Thursday, February 21, 2013

Astronomer of the Month: Mark Dickinson

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Mark Dickinson.

Tell us a little about yourself!

I'm Mark Dickinson, and I'm an Associate Astronomer at the National Optical Astronomy Observatory (NOAO) in Tucson, Arizona. I studied astronomy as an undergraduate at Princeton, then did my Ph.D. at UC Berkeley. After that, I was in Baltimore for 10 years at the Space Telescope Science Institute (STScI) and the Johns Hopkins University, first as a postdoctoral fellow and then on the STScI scientific staff, before I moved to NOAO in 2004.   When I first arrived in Baltimore, I shared an office with Harry Ferguson (now CANDELS co-PI), who was then also a postdoc at STScI. Bob Williams, then the director of the Institute, tapped us to help with the implementation of the original Hubble Deep Field (North) observing program. The HDF-North begat GOODS, which begat CANDELS, and so here we are...

What is your specific area of research? What is your role within the CANDELS team? 

I work mainly on the star formation histories of early galaxies - the way in which they formed stars and grew during the early phases of cosmic history, when the universe and the galaxies within it were young. When I started working in astronomy, we had almost no direct knowledge of galaxy evolution during the first half of cosmic history - the only evidence came from interpreting the fossil record of stars within present-day galaxies like the Milky Way. It's been an amazing transformation, driven by the technologies of new telescopes and instrumentation, and by clever observing techniques, which have led us to discover thousands of galaxies in the early universe, back to the era when the universe was less than 10% of its present age. This lets us make direct measurements of early galaxy formation and evolution - truly an opportunity to climb into the Way-back Machine and revisit the early days when the Universe and its galaxies were young, and directly measure how they were born and grew.   

I was involved in several of the major early multiwavelength surveys that led to the development of the CANDELS program, and am now the principle investigator for new observations of the CANDELS fields at far-infrared wavelengths using the Herschel Space Observatory - the largest telescope ever flown in space, which measures the energy from star formation and supermassive black holes at the centers of galaxies, energy that is often absorbed by dust and re-emitted at infrared wavelengths where we cannot measure it with ground-based telescopes. This project reveals the "hidden" side of galaxy formation - hidden by dust, and largely invisible to optical telescopes like Hubble, but which is a key element of galaxy growth over cosmic time.
What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

I remember becoming interested in astronomy in elementary school - I loved reading articles in National Geographic and other magazines about the planets and stars, and I remember seeing Comet Bennett, which was very bright in the sky early in 1970.  My parents took me out at night to a hillside in Connecticut, where I grew up, and I was just dazzled by this huge, strange thing in the sky. I can't say that I knew I would be an astronomer then, but I never lost my interest after that.
What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 
I've been incredibly fortunate, particularly with my education - I had every opportunity to dream and to learn, and nobody ever told me to stop dreaming and do something useful. If I had obstacles, they were mainly barriers of my own making - occasional laziness and lack of focus - but astronomy is a field that can reward mental wandering which (occasionally) leads to new inspiration.

Who has been your biggest scientific role model and why? 

That's hard to say, but I have to give credit to my Ph.D. thesis advisor, Professor Hyron Spinrad from U.C. Berkeley. Hy worked in many different areas of astronomy in his career, from stars to Mars to the most distant galaxies, but he was clearly driven by the fun of discovery - by the opportunity to see and find something new that had never been seen before. And, in the particular area of galaxy evolution, by the thrill of the chase - the wish to find the most distant, most exotic objects in the early universe.

What is it like to be an astronomer? What is your favorite aspect? 

I think I like the mix of planning and serendipity: you have to have a research plan, a goal for your projects, but the best part is when unexpected discoveries happen - when you find something you weren't looking for. That's when it all gets really interesting.

What motivates you in your research? 

Coffee. Oh, and the expansion of the universe, which is pretty cool if you think about it.

