Thursday, February 27, 2014

Astronomer of the Month: Eric Gawiser

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 Eric Gawiser.

Tell us a little about yourself!

Hi! I'm Eric Gawiser, an Associate Professor in the Department of Physics & Astronomy at Rutgers University.  We academics tend to walk a long, roundabout path to becoming a professor, and I'm no exception. I started out as an undergraduate at Princeton University, majoring in both Physics and Public Policy.  Grad school was at the University of California at Berkeley, leading to a Ph.D. in Physics; I studied theoretical cosmology in those days. Then I switched specialties, starting postdoctoral research in observational studies of galaxy formation at the University of California, San Diego. My next postdoctoral fellowship was at Yale University; during two years of that time, I had a joint appointment at Universidad de Chile and spent half my time living in Santiago, Chile. That was an incredible opportunity for an astrophysicist, as half of the world's biggest telescopes are located in the Atacama Desert in the northern Chile, and I had the chance to observe with most of those.   

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

My research focuses on distant galaxies. Projects range from trying to understand what makes certain types of galaxies form new stars to using these galaxies as cosmological probes that can reveal the nature of dark matter and dark energy. In CANDELS, my research group at Rutgers has been active in assessing the data quality coming in from the Hubble Space Telescope, offering advice on how to generate catalogs of galaxies from it, and in improving methods used to determine the distances to and masses of those galaxies. On March 1, the Astrophysical Journal will publish our first major CANDELS paper, led by Carlos Vargas (now a graduate student at New Mexico State University) and Hannah Bish (an undergraduate senior at Rutgers) - you can find it at:

In that paper, we used the exquisite data from CANDELS to show that a data analysis technique called "stacking" works pretty well on average but fails to reveal that distant galaxies called Lyman Alpha Emitters have masses that vary by a factor of 100. We discovered that these Lyman Alpha Emitters have much more rapid star formation than you would expect given their masses.  

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

My original interest was in math and physics, and I slowly gravitated towards astrophysics (pun intended, sorry!). Growing up, I wanted to become a scientist but was also interested in marine biology, as the idea of spending one's time doing research on tropical coral reefs seemed pretty enjoyable. I got to take an astronomy course in high school, which put it on my list of career possibilities, but I chose Physics instead as an undergraduate major. 

What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 

Academia can be a rough road. As you ascend from undergrad to grad school to postdoc to faculty, the competition for the next position becomes more and more intense. It's hard not to become cynical as you see certain people who get strong pushes from their Ph.D. advisor get the best job offers based more on reputation than merit and observe universities trusting each other's judgment more than their own in their hiring decisions. There are two stages in my career where I found it hard to get a job at the next level, but I have always believed in making your own luck -- whatever opportunity I received, I thought about what type of research I could do best with the collaborators and facilities available at that institution. In the long run, I found that creativity and hard work get recognized. The flaws of academia are now an opportunity for me, as when I hire postdoctoral researchers or Rutgers hires new faculty, I look for people who are stronger than their recommendation letters or worked for less famous advisors. There are incredibly talented people in this field, many of them in CANDELS, and my greatest enjoyment in doing research is getting to work with people who are clever, motivated, and team-oriented.
Who has been your biggest scientific role model and why? 

Out of many role models both current and historical, I would have to choose Galileo, who was my original childhood inspiration to become a scientist. He conducted creative experiments to figure out how the world actually works and was brave enough to stand up to incredibly powerful forces when his results disagreed with their propaganda. We need a lot more of that in modern society.

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

Making a new discovery in astronomy is an incredible feeling. I have had a few special moments in my career when I felt that I understood something that nobody on Earth had understood before. It's a scary moment too, as after the initial "Eureka!" moment you work hard to defend your discovery in a written paper and submit it for publication while hoping that the scientific referees and your colleagues will end up accepting your results. However, I never think I'm truly the first being to understand something, as I believe there are other intelligent civilizations in the universe whose astronomers are much more advanced than we are. 

What motivates you in your research? 

One of my graduate school mentors, Prof. Marc Davis of U.C. Berkeley, taught us that when it comes to research, you should either do something first or do it best. This acknowledges the reality of astrophysics research that we need more data beyond the first discovery of a new type of object to really be sure that we understand what's happening. I have always been motivated by the "do it best" half of this motto, seeking large samples of distant galaxies with high-quality data to analyze. CANDELS is perfect for that!  

