Thursday, March 20, 2014

Astronomer of the Month: Viviana Acquaviva

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 Viviana Acquaviva.


Tell us a little about yourself!

My name is Viviana Acquaviva and I work as an assistant professor of Physics at CityTech, one of 23 CUNY (City University of New York) campuses. I am originally from Italy and I completed all my studies there. I attended the University of Pisa where I got a Bachelors degree in Physics, and I moved to the International School for Advanced Studies (SISSA) in Trieste for my PhD. After that, I jumped (well, flew) across the pond and held two three-year postdoctoral positions, one shared between the University of Pennsylvania and Princeton University, and one at Rutgers University. I've been in my current position since 2012. I enjoy cycling and word games, and I'm quite the stereotypical Italian: I like to eat, drink wine, and zip around Brooklyn on my Vespa. 

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

I have explored several areas of research -- I effectively started as a theoretical cosmologist (and even did research with pen and paper -- lots of paper) working on inflation and the early Universe. During my PhD I worked mostly on the phenomenology of extended Dark Energy and Modified Gravity models. In my first postdoc I added some statistics and data analysis to my skill set and used them to analyze Cosmic Microwave Background (CMB) data, and in the last few years I became interested in the physical properties of galaxies and how to measure them in a rigorous, model-independent way. My latest projects focus on using data mining and machine learning techniques to help us navigate large data sets. In CANDELS, I am an active member of the SED fitting and photometric redshift team, and I try to help make accurate measurements of properties of galaxies such as mass, age, dust content, and star formation history.

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

I have a slightly unconventional background and I never really dreamed of becoming an astronomer, or thought I would become one, but I'm very glad I did (although I think I still lack many qualities a "real" astronomer should have, such as a solid knowledge of how to operate a simple telescope…). I started off thinking I'd want to go into some obscure branch of mathematical physics, became fascinated with cosmology in my senior year of college when I took a GR course, and sought an external advisor for my "Laurea" thesis (a meatier version of a senior honors thesis in the US) since there where no cosmologists where I was studying. When the time came to pick my PhD area, I was admitted to both the Astrophysics and High-Energy physics programs, and I bumped into the person who would become my advisor, Carlo Baccigalupi. I thought the research he was doing was super cool, I really enjoyed talking to him about science, and I decided I'd go for Astro. The rest, as they say, is history, although I still haven't had any formal training in Astronomy.

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

I can look back and identify three main roadblocks. The first came early in college -- I had been able to cruise through my high school years quite easily, and when I started college, I assumed that finding things difficult simply meant that I wasn't cut out for this field. I remember a tearful conversation with my mom in my junior year, where I was adamant (as adamant as a crying and distressed individual can be) about having chosen the wrong major and wanting to go into economics. She patiently listened and encouraged me to do whatever I felt was right. A few months later, thanks to the support of my classmates who kept telling me that in their opinion, I could understand physics pretty well and I was just having performance issues in tests, things began to get better, and I ended up graduating summa cum laude and happy as a clam. Thanks, mom. Thanks, mates.

Afterward, perhaps the most difficult waters to navigate were related to moving to the US and having to face a radically different work ethic and culture. I wasn't prepared for the competitiveness of this environment and I wasn't focused enough on my career. Up to that point, I had worked reasonable hours and had mostly had a "happy go lucky" approach, which, in all honesty, had worked pretty well. I had no clue what all these fellowships everybody was talking about were, and I had no intention or desire to work 12-hour days and every weekend, but suddenly I was surrounded by people who had been planning their next move for years and were determined to succeed, as well as very smart. Work was suddenly good -- perhaps too good, as if the number of hours spent at work, rather than the results, was something of which to be proud. My reaction (perhaps also because of lack of a stable mentor at that time) was to idle, and I got stuck in a weird mode where I wasn't really passionate about what I was doing, I wasn't productive, and these two evils kept feeding upon each other. It took some serious job (or lack thereof) scare and a great mentor to get back on track and realize that hard and systematic work was the cure.

