the subtle effects of weak gravitational lensing on the shapes of millions of galaxies, which provides a powerful cross-check on the supernova measurement and can help determine whether Einstein's theory of relativity is correct on very large scales.
3. Using Baryon Acoustic Oscillations as a measuring rod as a cross-check on (1) and (2).
4. Searching millions of stars for signatures of earth-size planets via gravitational microlensing.
5. Studying the "archeological record" of past star-formation in nearby galaxies by measuring the brightness and colors of individual stars.
6. Using measurements of galaxy clustering to study the relationship between galaxy properties and dark-matter halos.

This is why US astronomers identified a wide-field space telescope as the number one priority in their 2010 Decadal Survey of the field, and why the European Space Agency is considering building a wide-field telescope called Euclid.

The messy history of Dark Energy telescopes

The problem has been money. Well, money and astro-politics.  Well, that and the fact that science is always changing.

The push for a wide-field space telescope started around 1997, with a project called SNAP (the Supernova Acceleration Probe). It was conceived as an optical and near-infrared telescope with a mirror about 2 meters in diameter and a field of view about 0.7 square degrees - 230 times larger than Hubble's. It was geared at studying supernovae and foundered for a variety of reasons: the difficulty of getting NASA and the Department of Energy to work together; difficulty getting support from the rest of the astronomical community given the narrowly focused science goals and the lack of clarity about access to the telescope for astronomers not associated with the project; complexity of the instrumentation that drove up the projected cost of the mission; and finally (and perhaps most importantly) rapid progress in developing competing probes of dark energy (2) and (3) above.

To help understand the tradeoffs in different ways of studying dark energy, NASA and DOE set up a task force in 2005 to look at the competing techniques and try to recommend a path forward to get the most bang for the buck from both ground-based observatories and space telescopes.  Their work showed that techniques 1-3 were all complementary and argued persuasively that at least two techniques should be pursued. They defined a "Figure of Merit" for determining how well a specific set of measurements could constrain Dark Energy. Unfortunately, the figure of merit they chose was based on a set of assumptions which may or may not be correct (given that we don't understand dark energy), so the idea that we could just stack up proposed experiments beside each other and choose the one with the best figure of merit was widely rejected. There was a lively meeting on this topic at STScI in 2008. I highly recommend the panel discussion as entertaining and enlightening.

So in the middle of the last decade, NASA and DOE started a competition to select the best ideas for a Dark Energy mission (called JDEM at the time), but then called off the competition when it became clear that the money wasn't going to be immediately available, and when the European Space Agency started getting serious about doing something similar.

In 2010 the Decadal Survey gave the wide-field telescope a new name WFIRST (Wide Field Infrared Space Telescope) and gave it the top priority for a space mission following construction of JWST. The problem is that limited budgets have been limiting astronomers ambitions. The telescope has been whittled down to a telescope about 1.3 meters, which at near-infrared wavelengths provides an image quality that is barely sufficient for studying weak gravitational lensing. In the meantime, the Euclid mission is becoming more and more real, and it is starting to look silly for the US to send up a mission just to come in second on the Dark Energy measurements.

Anyone want a couple telescopes? Not even driven around the block...

So now along come two already-built 2.4 meter lightweight space telescopes.  It doesn't take more than a few seconds of thought for astronomers to come up with things for them to do.  

Image of a test-unit version of the new telescopes, taken from
Allan Dressler's June 4, 2012 presentation to the
Committee on Astronomy and Astrophysics
Credit: National Academies Board on Physics and Astronomy

Ah, but two telescopes, now that opens possibilities. My favorite right now is to do everything possible to get one of them up there fast. Don't try to build an all-singing, all-dancing set of instruments to do lots of different measurements. Just put big camera on board with a good selection of filters and start collecting the wide-area data. Do this as fast as possible. This would be good for measurements 2,4,5, and 6. Not so good for (3), and missing the crucial spectroscopic component for (1) -- although putting in some specialized filters could help.