What is your favorite astronomical facility? (This could include telescopes or super computers, for example) 

I have a deep fondness for the 4-meter diameter Mayall telescope at the Kitt Peak National Observatory. OK, I work at the National Optical Astronomy Observatory, which operates the telescopes at Kitt Peak (and also at Cerro Tololo in Chile). So maybe I'm biased, although ironically I observed at the Kitt Peak 4-meter *much* more often when I was a student and postdoctoral fellow than I do now since I came to work at NOAO. But still I really love that telescope - I got my training as an astronomer there and at the 3-meter Shane telescope at Lick Observatory, but the Kitt Peak 4-meter was really where I struck out on my own for the first time. Unfortunately now budget reductions at the National Science Foundation are threatening future continued operations at Kitt Peak, but there is still hope for a new future at the telescope with a giant new spectrograph funded by the Department of Energy to uncover the secrets of Dark Energy, the inexplicably strange phenomenon that has been discovered in the last ten years or so and which is apparently causing the expansion of the universe to accelerate.

Where do you see yourself in the future? What are your career aspirations? 

Astronomy has become a world-wide collaboration - astronomers work in international teams with colleagues from around the globe. I see myself continuing to travel world-wide to work with collaborators on exciting projects....I just wish that the economy seats in airlines didn't keep getting squeezed closer and closer together...
If you could have any astronomy related wish, what would it be? 
To have at least one good new idea for research every year.

What is your favorite, most mind-boggling astronomy fact? 

That the universe has a finite age, about 13.7 billion years, but is also infinite and (apparently) will expand at an accelerating rate forever.
Is there anything else you would like for the public to know about you or astronomy in general? 

Astronomy is an amazing science concerning a universe whose scale is vastly beyond anything in the realm of daily human experience - and yet, as humans we can actually take significant steps toward figuring it all out. We're learning things now that we hadn't even begun to imagine when I was a student, and that's not *that* long ago. And this process of discovery is a very human endeavor - we learn these things by working together with our colleagues and friends (sometimes our colleagues are our friends). Amazing.

Tuesday, February 19, 2013

Cosmic Clue: What's Killing the Massive Galaxies?

When I was a kid, my favorite board game was Clue. I loved being handed random, scattered pieces of evidence, and having to deduce through sheer logic and process of elimination, who, where, and how Mr. Boddy was killed. So it's not surprising that years later, as a galaxy formation theorist on the CANDELS team, I am drawn to a murder mystery on a cosmic scale, a mystery that is responsible for the most fundamental dichotomy in the galaxy population yet has puzzled astronomers since galaxies were first discovered: What kills the biggest galaxies in the Universe?

The existence of dead galaxies has been known since the time of Edwin Hubble. Dead galaxies are easy to spot as they are among the largest in the Universe, containing up to a hundred times more stars than the Milky Way. They are also very red, since they have not formed any new stars for a long time. This happens because stars in the sky are much like stars in Hollywood: The biggest ones are the hottest and shine the brightest, but they also die out the quickest (and most spectacularly). Since young, hot stars are blue (a consequence of their black body temperature), after they die out, the galaxy is left with nothing but plebian longer-lived redder stars like our Sun. The galaxy then becomes red and dead.

But why? What serial killer is responsible for systematically killing these erstwhile happily star-forming systems?

Red and dead galaxies, being large and bright, are well studied in the nearby Universe. They tend to be elliptical in shape, devoid of any cold gas, and reside in the densest regions of the cosmos such as groups and clusters of galaxies. These are valuable clues, but today's red and dead galaxies died long ago. In a "cold case", it's always tough to sort out the cause from the effects. Does losing a galaxy's gas cause it to become an elliptical? Or does becoming an elliptical cause it to lose its gas? And why does all this happen preferentially to the most massive galaxies, living in the densest regions? There are many clues, but no clear answer.

Elliptical galaxies often have strong X-ray emission associated with gas at many millions of
degrees. These images show a sample of elliptical galaxies, with the optical image showing
the stars on the right, and the X-ray image showing the hot gas on the left. The X-ray gas
often shows a lot of structure, indicating that it has been disturbed putatively by jets
from the galaxies' supermassive black holes. From the Chandra image archive.
For a long time, the answer seemed fairly obvious: Galaxies begin with a reservoir of cold gas, and once they use that up, they can't form any new stars, and they die. But upon closer scrutiny, this explanation doesn't hold water (or in this case, gas). Firstly, non star-forming galaxies form a tight red sequence in color-magnitude space, distinct from star-forming galaxies in the blue cloud, and the region in between (known as the green valley) is conspicuously devoid of galaxies. This means that whatever turns galaxies red happens quickly; if it was gradual, there would be a continuous distribution in color towards the red sequence, with no green valley gap. Hence galaxies don't die of "natural causes" by just gradually running out of gas. They are actively being murdered.