Inspiration is a funny thing, though. I have had my best research ideas while traveling, especially when sitting on a beach. 

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

It's not the biggest telescope in the world, but the Cerro Tololo Inter-American Observatory (CTIO) in Chile has played the biggest role in my research. I spent 38 nights there gathering data for the MUSYC survey. Summer nights are warm, and we take long exposures, so I relish the chance to walk outside and let my eyes adjust to the darkness and see thousands of individual stars plus the Large and Small Magellanic Clouds and the dusty center of our Milky Way galaxy. Combine that with the great data yielded by the wide-field MOSAIC-2 camera on its 4-meter telescope, and CTIO's as good as it gets! 

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

On a beach!  (In order to spur new research ideas, of course.) 
If you could have any astronomy related wish, what would it be? 

I think that astronomy deserves 10 times the level of public investment that it receives, and I would wish for it to receive that. Here is the argument: nothing that I know of brings humanity together across borders, cultures and languages more effectively than realizing that we all share one fragile planet orbiting 1 out of 100 billion stars in 1 out of 100 billion galaxies in the known universe. The gain to human civilization of fully recognizing this commonality would be immense - a reduction in war, increased trade, a greater focus on developing and sharing technology to solve common problems. These benefits would vastly outweigh the relatively modest costs of supporting new astronomical discoveries and increased efforts to share our findings with the public.  

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

We don't know yet if our universe is infinite in size or not. I think this is the greatest embarrassment of modern cosmology, but it's not obvious how to tell the difference. We know that our universe is very big, at least as big as 14 billion light years in every direction, and there hasn't been enough time since the Big Bang for us to see further to look for signs that we've seen everything there is to see! 

Is there anything else you would like for the public to know about you or astronomy in general? 

I've always found it critical to balance the intense time demands of an astrophysics career with the stress release of exercise. I play soccer whenever I get the chance. I used to be a competitive (though far from professional) triathlete in graduate school and recently came "out of retirement" to start doing triathlons again. In some ways, I am only truly relaxed when my brain has too little oxygen to think about research, and exercise does the trick! 

Thursday, February 13, 2014

Breaking the Galaxy Distance Record

In this loooong overdue post, I’m going to talk about what happened following the events of my previous post. In that post, I talked about how my research team and I used the Keck 10 meter telescope to obtain spectroscopy of 43 distant galaxies. To briefly recap, my group and I have been using CANDELS images to search for very distant galaxies (those that we see as they were within one billion years of the Big Bang, which gives them a redshift greater than 6). In a few previous posts, I’ve talked about some of the exciting things we’ve been learning in the distant universe, including how these galaxies get redder with time (as they build up their heavy elements; i.e. planet-making material), and whether galaxies can account for the reionization of the universe (yes!  we think).

In this previous post, we talked about how we use images to find these galaxies - essentially, since they are so far away, they are moving very quickly away from us, thus their light is redshifted due to the Doppler effect. Ideally, you would take a spectrum of every galaxy to search for redshifted emission lines to measure your redshift. However, this is impractical for samples of hundreds or thousands of galaxies. On the bright side, we can get a rough estimate of the redshift using imaging alone, and this technique has been well-documented over the past ~20 years.

The downside of this is that 1) the redshift is only approximate, and that makes everything else you learn a little more uncertain; and 2) its possible that some galaxies you think are really distant are actually close by galaxies that just happen to be very red. To get around this, we typically try to take spectra of a small portion of our sample, to verify that our contamination is small.  Fast forward, and this is why we went to Keck, to try to measure the redshifts for many of our distant galaxy candidates.

As I looked at the data we took at Keck, we found a very bright emission line from one of our distant galaxy candidates before we even left Hawaii. This left me feeling very optimistic!  However, as we continued to analyze our data, we found that the first line we saw would be the only line we would see - out of the 43 observed galaxies, we detected an emission line from only a single one. This may seem like a failure, but lets examine our detected galaxy a little more closely.