What about the second telescope? I happen to think measurement (3) -- Baryon Acoustic Oscillations (BAO) -- are extremely important. I also think that measurements of distant supernovae (1) will continue to be useful -- that's part of what we are doing on a small scale in CANDELS. Both require a spectrograph.  Measuring BAO involves measuring millions of redshifts; supernovae experts are hoping for thousands of supernovae.  Current thoughts on how to do this involve simply dispersing the galaxy light and letting all the little spectra overlap. This has been the plan for WFIRST and JDEM. Cheap but ugly. Micromirrors provide the opportunity block out some of the galaxies and the sky in between. Launching the second telescope later than the first would give the opportunity to try to equip the telescope with a micro-mirror spectrograph, which would really be doing the job right. It would open up huge possibilities for science in other areas, not just measuring BAO. A 2.4 meter telescope is probably overkill for BAO, but it could do the BAO measurements fast and have time left over for lots of other kinds of astronomy.

So make the first observatory a into simple, quick, cheap imager. Make the second telescope a new-technology wide-field spectrograph. 

Alternatively, equip the second telescope for ultraviolet observations. JWST doesn't have UV capability. Or go the other direction. If these telescopes can be cooled, then using the second  telescope to provide wide-field imaging out to longer infrared wavelengths to complement JWST would also be extremely useful.  I've also heard people suggest that one of the telescopes be outfitted with a coronograph for studying extrasolar planets.  I'm skeptical that these are the right telescopes for that, but then I don't know much about them.

Will we stumble again?

In my view our entire scientific enterprise (astronomers, NASA, DOE...) has been stumbling along for far too long on deciding what to do for a Dark Energy experiment and debating how much it is worth and how to fund it, and what else to do with it. SNAP wasn't the best conceived mission, but if we had just collectively decided to do SNAP or something like it circa 2000, it might have been in orbit by now gathering great data. Instead, we are still debating designs on powerpoint presentations.

So can we get on with it? Will NASA be able to find or free up enough money to get one of these telescopes up into orbit fast enough to remain competitive with Euclid? Will US astronomers put aside their differences quickly enough? Are there sufficient incentives to motivate the standard NASA contractors to look for quick low-cost ways to get one of the telescopes into orbit? Will the private sector help in any way?

Any comments?

Supernovae

For most stars, death is not so much an event as it is a process. A typical star will swell and contract as its nuclear fuels are gradually depleted, eventually shedding its outer layers into a gently expanding shell. The stellar core will be left behind as a pale remnant that fades into a dark, cool white dwarf. It makes a lovely display but doesn’t have much impact beyond its immediate surroundings (the death of a star is not good for any orbiting planets, but other nearby stars would hardly notice). A prominent minority of stars, however, will end their evolution in spectacular fashion with an explosion that can be observed from across the cosmos. This is a supernova: a powerful stellar explosion that can briefly outshine all the light from all the other stars in an entire galaxy.

Hubble Space Telescope image of Supernova 1994D in 
galaxy NGC 4526.  The supernova - visible in the lower 
left of the image - appeared in the outskirts of this dusty spiral 
galaxy, outshining millions of stars in the galaxy core. (original)
©NASA/ESA, The Hubble Key Project Team,
and the High-z Supernova Search Team

For decades, studying these spectacular events has led us to extraordinary advances in our understanding of the universe. Most recently, supernovae have made headlines with the award of the 2011 Nobel Prize in Physics "for the discovery of the accelerating expansion of the Universe through observations of distant supernovae" (a.k.a. dark energy). Our own colleague and the head of the CANDELS supernova team, Adam Riess, shares that prize with astronomers Saul Perlmutter and Brian Schmidt.

In today's post I'll first sketch out some recipes for how to brew a supernova. Then I'll scratch the surface of modern supernova science, describing how supernovae play three important roles in the astronomer's toolbox: as laboratories, factories and light houses. In future posts I'll come back to say a bit more about how CANDELS supernova discoveries are helping us better understand supernovae, dark energy and the universe.

How to Brew a Supernova

Astronomers divide supernova explosions into two broad categories. The first set includes several flavors of supernovae resulting from the death of giant stars, called core collapse supernovae. The second set I will call white dwarf supernovae, referred to by astronomers as Type Ia ("Type one-A") supernovae. These are extremely useful for cosmology - more on that below. If you are setting out to make a supernova explosion, these two categories require two very different approaches.