Even more damning for the "running out of gas" explanation is that red and dead galaxies actually are surrounded by a halo of hot gas, typically at a few million degrees. We can see this gas via its X-ray emission with telescopes such as the Chandra X-ray Observatory. And here's the rub: This gas should be cooling! By any reasonable estimate, the energy loss rate of this gas implies that red and dead galaxies should be acquiring tens to hundreds of solar masses per year of fresh cold gas to fuel star formation. But this is clearly not happening -- we see no cold gas in these galaxies, and no star formation. Something is keeping this gas hot, and not allowing a dead galaxy to revive itself.

So it appears that we may need two killers: One to quickly quench a galaxy's star formation by removing the cold gas, and another to prevent any new cold gas from falling on the galaxy. The plot thickens!

Do astronomers have any suspects? Well, one simple rule in astronomy, analogous to "follow the money" in a terrestrial investigation, is to "follow the energy". Removing cold gas from a galaxy takes a lot of energy. So does keeping an entire halo of hot gas hot. What agent has the ability to provide such enormous amounts of energy?

When viewed this way, the list of suspects narrows dramatically, basically down to one single agent. It is an improbable agent, one already shrouded in mystery as the ultimate harbinger of doom in the Universe:  The Supermassive Black Hole.

It was only around 15 years ago that it was realized that most sizable galaxies contain a supermassive black hole in their center, with masses that can exceed a billion Suns. But the radius of a black hole is miniscule when compared to that of the galaxy. And the vast majority of black holes seem to be fairly inert, like the one in our Milky Way, with only a small fraction emitting any significant amount of energy in a so-called active galactic nucleus, or AGN, phase. So how can these black holes, as dark and malevolent as they may seem, be responsible for killing an entire galaxy whose mass is a thousand times larger? The poor, persecuted black hole pleads innocence!

Not so fast, says the prosecution. Black holes act like a cosmic drain, drawing in any unsuspecting mass near the galaxy's center to be devoured into its space-time singularity. But it turns out that black holes are sloppy eaters. A significant portion of the mass that approaches the black hole, by being accelerated close to the speed of light, is converted into pure energy, and is released back out into the galaxy instead of being swallowed. This follows Einstein's famous relation, the energy released is the mass times the speed of light squared. Since the speed of light is a large number, a little bit of mass going in can mean a lot of energy coming out, perhaps enough to kill a galaxy!

This movie shows a numerical simulation of how a galaxy merger might trigger an AGN that removes the gas from a galaxy. The gas distribution is shown for two spiral galaxies, color-coded by temperature. As the two galaxies collide owing to their mutual gravitational attraction, a black hole is fed, which injects energy into the surrounding gas and evaporates it away. Mergers can also transform the galaxy's morphology from spiral to elliptical, which can explain why dead galaxies are usually ellipticals. Movie by T. Di Matteo, V. Springel, and L. Hernquist from Nature, 433, 604 (2005).

Energy released from black holes is called AGN feedback, and it's been observed. It is seen to be strong during galaxy mergers, when the disorder of the collision results in a sort of feeding frenzy for the central black hole, which can eject large amounts of gas. It is also seen to happen intermittently in galaxy clusters, where the cluster gas shows bubbles in X-ray gas that astronomers suspect have been inflated by powerful jets from the central black hole that are only active about 10% of the time. Crude estimates suggest that the amount of energy released in these events could plausibly provide enough energy to kill a galaxy. Heuristic models that include these effects such as the semi-analytic models of CANDELS theorist Rachel Somerville are able to predict a population of red and dead galaxies mostly as observed, which is encouraging. So it seems to be a plausible scenario.

But plausibility wouldn't yield a conviction in a court of law, and it doesn't hold up in a court of science, either. The energetics are a necessary but not sufficient condition for black holes to kill galaxies. Closer examination reveals many puzzling aspects in this story, such as: How does the energy released by the black hole get distributed on such large scales? How does that energy know to go into exactly the gas needed to kill a galaxy, and not other gas? Why does the black hole start putting out all this murderous energy only once the galaxy is massive and living in a dense environment? Theorists have struggled to come up with well-justified answers to these questions, so the case against black holes remains full of, well, holes.