This image shows a region of the CANDELS GOODS-North field, just above the handle of the Big Dippler.  Highlighted is z8_GND_5296, the most distant spectroscopically confirmed galaxy in the universe.  The galaxy looks very red in this image, as it is so distant (and thus moving so quickly away from us), that it is only detected in Hubble's reddest filters.  Image Credit: V. Tilvi, S. Finkelstein, C. Papovich, A. Koekemoer, CANDELS and STScI/NASA.
The emission line we saw was the Lyman alpha line from hydrogen. This line is emitted in the ultraviolet, but we saw it all the way in the infrared, meaning that it has a very high redshift.  In fact, the measured redshift of this galaxy is 7.5, making it the highest redshift spectroscopically confirmed galaxy*** (the previous record was at 7.2). That's exciting in itself, but the galaxy had more in store for us. Using how bright it is in the CANDELS imaging, we can measure how fast this galaxy is converting hydrogen gas into new stars, and we found that its “star-formation rate” is an insane 300 solar masses per year; this is 150 times faster than the Milky Way!!! From what we (thought we) knew at high redshift, if you found a random redshift seven galaxy, you would have expected it to be forming stars at around 10 solar masses per year, so this galaxy is forming stars 30 times faster than its peers.  

Our spectrum from the MOSFIRE spectrograph on the Keck 10 meter telescope.
The white blob in the top panel shows Lyman alpha emission from z8_GND_5296. 
At the observed wavelength, this corresponds to a redshift of 7.5078. The bottom
panel shows a cross-cut of the top spectrum (what we call a one-dimensional spectrum),
which shows the galaxy's flux versus wavelength. You can see the peak
corresponding to Lyman-alpha emission (highlighted by the red line).
There are a number of other peaks too, which all correspond to the position of emission
lines from our own atmosphere. These are very bright, and we try to subtract
them out, so what you see here are residuals. The lines are difficult to
subtract completely, because their intensity changes rapidly with time.
Not only has this level of star factory not been seen at these redshifts before, but it was also a complete surprise to theorists, who do not see such galaxies in their models. While this galaxy could just be a weirdo, we don’t think thats the case. The previous record redshift holder I mentioned, at z=7.2, has a star-formation rate of 100 solar masses per year. Smaller, yes, but still very high. And, it is located in the same region of the sky as our galaxy.  What are the odds?!? What we think we’re learning is that these extreme star factories are much more common in the early universe than previously thought, so now we need to get with our theorist friends and try to figure out why that is.

As for the other 42 galaxies we didn’t see? The jury is still out. It may be that the gas between galaxies is becoming neutral (as would happen if we’re entering the epoch of reionization), and this neutral gas “fog” is screening us from seeing the Lyman alpha photons. Or, it could be that these distant galaxies are becoming increasingly rich in gas themselves, preventing these Lyman alpha photons from escaping. Only time and further study will tell, but we’re hot on the trail!  If you're interested in all the details, you can see our paper, which has been published in Nature, here, and our official press release, which is here.

***Often in the news there are articles about the most distant galaxies in the universe - some of these are spectroscopically confirmed like our galaxy here, while others are candidate galaxies, meaning that their redshifts have not been verified. While many of these candidates turn out to be real, measuring the redshift spectroscopically is the gold standard for galaxy distance measurements. A case in point is our recent blog post, which mentions a galaxy with a redshift of close to 11 from the CLASH survey. This galaxy has not been spectroscopically confirmed (though Hubble will try to do it in a few months). However, in the particular case of this galaxy, I think its highly likely that its real, as not only are its colors that expected of such a distant galaxy, but the positions of the lensed images are what you would expect for a galaxy at the estimated redshift.  Hopefully Hubble will measure a redshift, and, if not, then we’ll have to wait a few years for the next generation of telescopes.