Making a core collapse supernova is relatively simple. All you need is a very massive star.  At least 8 times the mass of the sun, and the more mass you pile on the more interesting it gets. Let's suppose you want to see some real fireworks, and go with 20 times the mass of the sun. This star of yours will age very rapidly (in astronomical terms), requiring only about 10 million years from cradle to grave (for contrast, our own sun is now ~5 billion years old, and will go on essentially unchanged for about another 5 billion more).  At birth, this big baby of yours operates much like our own sun (on steroids), with a powerful nuclear fusion engine in its core, burning up hydrogen atoms and turning them into helium. After 8 million years your star runs out of hydrogen and has to start burning helium instead, producing an "ash" of oxygen and carbon. That will keep it going for another million years, until it runs out of helium and has to start burning carbon. The carbon stage lasts only about a thousand years, before your star turns in rapid succession to neon, then oxygen and then silicon. Finally, after burning silicon into iron (for only about two weeks) your star hits the end of the road:  it cannot produce energy by fusing iron together, so the central engine of nuclear fusion fails. The core cools down, the outer layers start to collapse, and the whole star falls in on itself. The core itself has already been compacted into a dense, nearly incompressible sphere, so when the loose gas from above the core falls onto that hard surface... it bounces. That bounce sets off the supernova explosion, tearing off the outer layers and lighting them up with a glow that we can see from billions of light years away. (There's a lot of interesting and controversial physics I'm glossing over here. For example, we don't know precisely how the energy of collapse gets transformed into the energy of explosion. )

Artist's conception of a possible pre-supernova binary star
system: a white dwarf cannibalizing its giant stellar companion.
(original) © ESA and Justyn Maund (Queens Univ. Belfast)

Another artistic impression: two white dwarf stars spiraling
in toward a collision, emitting gravitational waves as the
orbit decays.  (original) © NASA, Tod Strohmayer (GSFC),
and Dana Berry (Chandra X-ray Observatory)

Now, the other option for your home supernova construction kit is a Type Ia, or white dwarf supernova.  Here the recipe is not so clear. We know that you need a binary star system, with two stars locked in a close orbit.  One of these stars must be a very dense white dwarf star: as massive as the sun, but much cooler, and as small as the Earth. Somehow this white dwarf star has to steal a lot of mass from its companion. This could happen by slow accretion: over millions of years the white dwarf slowly cannibalizes its neighbor, swallowing gas from the outer layers and engorging itself. Or it could happen with an orbital death-spiral: the companion star is another white dwarf and the two are locked in a decaying orbit, dancing closer and closer as they lose orbital energy through gravitational waves until eventually they coalesce and merge. We don't yet have any clear evidence which of these two scenarios is correct (perhaps they both occur).   Regardless, the end result is a white dwarf that has acquired more mass than it can handle. It is already too dense for further collapse, so instead it heats up rapidly, reaching a temperature where it can suddenly ignite thermonuclear fusion of carbon atoms. This compact star can't handle the sudden rush of new energy, so it sets off a thermonuclear explosion that ignites the whole star like an atomic bomb.

Now that we know (more or less) how to make a supernova, what can we do with them?   Supernovae play three important roles in the astronomer's toolkit:

1. The Stellar Lab

An entomologist who wants to know how an insect breathes can go catch some insects, open them up and examine their parts. An astronomer who wants to know how the interior of a star works does not have the luxury of slicing it open to peer inside. Instead, we have to make do with the laboratories that the universe has provided for us. Supernovae make exceptional stellar labs, as they very obligingly open themselves up, spewing out a wealth of information about their interiors that becomes accessible to us. We study the changing light of the explosion and the expanding shell of ejected material, measuring the speed, shape, color and content. Comparing these observations to computer models can tell us about the star's pre-explosion structure and its life cycle. Each supernova gives us a truly unique lab for learning about the physics of nuclear fusion, explosions, and energy transport.

2. The Atomic Factory

All stars have at their core a nuclear furnace, steadily burning hydrogen into helium and eventually making some heavier elements such as carbon, nitrogen and oxygen. Those heavier elements are extremely useful to have around if you ever want to construct a planet, especially one with things (carbon) that breath air (oxygen and nitrogen) and drink water (hydrogen and oxygen). The vast majority of stars, however, are extremely stingy about releasing their elements. The heavy elements are all created deep in the stellar core, and in a typical star like our sun that core remains intact as the star slowly dies. After spending billions of years constructing those precious carbon atoms, they all end up trapped inside a cold fading core for the rest of the life of the universe.

Supernovae, however, have much more powerful nuclear furnaces - especially during the explosion. They are able make many more interesting elements, going well beyond carbon and oxygen to produce everything else in the periodic table: gold, silver, nickel, plutonium, etc. What's more, the supernova explosion sends those elements out into empty space, polluting the cosmos with a spray of atoms. Eventually those little bits of supernova stuff will cool and settle down, and some of it will coalesce into new stars and form planets with small curious creatures who read and write blogs. This is basically the only mechanism that our universe has for generating and distributing the heavy elements that form the building blocks of planets and life. As Carl Sagan was fond of saying: "we are all star stuff."