And then of course, a few years later, there was "the faculty job" issue. I don't have much to say about how to overcome this one, because getting a job requires luck. I don't mean that I don't deserve the job that I got, but I am aware that it could have gone another way and whoever was next on that shortlist is probably as good as I am. I can share a few pointers for anxiety-control during this time, though. I tried to keep my options open during the process, by identifying valid alternatives and trying to build some additional skills. For example, I took a graduate course in machine learning in my last year as a postdoc, knowing that this was a field that fascinated me and maybe it would help me find a job I'd like outside academia. I also decided to only apply for positions I really wanted; geographical constraints were an issue for me, and I only selected a handful of locations where I thought I would be happy living. Where to set this boundary is a very personal decision, but I do strongly believe in only spending energy and time toward the things we want - a job we don't like isn't better than the alternative, even if the alternative is unknown. Of course, I had no significant other or children at that time, so it was easy for me to decide to play it out this way. And finally, in times of disappointment, I found it useful to take a break, travel and just get out of it all. Placing myself in an environment where nobody saw the big deal in not getting a faculty job in a particular place was a great help in regaining perspective.
 
Who has been your biggest scientific role model and why? 

I don't know if I have one specific person in mind, I was fortunate to work with many incredible scientists and I like to think I learned something from each of them. The person who had the biggest impact on my career was probably Eric Gawiser, another CANDELS team member and a fantastic mentor for me throughout the years. He offered me a job in a field very different from the one I had been working in up to that time, and let me have time to build up my knowledge in the new field. I had months where I was allowed to just study statistics books and work on code, and I remember them very fondly. But all the advisers I had have qualities that I admire and taught me something that I hope to replicate with my own students and mentees. Sabino Matarrese, my "Laurea" thesis advisor, believed in me even if I came from another University and he had no idea of what my GPA was; Carlo Baccigalupi taught me cosmology from the very beginning an put up with my "enticing" combination of inexperience and arrogance; Licia Verde will forever be my icon of hard, passionate work and lack of fear in asking questions; David Spergel taught me to always be open to new subjects and listen to all talks and all people.

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

My daily job involves a mix of teaching, research, and service work. I teach two or three classes per semester; fortunately they are small classes and usually it's introductory Astronomy, so I don't need a ton of preparation time by now. I have about two full days a week for research, and I try to divide them among my own (let's say, first-author) project, and projects with collaborator and students. I serve on various committees both for my department and my college, and I participate in a number of outreach activities, from Planetarium trips to public lectures. NYC is a very special place for that; the last talk I gave was within Astronomy on Tap, a series of events set up in various bars across NYC, and in May I will give a lecture on space in a black-tie event in a club who had Mark Twain among its early members. I look forward to talking about space while sipping champagne and wearing an evening gown! My favorite aspect of academic life is the freedom to choose what you want to work on and the flexibility in organizing my day. These factors are not exclusive to Astronomy, but I think they are just an incredible luxury. I like being around students -- it's tiring but very rewarding, and I hope that being in contact with young people will keep me young at heart and prevent me from getting bored, my greatest fear of all times. What are for me the best things about our field in particular? 1. I know tons of cool factoids (especially now after a few years of teaching -- Yes, I'm serious, a paper clip of neutron star matter weighs as much as Mount Everest, and that's why you need to know scientific notation!), and 2. our job title is quite awesome (Hi, I'm an astrophysicist, and a woman! Ta-da!).

What motivates you in your research? 

Probably the fact that there are constantly new challenges. It's interesting that what keeps me motivated is also what makes me very frustrated at times: working on open-ended questions requires a good dose of self-discipline and humor. I was born with at least some of the latter but I constantly have to work on the former… Also, I've always been one to enjoy the trip more than the destination, so I'd say that my motivation comes more from the fact that I love sitting at my desk and dealing with the puzzle of the day than from the desire of solving long-standing problems in Astronomy.

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

I am going to go with the Hayden Planetarium for this one. I hold a visiting scientist appointment there, and I just find it an incredibly inspiring place where to be. I went observing at Magellan once (I was the third astronomer on the trip and my job was basically to look around and not touch anything) and that place was magical and unforgettable - but then again, I have nothing else to compare it to, so perhaps it wouldn't be right to call it my favorite telescope.

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

Because of the luxury of a (hopefully) permanent position, I have been trying to shift my mental focus on longer-term objectives rather than short-term ones -- asking myself for real, what would I want to accomplish in the next few years? In terms of research, I would really like to focus on machine learning and data mining techniques and their applications in Astronomy. I also want to see some of the underprivileged undergraduate students I supervise enter a good graduate program, and I want to help my institution strengthen the programs currently in place for undergraduate research.
 
If you could have any astronomy related wish, what would it be? 

Turn off all the lights in NYC for a day, and show all my students what the sky really looks like. They were the ones who told me that in LA, during a massive black-out in 1994, many people called 911 reporting "a weird object" in the sky: it was the Milky Way.