Wednesday, February 5, 2014

CLASH : the Cluster Lensing And Supernova search with Hubble

In 2010 the Hubble Space Telescope launched three bold new initiatives that came to be called the Multi-Cycle Treasury programs. One was the CANDELS program, the parent of this blog.  Another was the Panchromatic Hubble Andromeda Treasury program (PHAT), a deep and detailed study of the nearby galaxy M31, led by Julianne Dalcanton of the University of Washington. The third program was called CLASH: the Cluster Lensing And Supernova survey with Hubble (tortured acronyms were a prerequisite for approval of the HST time). The CLASH team (not to be confused with The Clash) is led by Marc Postman from the Space Telescope Science Institute, and includes about 50 astronomers at some 25 institutions around the world.  This survey is in many ways a close sister to the CANDELS program, and indeed there is significant overlap across the two groups, especially in the supernova search component, which has been a joint CLASH+CANDELS effort.
Galaxy cluster MACS J1206.2-0847 (or MACS 1206 for short) as viewed
through Hubble in the CLASH program. Credit: NASA, ESA,
M. Postman (STScI), and the CLASH Team
The CLASH program takes a deep look at 25 massive galaxy clusters. These are collections of galaxies (a few hundred in each), hot gas (heated to above 10 million degrees), and dark matter (more on that mysterious stuff below). The clusters in the CLASH sample sit at redshifts between about 0.2 and 0.9, so we are seeing them at a fairly recent epoch in terms of cosmic history (the universe was already more than 6 billion years old when the light we see left these clusters). Several of these clusters have been studied in great detail, but the CLASH program has opened up a new window to look in at one of the great mysteries of the universe: the nature of dark matter.

When Hubble looks at a galaxy cluster in the CLASH survey, it captures the ultraviolet, optical, and infrared light emitted by billions upon billions of stars in the many galaxies that live within the cluster. Astronomers have long known, however, that these stars make up only a small fraction of the total contents of these clusters.  Far more important is the hot gas in the Intra-Cluster Medium (ICM). This superheated gas (mostly Hydrogen and Helium) has been stripped away from the galaxies by tidal gravitational forces and the effects of ram pressure. The gas is so hot that it emits x-ray radiation, which can be observed using x-ray observatories like Chandra and XMM-Newton. The mass of gas in a typical galaxy cluster is almost 10 times greater than the total mass of all the stars in all the member galaxies. However, even after counting up all of the stars and gas, we still have only captured about 10% of the total mass of the galaxy cluster.  The other 90% is (presumably) in the form of dark matter.
"Dark matter" is the name we assign to all the mass in the universe that does not emit any light. There are a number of theories as to what this dark matter could be, and the most promising idea right now seems to be that it is some form of elementary particle that does not interact with other matter -- except through the force of gravity. In galaxy clusters, we have two primary lines of evidence that reveal the presence of a large concentration of dark matter. First, the motions of the galaxies in the cluster show that there must be a large central mass pulling the galaxies in and through the cluster (more mass than we can account for in stars and gas). Second, we see the effect of the dark matter on background galaxies through gravitational lensing.
Einstein's theory of relativity tells us that the force of gravity is in fact a warping of spacetime. This distortion of the fabric of our universe affects all forms of matter -- as we see in the motions of planets, stars and galaxies -- and it also affects light itself. In the CLASH clusters, the warping is sufficiently strong to bend the pathway of light rays passing through the cluster. This results in a lensing effect, as light rays are distorted and redirected such that they focus on our location here in the Milky Way. We see the extraordinary evidence for this lensing in the form of absurdly stretched galaxies, long arcs, and impossibly bright background sources that have been distorted and magnified by the cluster's gravitational lens.
The principal aim of the CLASH program is to use these lensing artifacts to construct detailed models of the matter content of each of the 25 clusters. The cluster models are built by piecing together these distorted background sources to make a map of the dark matter that Hubble cannot see. Adding in evidence from the star light and the x-ray gas emission provides a complete picture of all the content in the cluster. With all of this information, the CLASH team has been able to improve our understanding of how these clusters are formed, and even to put new constraints on the nature of the dark matter fluid that dominates the cluster.

The tiny red blob (just a fraction of the size of our Milky Way) is among
the most distant galaxies ever observed. The object is observed just
420 million years after the big bang, and is only visible to the Hubble
Space Telescope due to the magnification from the massive galaxy
cluster MACS0647,  which lies in between us and the distant
proto-galaxy. Credit: NASA, ESA, M. Postman and D. Coe (STScI),
and the CLASH Team 
The gravitational lenses in the CLASH clusters also provide a powerful tool for looking back into the very early universe. The CLASH team has twice discovered objects behind the gravitational clusters that are among the most distant galaxies ever seen, first in April 2012, and then again in November 2012. These very distant background sources would normally be far too faint for even Hubble to see, but the intervening cluster acts like a natural telescope, focusing the light from the far-off galaxies so that Hubble can just barely detect them.

The science work of the CLASH team is still in progress, and we expect many more exciting discoveries are yet to come. Hubble is not done with deep galaxy cluster surveys, either, as the new Frontier Fields initiative has already begun to follow in the footsteps of CLASH.