3. The Cosmic Light House

Theoretical physicists have crafted some wonderful and exotic models of the universe, and it is nice to test those from time to time. One of the best methods for testing cosmological models is to measure distances to far-away objects and map out the geometry of the observable universe. To do this, one can use a tool that we call a "standard candle": some class of objects that all have the same intrinsic brightness. If you see a faint star in the sky, you can't know at a glance if it is nearby and naturally dim (like a firefly), or if it is actually quite bright, but appears faint because it is very far away (like a distant light house). For astronomers, when we observe a standard candle that appears faint, we can immediately determine its distance because we know already that it's a light house, not a firefly.

It happens that white dwarf supernovae (Type Ia) are excellent standard candles. They all have very similar intrinsic brightness, and they also happen to be extremely bright, so we can find them at great distances. This characteristic is what enabled the 2011 Nobel laureates and their collaborators to discover dark energy in 1998. They measured the brightnesses of distant supernovae and found them to be fainter than expected, unless they introduced this peculiar accelerating expansion, driven by an unknown and unseen force. In a future post I'll come back to explain how the CANDELS supernova team is now pushing these supernova discoveries out to record distances, finding these cosmic light houses at distances of more than 9 billion light years.

Galaxy Masses and the "Parking Lot" Problem

I recently helped to organize a small workshop on galaxy formation at the Leiden Observatory on May 14-15, 2012, entitled "Theory Goes out on a Limb: Theoretical Predictions for z > 1 Galaxies" (Harry Ferguson of CANDELS and Marijn Franx of Leiden were co-organizers). The goal of the workshop was to feature theory more than is usual at astronomy conferences, in an effort to stimulate theoreticians to make concrete predictions about what properties of galaxies should be observed at moderate to high redshifts.

Illustration of a spiral galaxy embedded in a dark
matter halo, with infalling gas:
Credit: NASA; ESA; A. Feild, STScI

I was struck by one of the major themes that emerged at the workshop, namely, the persistence of a problem that has been with us for several years now and refuses to go away. The problem is the fact that our theoretical predictions for matching the number of galaxies of different masses are increasingly bad back in time -- models tend to UNDERpredict the number of massive galaxies but OVERpredict the number of small galaxies. The discrepancy is at the level of factors of 2-10 depending on mass, big enough to be quite disturbing.

A new insight (for me) was that the fault can plausibly be attributed to the fact that most models take gas falling into halos of galaxies (drawn in by gravity) and rather quickly and efficiently turn it into stars. We understand this pattern of gaseous infall quite well from theory, and there is little opportunity to alter it -- it is driven by the growth of halo structure in the universe, which is well understood. But this pattern of gaseous infall is failing badly to reproduce the mass growth of galaxies. To save the theory seemingly requires major modifications to the sequence of events whereby galaxies take infalling gas and turn it into stars.

Though details are lacking, we understand in a general way the fix that is required.  Essentially, we have to save up a lot of the gas early on in some kind of inert form and then turn it into stars a few billion years later. My favorite name for this solution is the "parking lot", wherein gas has to be put into a reservoir where it does nothing for some time but then becomes available for star formation after a few billion years. This latter part is the tricky thing -- it's pretty easy to drive gas out of the galaxy entirely by using energy produced by the first generation of stars in "feedback" mode. Early stars can produce a lot of energy (or momentum) to drive gas out in the form of intense radiative from young hot, stars and blastwaves from exploding supernovae. Such feedback has been known for some time.  But feedback is very easily overdone -- if too intense, it can drive gas out of the galaxy entirely and render it permanently unavailable. In other words, successful feedback has to walk a narrow knife edge in which there is sufficient feedback to eject the gas and keep it ejected for some time but not so much as to lose it altogether. And then the gas has to return on just the right timescale to make stars later and match the observations.

It was my impression from the meeting that there is no generally agreed-on method to accomplish this. Without a solution to this problem, we are lacking a fundamental theory for galaxy masses, and, without a theory for masses, we cannot predict the galaxy content of the distant universe to any useful accuracy.

Perhaps a better title for the workshop  would have been:

 

         "Theory Goes Out on a Limb...and Saws It Off."