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

Tough one! I guess the fact that most mind-boggling facts can be explained really simply and understood by everyone. In one of the Astro labs, we count the galaxies in a picture of the HUDF and then to calculate the number of galaxies in the Universe based on how many HUDFs-sized fields there are in the sky, and the number comes up surprisingly accurate (and definitely mind-boggling). The fact that so much of the history and fate of the Universe can be described in terms of six numbers and a couple of differential equations is also pretty amazing to me.

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

I talked about myself enough, and I can't say anything new about Astronomy… so I'll close with a suggestion: if you can, take a trip to a dark location, look up, and wonder. Caveat: your kids might get a lot more passionate about science after this trip!

Thursday, March 6, 2014

CANDELS Results Highlighted in Other Blogs

The recent CANDELS paper (Morphologies of z~0.7 AGN Host Galaxies in CANDELS: No trend of merger incidence with AGN Luminosity) by Carolin Villforth (see her blog post on it here) has been discussed in a couple of other blogs. The first is a post by Tanya Urrutia and the second is a post on Astrobites. Check them out!


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:
http://arxiv.org/abs/1309.6341

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. 

Thursday, January 30, 2014

More about CANDELS at the AAS

As mentioned by Liz McGrath in her recent blog post, the American Astronomical Society held its 223rd meeting  in early January. Altogether there were 37 presentations that had the word CANDELS somewhere in their title, author list, or abstract, and probably several others that used CANDELS under the radar. I thought it worth sharing a few highlights.

The meeting opened with the Kavli lecture by Bob Williams, the former director of the Space Telescope Science institute and president of the International Astronomical Union, who recounted the history of the original Hubble Deep Field and the science legacy that flowed from that first observation. One of the things he emphasized is how the making the data completely non-proprietary helped build the scientific momentum. Instead of observing separate spots in the sky and keeping the data to themselves, astronomers interested in the distant universe were suddenly much more willing to point telescopes to the same spot and share the data. CANDELS carries forward the legacy of this culture change; all of the Hubble data and most of the data from other telescopes are available to everyone.

A figure from Steve Finkelstein's talk, showing the number of galaxies found at redshifts
z=7 and 8 from the ultra-deep survey fields (blue) and CANDELS (red). Absolute magnitude
is a measure of brightness, with smaller numbers (more negative) indicating brighter galaxies.
CANDELS is great for finding brighter (rarer) galaxies, while the ultra-deep fields
excel at finding fainter (but more common) galaxies.
On Monday afternoon, the CANDELS and CLASH projects were featured in a special session. This was an opportunity for astronomers from both projects to give a top-level summary of the results. The CANDELS talks were given by Jeyhan Kartaltepe, Steve Finkelstein, Yu Lu and Steve Rodney. Jeyhan summarized the results on galaxy morphology, with particular attention to the CANDELS visual classifications and the evidence that ultraluminous infrared galaxies are associated with galaxy collisions and mergers, even at high redshift. Steve Finkelstein summarized the state of research on the most distant galaxies. He showed a nice diagram that helps to put into perspective the contributions of the "deep but narrow" fields (the Hubble Ultradeep field and associated parallel observations), and CANDELS. He also talked his discovery of what is currently the record-holder for the most distant spectroscopically confirmed galaxy, at redshift z=7.51. Yu Lu summarized the effort of part of the CANDELS theory group to compare semi-analytical models of galaxy formation. This is the most detailed side-by-side test of such models yet carried out and will be of great utility in helping us understand what we are learning from CANDELS. Finally, Steve Rodney partnered with Or Graur of the CLASH team to give a summary of the supernova program, including the first estimates of the evolution of supernova rates from both surveys.

A figure from Guillermo Barro's talk about the evolution of compact galaxies at high redshift.
The vertical axis shows how active the galaxy is in forming stars. Specifically, it shows
the ratio of each galaxy's star-formation rate, to its total stellar mass. The horizontal axis
shows how densely packed the stars are together. Dense galaxies are to the right,
diffuse galaxies are to the left. Guillermo is finding that galaxies tend to make the
transformation from being diffuse to being compact while they are still forming stars;
the quenching of star formation happens later.
The next day, CANDELS figured prominently in the Hieneman prize lecture given by Rachel Somerville. Rachel has been one of the leaders of the theory effort in CANDELS and recounted the progress in trying use the observations to get at the detailed physics of galaxy formation. One of the more recent results she showed in her talk was the success that she and Lauren Porter have had in matching the trends that Guillermo Barro and Christina Williams see in the evolution of compact galaxies. Both Guillermo and Christina spoke that morning in a session about galaxy surveys that was almost entirely populated by CANDELS talks.