This is where CANDELS comes in. CANDELS will provide us many more clues than we had before.  For the first time, we will be able to probe back to when the first red and dead galaxies began to appear -- when the murders were fresh -- about 2-3 billion years after the Big Bang. Since red and dead galaxies are relatively rare, one needs a wide survey area to be able to find them, and since the first red and deads appeared long ago, one needs very deep imaging to see so far away. This combination of wide and deep is exactly what CANDELS is designed to provide. At the same time, CANDELS can also be used to track and identify black holes over much of cosmic time, with a little help from its survey friends. Using this data, we hope to directly associate the killing of galaxies with black holes, in essence try to catch the culprit red-handed.

But early results from CANDELS have only deepened the mystery. Work by Dale Kocevski and collaborators has shown that, at higher redshifts, the correlation between galaxy mergers and AGN is not nearly so clear as it is today. David Rosario led a study that showed that galaxies with AGN are not obviously distinguished from galaxies that don't, a puzzling result if AGN are supposed to be a harbinger of turning galaxies red. Jen Donley found that AGN at earlier epochs are increasingly surrounded by lots of obscuring gas and dust, which is odd if AGN are supposed to be removing all the gas. CANDELS has seen the first red and dead galaxies appear long ago, but they are strangely compact, unlike anything we see nearby, adding a new puzzling twist to the saga.

So far, we have yet to find any smoking gun evidence pinning the murder of galaxies on supermassive black holes, all while the black holes sit smugly smirking at our hard detective work.  Or perhaps we are in fact falsely accusing the poor black hole, and there is some other murderous agent responsible.  The end of this game of Clue still appears to be far away, which means that for CANDELS astronomers, the fun is just beginning!

Thursday, February 14, 2013

Our First Google Hangout

Last week, members of the CANDELS team participated in a Google hangout hosted  by Tony Darnell and Alberto Conti at the Space Telescope Science Institute. You can follow their regular Google Hangouts at #SpaceFan on Twitter or through their YouTube channel at the above link. Several team members, including Steven Finkelstein, Romeel Dave', and Darren Croton, a few of our regular bloggers, Joel Primack, and Cathy Caviglia discussed CANDELS and their own research using CANDELS data. 

Check out the video below and let us know what you think! We will try to do more of these in the future covering a range of science topics.

Friday, February 8, 2013

Star Formation in the Mountains

A view of the mountains surrounding Sesto, Italy. Photo by Dale Kocevski.
No two snowflakes are alike, and yet forecasters are pretty good at predicting snow. No two mountains are alike, and yet geologists can tell us quite a lot about how mountain ranges form and erode. Similarly, no two galaxies are alike and yet astronomers would like to understand how galaxies as a whole form and evolve. So what better place to talk about this topic than in a snowy mountain range! Last week a group of about 40 astronomers met in the small town of Sesto, Italy, nestled in the Dolomite mountains right near the Austrian border. The title of the workshop was "Star Formation Through Cosmic Time," and the focus was on trying to link together what we are learning about star-formation in very distant galaxies from Hubble observations like CANDELS to observations at infrared wavelengths from the Spitzer and Herschel observatories. This is important because more than half of the energy produced by stars in distant galaxies is absorbed by dust and re-emitted as infrared radiation