Venus Transit

Venus in transit in the optical (upper image),
ultra-violet (lower left image) and
extreme ultra-violet (lower right image),
image credit: NASA/LMSAL

What is a Venus transit? Have you ever seen a solar eclipse? A solar eclipse happens when the Moon moves in between Earth and Sun and blocks out the sunlight diving bits of the Earth into shade. One was just visible from in the US and Asia. A Venus transit is in principle the same thing, just swap the Moon for Venus. So essentially, Venus moves in between us and the Sun and blocks out a bit of its light. Since Venus, although itself bigger than the Moon and nearly as big as the Earth, is so much further away from us than the Moon, it's only a little speck in front of the sun and blocks out only a little bit of light. Doesn't seem too spectacular?

Venus Transit June 5th, 2012,
image credit & copyright: Pete Marenfeld

Well, it is spectacular! First of all Venus transits are a rare occasion and don't happen about twice a year as solar eclipses do. In fact, the last Venus transit was in 2004 and the one before that in 1882. The reason for these long time spans between transits is that the orbits of Earth and Venus around the sun do not lie in the same plane (see figure below). The orbit of Venus is tilted by about 3.4 degrees. So very often Venus just misses the Sun and travels either above or below it when it passes between us and the Sun. Only 6 transits have been observed scientifically so far, the first one being in 1639. 

Illustration of Earth and Venus orbits around the sun

Illustration of transit time measurements

Second, you can do awesome stuff with a Venus transit. It allows you to calculate not only the distance between Earth and Venus, but also the distance between the Sun and Earth! Now, some math is involved, but it's really quite easy. What's needed is a record of the time when Venus enters the sun and exits the sun from two different places on Earth. With the time difference between each of those events in each place and knowing the distance between your two observation spots a simple triangulation does the trick. (You can find a detailed description here.) This method for the determination of distances is also called the parallax method.

The first to suggest using this method to get a handle on the absolute distances between objects in our solar system was astronomer Edmond Halley (yep, Halley's comet is named after him) in the 18th century. Until then astronomers only knew the relative distances between planets in our solar system, e.g. that Venus is 0.7 times as far away from the Sun as the Earth is. Even during transits today groups of people all around the world combine their data to repeat this experiment, although more for educational than scientific purposes. However, the observations of transits is one of the modern methods to detect extrasolar planets around other stars by observing the dip in brightness of the star's light when the planet passes in front of it.

Astronomers in awe at NOAO during the Venus transit June 5th/6th 2012

Hanae Inami, new CANDELS member,
takes a picture of the projected sun,
credit & copyright: Jeyhan Kartaltepe

Astronomers at NOAO watching the
Venus transit (me at the right),
credit & copyright: Jeyhan Kartaltepe

You might think that only stellar or solar astronomers are interested in observing a Venus transit. I can assure you, that most astronomers were pretty awed by the spectacle. Many of us gathered outside at the National Optical Astronomy Observatory in Tucson to watch the tiny disk of Venus move in front of the sun at a few minutes past 3 pm with sun-viewing glasses and through telescopes with special filters. In fact, there were so many of us that you can't even make out the telescopes in the pictures!

Astronomers at NOAO watching the Venus transit, credit & copyright: Jeyhan Kartaltepe

Now the all important question: Did YOU get to see the Venus transit on June 5th (June 6th in Europe)? If yes, lucky you and I hope you viewed it safely! If not, you'll now have to wait for more than 100 years (105 to be precise) before the next one happens. So until medicine advances drastically in the next few decades in prolonging your average life expectancy or cryogenics can freeze you and wake you up again in time, you won't see another one in your lifetime!

If you want more information, check out the Bad Astronomy Blog Post on the Venus transit or Karen Master's Blog post about it!

Cooking up Galaxies

Armed with just some physical concepts and a powerful computer, can we build a mathematical model of galaxy evolution that looks like what we see through our telescopes? That was the question on the minds of thirty or so astronomers who met May 14-15, 2012 at the Lorentz center at Leiden university in Holland.

The resounding answer was no. Which is fine. We knew that going into the meeting. Otherwise there wouldn't have been much to talk about. The important thing is to try to understand why the models fail. Of course, if we all agreed on that, then there also wouldn't have been much point in holding the meeting. So the topic of discussion and debate was the stress tests for the models -- which observations were particularly useful, which aspects of the models seemed most dicey, and were there any new ideas for addressing them?