In addition to the talks, there were quite a few posters. Posters are tacked to display boards in a big exhibition hall, and stay up all day. On the plus side, this gives people an opportunity to wander by and discuss the research, which is harder to do in a session packed with 5-minute talks. On the minus side, there are so many posters that it impossible to look at them all. CANDELS was very fortunate to have most of our posters on the first day located right near the entrance to the hall, so there was lots of opportunity for people to see them.

It was also great to see work by other groups using the CANDELS data. One of the press releases from the meeting was from Garth Illingworth, discussing finding some suprisingly bright galaxies in the CANDELS GOODS-N data that might be at redshift z>9. These are not yet spectroscopically confirmed, but if they are really at high redshift, then they indicate that star-formation was already proceeding vigorously when the universe was only 500 million years old.

Monday, January 27, 2014

A New Type Ia Supernova in M82

Just a couple of days ago, a dim, but quickly brightening, supernova was discovered in M82, the beautiful "cigar galaxy." At "only" 12 million light years away, this is the nearest supernova to Earth since 1987 and the nearest Type Ia supernova since 1972. With the enormous changes in our imaging technology since then (including the launch and subsequent improvements to the Hubble Space Telescope), this is a fantastic opportunity for precision measurements of one of the brightest and most mysterious explosions in the universe.

The new supernova in M82, discovered by students at the University College London
Observatory.  Photo by Adam Block/Mount Lemmon SkyCenter/University of Arizona


Discovering more about the nature of Type Ia supernovae has been one of the primary goals of the CANDELS project. These supernovae begin as stars like our sun, which have shed their outer layers at the end of their lives and become white dwarfs. White dwarfs are the extremely dense cores of a burned-out star, and although they're only the size of our earth, they have the mass of our entire sun.  The detonation happens when a nearby star adds even more mass onto this dwarf -- when the weight becomes too much, nuclear fusion ignites it and a supernova occurs.

In CANDELS, we study the most distant Type Ia supernovae that we can find, the farthest of which stands at over 10 billion light years away. Our supernovae tell us about the early expansion of the universe (and its Dark Energy), the chemical evolution of the universe, and how quickly supernovae form and explode around 8-10 billion years ago -- at the peak of star formation in the universe.

This nearby galaxy offers a completely different, and rarer, perspective. In 1972, when the last Type Ia supernova this close to Earth exploded, it was still a year before anyone proposed the idea that these supernovae were formed in binary star systems. It was 12 years before someone realized that both stars could be white dwarfs, and 18 years before supernovae could be studied from space with the Hubble Space Telescope. It was over 25 years before such supernovae were used to discover that Dark Energy was accelerating the expansion of our universe.

Motivated by the knowledge and technology gained since the last close Type Ia supernova went off, scientists will be asking an entirely different set of questions this time around. First, we'll be looking for a giant companion star that could have fed mass onto the white dwarf. If a companion star is visible, this would be the first direct evidence that a system with one white dwarf can lead to a supernova; if a companion star is not found, the theory that two white dwarfs can make a Type Ia supernova will gain credibility.

Artist's conception of the single-degenerate (one white dwarf)
theory of Type Ia supernova explosions, wherein
a white dwarf accretes mass from its companion
star.  (original) © ESA and Justyn Maund (Queens Univ. Belfast)
Artist's conception of the double-degenerate theory
of Type Ia supernova explosions, in which two white dwarfs merge
together as they emit gravitational waves. (original) © NASA,
Tod Strohmayer
(GSFC),  and Dana Berry (Chandra X-ray Observatory)

Second, scientists will be studying the geometry of the supernova from the fraction of polarized light emitted. Polarization, the orientation of a light ray's electric field, is entirely random when it originates from a spherically symmetric star. However, if one side becomes longer than the other, the light's polarization will have a preferential direction that can be measured on Earth. As the outer layers of the M82 supernova expand, they will become transparent and expose the inner material. Over the next month, scientists will be able to measure the shape of different layers and examine the three-dimensional explosion. With this structural information, we'll learn more about how supernova detonation occurs; specifically, how nuclear fusion begins and spreads through the layers of the white dwarf.

The location of M82 on the night sky from Sky and Telescope.
A more detailed chart is available here
Lastly, Type Ia supernovae are nearly uniform in brightness, serving as excellent distance indicators for most of the visible universe. CANDELS supernova principal investigator Adam Riess -- among others -- will be measuring the distance and doppler shift velocity (the reddening of its light) of this supernova to determine how fast the local universe is expanding -- and infer the amount of the mysterious Dark Energy that surrounds us.