Most galaxies seem to form stars at a rate that is proportional 
to the number of stars that they already have. Some astronomers 
are calling this the main sequence of star-forming galaxies. 
Other  galaxies fall off the sequence. The red and dead ones
or quenched or quiescent ones aren't forming many stars at all. 
On the other hand there are some galaxies forming stars at much 
higher rates, which we call starbursts. Then there are a few galaxies 
that are still forming stars, but at lower rates than on the
main sequence. These populate the green valley,  although shutting 
down star formation isn't the only way to end up with greenish colors, 
so the green valley is sort of a hodgepodge of various
kinds of galaxies. 
A lot of the discussion at the meeting centered on the "Main Sequence of Star-Forming Galaxies" and on the galaxies that depart from that sequence. The "main sequence" is a term that was coined by CANDELS team-member Kai Noeske a few years ago and seems to have caught on. He noticed that most galaxies that are forming stars are forming them at a rate that is roughly proportional to their existing stellar mass. We don't understand in detail why this should be the case, so one item on the agenda was to discuss the evolution of this main sequence and the link between galaxies on the main sequence and galaxies that fall off it. The galaxies that fall off it fall into two classes: those that are forming stars at much higher rates ("starburst galaxies"), and those that have more-or-less stopped forming stars. Several people at the meeting talked about the starburst sequence. Depending a bit on how you define it, it looks like starbursts account for about 10-15% of all the cosmic star formation. I'm not sure the evidence that there are two separate sequences is all that compelling, but it is impressive that by assuming there are two sequences, it is possible to explain the evolution of the infrared luminosity function of galaxies, and to infer something about the evolution of the gas and the evolution of the heavy elements in galaxies. This is very handy because it can help inform us what to expect (and what to go look for) with two powerful radio telescopes that are just coming online, the JVLA and ALMA.

There were several talks about the ability of theoretical models to explain these two sequences. Currently, they seem to get the qualitative behavior right (there is a main sequence), but the quantitative behavior wrong (e.g. the proportion of stars forming in starbursts was about a factor of two too low in one of the models discussed). The failures of the model are almost certainly connected to the feedback of energy into the gas that is too cool to form the stars. This feedback can come from the stars themselves, particularly when they explode as supernovae, or from gas funneling into central massive black holes in the centers of galaxies. Supermassive black holes probably go through periods when they are not accreting a lot of gas, and other periods when they are. When they are actively accreting, they are called Active Galactic Nuclei (AGN) and emit a lot of high-energy radiation such as X-rays. However, if they are surrounded by dust, those X-rays can be absorbed and re-emitted as infrared radiation. There were discussions about new ways to identify AGN using infrared radiation as well as discussion about the properties of the host galaxies surrounding the AGN. CANDELS observations have revealed that distant AGN hosts don't really look any different than galaxies that are not hosting AGN, so that probably means that whatever is causing the gas to funnel into the black hole is not affecting the overall shape of the galaxy. That's a bit of a problem because it seemed quite likely that mergers between galaxies were a key way of getting the gas into the center.

Another interesting question is whether AGN prefer to be in star-forming galaxies or in galaxies that are shutting down their star formation. If feedback from AGN is important for quenching star formation, than one might expect that the galaxies that host AGN might look like they are starting to shut down. You might expect the "green valley" of galaxies in the diagram above to be populated by galaxies with AGN in their centers. The jury is out on this. Dale Kocevski showed evidence that the AGN hosts in CANDELS span the full range of star-forming activity that is seen in galaxies of the same mass. On the other hand another CANDELS member, David Rosario, showed evidence from far-infrared data that AGN hosts are drawn from a population of normal actively star-forming galaxies, and tend to avoid weakly star-forming, quenched or quiescent galaxies. So the observations are giving us somewhat contradictory information, and it is going to take some work to see how to reconcile these results.

One of the things that CANDELS provides is a good way to find and study quenched or quiescent galaxies at great distances. Several talks focused on the numbers of these galaxies. We are now finding massive quiescent galaxies when the universe was just a few billion years old. These galaxies are much more compact than massive non-star-forming galaxies today, so a couple of questions arise: (1) can they grow into their high-mass cousins by just acquiring stars in their outskirts by merging with surrounding galaxies and (2) can we find galaxies on the star-forming sequence that have enough stars jammed into their centers to be the likely progenitors of the quenched galaxies. The answers to these questions are tentatively yes: the densities of stars in the centers of the very distant quenched galaxies are pretty comparable to the central densities today, so adding stars to the outskirts probably works. And there appear to be enough compact galaxies on the star-forming sequence to form the galaxies on the quenched sequence if the star-formation shuts down on a reasonable timescale. On the other hand, CANDELS observations are finding fewer quenched low-mass galaxies than theory predicts, so that may be a problem. 