The ingredients

To start cooking galaxies on the computer, everyone starts with a mix of dark matter (a bit more than 25%), dark energy (a bit less than 75%), and a frosting of normal baryonic matter (the stuff atoms are made of). They assume that the universe is expanding, and that the expansion rate was slowing down for the first few billion years, but has been speeding up since then due to the pressure of dark energy. They fill the universe with an almost perfectly uniform soup of dark and baryonic matter, with just tiny fluctuations in density with an amplitude and scale set to match observations of the cosmic microwave background and expectations from inflationary theory. They set the universe expanding and then watch as gravity starts to pull material towards the centers of the densest fluctuations.

Okay, this already sounds pretty dicey. No one knows what dark matter and dark energy are. Nonetheless, these basic assumptions explain the propensity for galaxies to cluster together in groups. And the particular assumption that the dark matter is "cold" -- which means it was moving at speeds much lower than the speed when the universe was about 400,000 years old -- explains the clustering of galaxies (measured using correlation functions or power spectra) in detail. The most magnificent illustration of this recently has been the detection of the "baryon acoustic peak" --- the strongest feature observed in the spatial power spectrum of the cosmic microwave background --- in the clustering of present-day galaxies. That said, one of the reasons that astronomers are really interested in understanding galaxies is to use them to help understand fundamental physics, such as dark matter. It's just frustratingly indirect.

Cooking instructions

Most of the meeting was focused not so much on the ingredients, but in how the galaxies evolve -- how gas cools, condenses into stars and black holes, and cycles energy back into the gas to prevent further cooling. The basic challenge is that the gas that collects around the densest concentrations of dark matter tends to cool too fast. If this is what actually happened, the universe today would be filled with galaxies much smaller than the Milky Way -- hundreds of times more dwarf galaxies than we actually see. So something must have kept the gas in galaxies from cooling too fast. For galaxies about the size of the Milky Way, it's quite likely that this something was the the energy released by stars (and especially the violent deaths of stars in supernova explosions). 

 

Astronomers generally agree on how much energy stars release, but no one has yet been able to cook up perfect galaxies on a computer (following all of the systematic trends of real galaxies). They tend to be too small and to rotate too rapidly. When these problems are fixed by tuning the feedback recipe, then other problems arise. When models are tuned to match observed galaxies at the present day, they generally run into trouble matching the observations of distant galaxies. In a recent experiment called the Aquilla project, experts in simulating galaxies all started their codes with exactly the same initial conditions. The galaxies that they produced ended up looking completely different. And none of them looked quite like the Milky Way. On the other hand, the highest resolution simulation to date, the Eris simulation, does end up looking a lot like the Milky Way. Perhaps this is because in this simulation, the energy from star formation can be deposited where it is needed, in the dense lumps of gas that formed the stars. But with only one simulation at this resolution, we don't yet know if the trends among galaxies can be reproduced. 

Lumps in the sauce?

Distant clumpy
galaxies observed
by Hubble

There was quite a lot of interest and debate at the meeting about giant clumps of star-formation in distant galaxies. Such giant clumps were very common in galaxies when the universe was less than four billion years old, but are quite rare today. We do not know for sure if these clumps represent merging galaxies, or if the clumps formed from gas that had already settled in a rotating disk. Some people at the meeting thought the clumps could be just an artifact of the computer models (due to some of the approximations used). Others thought that the clumps were an essential feature of galaxy evolution, and that their existence could be predicted without a sophisticated computer code. They must be fairly long-lived features or else they would be uncommon. But there was a lot of debate on whether they survive long enough to sink to the centers of galaxies, forming the central bulges we see today. For the observers, just finding clumps in distant galaxies and trying to agree on their properties is proving to be a challenge. On the bright side, the simulations can be used to guide our measurement techniques, even if the simulations themselves don't produce very realistic galaxies. 

Some computer simulations form giant
clumps of star formation.

Is it soup yet? 

As is often the case in meetings like this, one is left with both hope and despair. There were some remarkable successes (for example in explaining the evolution in the sizes of certain types of galaxies over the past few billion years), and in reconstructing the average star-formation histories as a function of dark-matter halo mass. But there were also sobering comments from experts who pointed out that different modelers sometimes wildly disagree on the equations that govern the same physical process (such as rates of cooling of gas in dark-matter halos), and that there are physical processes that no one really knows how to model well (such as recycling of gas that was ejected from a galaxy but later falls back in). There is plenty of homework before the next such conference.