This supernova is particularly rare in that it offers opportunities not only to scientists, but for anyone with access to a dark night sky. It will brighten for approximately a week and a half, and at its peak it will be visible near Ursa Major (the Big Dipper) to anyone with a set of binoculars. Although it's impossible to predict when the next close supernova will be, I'm looking forward to seeing an exploding star with my own eyes - it may be 40 years before there's another opportunity.

Wednesday, January 22, 2014

223rd AAS Meeting in Washington, D. C.

Twice a year, the American Astronomical Society holds professional meetings covering a broad range of research and education topics. This January’s meeting was held in Washington, D. C. at the National Harbor. I wouldn’t be surprised if it broke previous attendance records, with almost 3200 people on the official registration list. 

Rachel Somerville giving the Heineman Prize Lecture.
Photo credit: Joson Images/ AAS
CANDELS scientists had a very strong representation at this meeting. Rachel Somerville won the Heineman Prize and gave a lecture entitled, “The Formation of Galaxies and Supermassive Black Holes: Insights and Puzzles.” Meanwhile, Jennifer Lotz held a press conference on the release of data from a new ultra-deep, wide-field imaging survey that she is leading known as the Hubble Space Telescope Frontier Fields (see the image from their press-release below).  There was also a CANDELS special session which included 4 talks and 10 posters on CANDELS results, as well as 22 other CANDELS-related talks and poster presentations throughout the course of the meeting.

HST Frontier Field Abell 2744.  Image credit: NASA,
ESA, and J. Lotz, M. Mountain, A. Koekemoer, and
the HFF Team (STScI).
People go to the AAS for a variety of reasons. As one of the largest gatherings of astronomers, it is impossible to see every science talk of interest. For this reason, I find AAS meetings are more of a place to have discussions with your colleagues and the larger astronomical community about priorities and goals for the future. Many of these larger, community-oriented discussions take place during “town hall” sessions.  Sometimes difficult decisions need to be made, especially in the current funding climate. At this meeting, astronomers grappled with the likelihood that many of our beloved facilities will either need to find private partners to sustain operations costs, or be shut down in the next couple years. This is because NSF does not have the money to fund them while continuing forward with important projects like the James Webb Space Telescope and the Large Synoptic Survey Telescope. If you know anyone with a few hundred thousand dollars to spare, you can buy yourself some quality time on top-notch facilities!


Other important discussions include networking with more senior astronomers, especially when you’re on the job hunt. Jon Trump wrote about this aspect of AAS in a previous post.  However, the largest growing demographic at these meetings is young student researchers. This is a great place for them to showcase their work and gain experience talking with people about their research. For me, this was my first AAS meeting where I was on the “other side”, serving as faculty advisor to an undergraduate student who is applying to graduate school this year. As my first thesis student, I encouraged her to come present her results, while I tried my best to introduce her to people doing exciting science as well as folks on various graduate admissions committees. The AAS can be a bit daunting to newcomers, so it’s the advisor’s role to help facilitate discussion and provide a role model for students just starting down this career path.

Neil deGrasse Tyson at the AAS.
Photo credit: Joson Images/ AAS
In addition to science talks, there were a number of sessions on astronomy education research, which is another growing aspect of the AAS community. One session I attended on how to improve student outcomes in Astro 101-style courses, was standing room only. It’s great to see so many professional astronomers care so deeply not just about their personal research, but also about how to improve their approach to teaching science to non-science majors. While this is a significant part of many of our jobs, it may surprise readers to learn that most of us were never trained as teachers. Therefore, these sessions are particularly important for learning how to be effective instructors.


Astronomers also like to have a bit of fun in these meetings. At this meeting there was a special talk by Neil deGrasse Tyson, who was surrounded by hundreds of adoring, geeky fans (a.k.a, professional astronomers) as he talked about how to use twitter to engage the public in science. There was also the infamous AAS “after-party”, which was a bar-hopping extravaganza, complete with astro-themed cocktails and a mechanical bull (although I never did see anyone attempt the bull)!

Space Shuttle Discovery at the National Air and Space
Museum.  Photo credit: E. McGrath
Finally, with the meeting being held in Washington D. C., I took advantage of the opportunity to do some astronomy-themed sightseeing. I visited the National Air and Space Museum hangars located near Dulles airport, where I got to behold the impressive Space Shuttle Discovery, as well as a Mars Pathfinder prototype. The shuttle was even more impressive in person than I could have imagined—definitely worth a visit next time you're in D. C.