Harry Ferguson talked about some of the difficulties of inferring the
star-formation histories of high-redshift galaxies. Photo by Dale Kocevski.
There was also a lot of discussion about the star-forming histories of galaxies. We can estimate the stellar masses of galaxies in a variety of ways, and lots of checking suggests that these measurements are pretty robust; for an individual galaxy the estimates based on existing data are probably within a factor of two of the true value. Estimating star formation rates is much more difficult, but if you have information from the far-infrared together with infrared from the ultraviolet part of the spectrum, then it is also possible to make pretty good estimates. So putting those together, astronomers can estimate the total number of stars forming per year, and can do this at various "lookback times" from the present day to about 12 billion years in the past. Astronomers can also estimate the amount of stellar mass present at each of these lookback times. Now the stellar mass at later times ought to agree with what we infer from the rate of star formation at earlier times. This has been a problem in the past, but it now looks like the problem has been resolved with better estimates of star formation rates and stellar masses. So that's good news. On the other hand, the very fact that these estimates agree means that there can't be a lot of galaxies missing from the census of either star-forming or non-star-forming galaxies. That's a bit weird because galaxies can disappear from the census pretty easily if they become very dusty, or fade enough between bursts of star formation. Some theoretical models predict a lot of bursting and a lot of dusty galaxies, so these models might need to be revisited. 

We can also look in detail at the measurements of galaxy colors and spectra and try to infer a bit more about their individual histories of star formation. A lot of discussion at the meeting was about the difficulties involved in doing this. Unfortunately, the current state-of-the art is that when you use all of the information provided in the spectrum to try to estimate the star formation rate, you probably get a worse estimate than if you ignore the optical and near-infrared portion of the spectrum and just use the information from the ultraviolet or the far-infrared (or better yet, both). This is probably because we don't have the correct star formation histories in our models, but we need to find a way to introduce more realistic star-forming histories without "over fitting" the data. 

The useful thing about small workshops is that people are more willing to admit what they don't understand. That tends to make for very fruitful discussion and provides the fodder for new projects. On that score, the meeting was very successful. 

Tuesday, February 5, 2013

AAS and the Job Hunt

A wordle made up of the 250 most common words from the AAS
abstract book. Image credit: Jim Davenport
Being an astronomer is a pretty cool job... I feel rather fortunate to get paid for what I do. But how do you manage to turn astronomy into steady work?

For many astronomers, the path to finding a job includes a stop at the annual American Astronomical Society (AAS) meeting. The conference provides several dimensions to help recent Ph.D. graduates and postdocs find a job: interviews, workshops, networking, and simply presenting your work in front of a wider audience. As a junior postdoc and recent Ph.D. graduate myself, I fall squarely into the AAS job-hunter category.

With so many astronomers (over 3000 this year) traveling to the AAS, it ends up being a convenient place for many employers to hold interviews for prospective job candidates. This includes both tenure-track faculty jobs and postdoctoral research positions. Like in any career, an interview gives employers a chance to get to know their candidates face to face. Personally, I find these interviews to be exciting opportunities: it gives you a chance to brainstorm new research ideas with experienced senior astronomers.

The AAS also offers several workshops for resume-building, grant writing, and interview tips. I haven't actually attended any of these workshops, but I do have a lot of experience with the networking side of the AAS. Sometimes this means finally meeting a colleague in person when you've only swapped emails before, and sometimes it means trying to spearhead a new observing campaign or data analysis plan. Networking also provides opportunities to meet astronomers who might be offering attractive postdoc jobs in the near future. The AAS is also one of the few meetings where industry and science policy professionals attend: if you are an astronomer seeking a non-academic job, the conference provides a rare outlet to network with potential employers.

With over 3000 astronomers, the AAS is simply the largest astronomy conference of the year. This means your talk or poster presentation will have a much larger audience than at smaller conferences. For this reason, the AAS is one of the few meetings where astronomers are unusually well-dressed. I suppose you never know which future employers might be at your talk, and it doesn't hurt to show that you are serious about your work.

The search for a job as a professional astronomer is not easy: there are many more qualified applicants than there are jobs (both temporary postdoc and permanent faculty). But the AAS provides a rare opportunity for face-to-face time with potential employers. For me, this is the biggest service the meeting provides, and it makes it an important meeting for recent Ph.D. graduates and young postdocs.