What types of galaxies are there?

The Milky Way, credit:B. Fugate (FASORtronics)/ESO

If you have ever seen a night sky in a very dark area, chances are you've seen this white-ish band across the sky. This band is the Milky Way (see picture to the left), the galaxy we live in, our home. It stands out in the night sky because it consists of many, many stars. Like our sun is just one star in many billions in our own galaxy, the Milky Way is just one galaxy of many billions in the entire Universe. And like the people on our planet, galaxies come in many different shapes and sizes. Here, I will give you a very basic overview over the different types of galaxies we see in our local Universe.

Spiral Galaxies

Face-on spiral galaxy

Edge-on spiral galaxy with prominent dust lanes

Most galaxies in the Universe have a very structured appearance, with spiral arms, clumpy star-forming regions, dark dust lanes, and sometimes bars in their centers. Because of their structure they are called spiral galaxies. Spiral galaxies have a disk, a bulge, and a halo. The disk contains mainly gas, dust and young stars (and young means a few million years) which shine blue due to their hot temperatures (much hotter than our Sun). Star-forming regions, the birthplaces of new generations of stars, are located mainly along the spiral arms. The stars, dust, and gas in the disk orbit around the galaxy's center. The bulge in the center of the galaxy on the other hand consists of mainly old stars which are much cooler and thus red. This gives the bulge its distinct red colour. The halo contains only a small number (compared to the many stars in the disk) of old stars which often reside in clusters. While the spiral arms are best seen in galaxies that face us (we say the galaxy is face-on, like the one to the top right), we know they are very flat disks from the ones that are edge-on (like the one on the bottom right).

Our own galaxy, the Milky Way, is such a spiral galaxy. You might ask, how do we know that seeing that we are sitting right in the galaxy and can’t look at it from the outside. Well, if the Milky Way were not a flat, disky galaxy you would not see that white-ish band across the night sky because the stars would not be concentrated in a thin disk but be more evenly distributed across the sky. From the picture of the Milky Way (first image), you can recognize dark structures in the white-ish band of stars. These are created by dust and gas between us and the stars which block out the light of these stars.

 

Elliptical Galaxies

Elliptical galaxy 

In contrast to the structured spiral galaxies, there exist galaxies that are very smooth, in which the stars are evenly distributed and that have pretty much no structure at all (like the galaxy on the left). Because they are of elliptical shape, we call them elliptical galaxies. Most ellipticals are red in colour because they consist of mostly old stars and they do not form new stars. It is also believed that they contain very little to no dust and gas, unlike spiral galaxies. Elliptical galaxies are in many ways very similar to the bulges of spiral galaxies.

Both the biggest and smallest galaxies in the local Universe are elliptical galaxies. The light in the largest ellipticals takes more than 2 million years to travel from one end of the galaxy to the other. In comparison, the light from one end of the Milky Way needs about 100,000 light years to travel to the opposite end. The smallest ellipticals are also the faintest galaxies; they are called dwarf ellipticals. There are a few such dwarf galaxies around the Milky Way which are likely to be swallowed by our own galaxy over time.

Irregular Galaxies

Small (left) and Large Magellanic Cloud (right)
credit: ESO/S. Brunier

As you might have expected there are also galaxies in the local Universe that don’t really fit in either of the above categories. Their shape is usually irregular and they are forming a lot of stars. We call them irregular galaxies. The most prominent examples are the two Magellanic Clouds, our intergalactic neighbours.

Hubble Sequence

Classification scheme for galaxies

However, even among the elliptical and spiral galaxies appearances vary. For example, an elliptical galaxy can be nearly spherical or very elongated. The spiral arms of spiral galaxies can be tightly wound or rather loosely wound, its bulge can be larger or smaller. There can also be a bar in its center. The first to order galaxies not only by elliptical (E) or spiral (S) type was the astronomer Edwin Hubble. In this classification scheme (see figure to the left) a number is used to describe the roundness of ellipticals, e.g. an E0 galaxy is pretty much round, while an E7 galaxy is extremely elongated, like a cigar. Spirals are divided into two subgroups, those with a central bar (SB galaxies, barred spiral galaxies) and those without (S galaxies). Our Milky Way is a barred spiral. Small letters describe the size of the bulge and the tightness of the spiral arms, e.g. a Sa galaxy has a relatively large bulge and very tight spiral arms, a Sc galaxy has a smaller bulge and the arms are very loosely wound. S0 galaxies build the bridge between the two basic types. These are galaxies that look like ellipticals but have a disk, although without spiral structure. Because of the shape of the diagram this classification scheme is also often referred to as Hubble’s Tuning Fork. Morphological classification schemes like this are still in use today, most famously in the citizen science project Galaxy Zoo which allows the general public to classify real galaxies.

Galaxy Surveys

An increasingly popular method in astronomy to study the galaxy population in the Universe is to take a large survey, i.e. to look at many galaxies instead of just a few. This helps us to make more general statements about the galaxy population by averaging the properties of a large number of galaxies. Thanks to the Sloan Digital Sky Survey, which mapped more than a quarter of the sky, we know a lot about the galaxies in the local Universe. However, many astronomers are interested in the formation and evolution of galaxies over time which in part means to answer the question: Why do galaxies in the local Universe look the way they do? To answer this question we need to study galaxies that are much further away and which existed when the Universe was only a few billion years old (in comparison, the Universe today is about 13.5 billion years old). With the Hubble Space Telescope we are able to find far away galaxies which appear much fainter than close-by ones and map the distant Universe. From this we already learned that distant galaxies can look much different compared to the ones we see nearby (see for example the image of the Hubble Deep Field). However, the light of distant galaxies gets stretched and shifted to the red on its way to us (this is called redshift) and thus what we can observe of distant galaxies by analyzing images is difficult to compare to what we observe locally. With CANDELS we are able to observe far away galaxies in the same light as local galaxies. This will allow us to directly compare galaxies and to understand how distant galaxies evolved.


In future posts we will tell you more about the distant Universe, the shape of galaxies, interactions between galaxies, and the properties of galaxies. So stay tuned!

Meet the CANDELS Team

The CANDELS collaboration is made up of over one hundred individuals with members from all over the world, including the United States, Canada, the United Kingdom, Italy, France, Austria, Germany, Spain, India, Israel, and Chile. These members work at a wide range of institutions, including large research Universities (e.g., University of California, Santa Cruz) and national facilities and telescopes (e.g., Space Telescope Science Institute, National Optical Astronomy Observatory, and NASA).

Not only does this group represent a wide range of countries and cultures, CANDELS is a coming together of members from several other collaborations working in each of the different deep fields in the survey. CANDELS has many senior researchers as members, but junior scientists, including many postdoctoral researchers (aka, postdocs) and graduate students, make a large contribution to the work. A number of undergraduate students are even working with CANDELS data, getting one of their first tastes of scientific research. Through this blog, you will meet a number of these scientists, as different people will write posts introducing themselves, explaining their research and describing their scientific results.

When CANDELS team members are not hard at work analyzing data and writing papers, you can often find them all over the globe, collecting data on observing runs, meeting with collaborators around the world, and presenting their results to the scientific community. Once a year, the CANDELS team gets together for a collaboration meeting, to discuss ongoing work and plan for the next year. Last year, our meeting was held at the University of Edinburgh's Royal Observatory (shown in the picture above). This September, we will be gathering at the University of California, Santa Cruz. Stay tuned to hear more about the exciting exploits of this unique collaboration!

About the CANDELS blog

In late 2009, the Hubble Space Telescope began an ambitious program to map five carefully selected areas of the sky with its sensitive near-infrared camera, the Wide-Field Camera 3. The observations are important for addressing a wide variety of questions, from testing theories for the birth and evolution of galaxies, to refining our understanding of the geometry of the universe.

This is a research blog written by people involved in the project. We aim to share some of the excitement of working at the scientific frontier, using one of the greatest telescopes ever built. We will also share some of the trials and tribulations of making the project work, from the complications of planning and scheduling the observations to the challenges of trying to understand the data. Along the way, we may comment on trends in astronomy or other such topics.

CANDELS stands for the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. It builds on the legacy of the Hubble Deep Field, as well as the wider-area surveys called GOODS, AEGIS, COSMOS, and UKIDSS UDS. The CANDELS observations are designed to search for galaxies within about a billion years of the big bang, study galaxies at cosmic high-noon about 3 billion years after the big bang - when star-formation and black hole growth were at their peak intensity - and discover distant supernovae for refining our understanding of cosmic acceleration. You can find more details, and download the CANDELS data, from the CANDELS website.

You can also use the Hubble Legacy Archive to view the CANDELS images, such as this one from the UDS.