Exploring Galaxy Evolution with CANDELS Morphologies

The most massive galaxies in the Universe are important probes of galaxy formation as they provide insight into the physical processes which govern the evolution of galaxies at the extreme high mass limit. By studying these most massive systems across cosmic time we can provide rigorous tests for our understanding of how mass is assembled in the Universe.

Example 6x6 arcsecond image stamps of the
bulge+disk decomposition of one of our
objects with significant bulge and disk
components. The residual image illustrates
the goodness of fit of the combined model.

It has recently been discovered that massive galaxies at z>1 have surprisingly smaller sizes than similarly massive galaxies in the local Universe and in the most extreme cases are up to a factor of 4 times smaller. In fact, it has been shown that the most compact of these high redshift galaxies also display the least amount of ongoing star-formation, which raises the question of how such compact systems at z>1 can grow to reach the sizes of comparably massive local galaxies if they have very little continued star-formation. There are several proposed mechanisms which may explain how these galaxies can grown in size, with very little mass increase from high redshift to the present day, such as through minor merger events or from AGN driven gas expulsion, which causes the system to expand. The exact physics responsible for this required growth remains debated, but it is clear that the morphologies of these massive galaxies can provide us with important information about their evolution.

In the local Universe galaxy morphologies can be classified by the Hubble sequence and they display a well-known correlation between colour and morphology, with spheroidals being predominantly red in colour due to the fact that they have very little ongoing star-formation and, conversely, disks being blue, but at higher redshifts the case is more complicated.

The unparalleled high resolution H-band data from CANDELS has allowed us to conduct a detailed study of  ~200 of the most massive galaxies with M>$1x10$¹¹ (i.e. 100 billion) solar masses at 1<z<3 (when the Universe was 1/2 to 1/6 its current age) in the UDS field, where we were able to decompose the rest-frame optical morphologies of galaxies into their separate bulge and disk components for the first time at these high redshifts for such a large sample size (see the CANDELS paper here). By conducting this decomposition we were able to explicitly explore how the sizes of the different components evolve within this redshift range, and compare this to studies in the local Universe. In doing so we found that the bulge components appear to display a more dramatic evolution in size than the disks.  This can be seen in the figure below both from the number of bulges which have sizes significantly smaller than objects in the lower Universe, and in the difference between these sizes, which is more extreme for bulges than disks. 

Size-mass relations for the separate bulge and disk components. Left: bulge components over-plotted by the red solid line with the size-mass relation found in the local Universe for spheroidal galaxies, and by the red-dashed line which is the corresponding error on the local relation. Right: disk components over-plotted in blue by the size-mass relation found in the local Universe for disk galaxies. From these plots we can see that while some bulge components lie on the local relation, the majority of them fall below it, whereas for disks a larger fraction are consistent with the local relation, and for those components which fall below the difference in size with local values is not as great as for the bulge components.

In addition to how massive galaxies evolve in size, decomposing objects into their separate bulges and disks also allows us to explore how the overall morphologies of galaxies evolve with redshift. In the local Universe, the majority of massive galaxies are pure bulge systems, but from this study we found that not only do massive galaxies become increasingly mixed systems with significant bulge and disk components with higher redshift, but that by z~2, they have predominantly disk-dominated morphologies. This suggests that not only is 1<z<3 a crucial era in cosmic time when global star-formation in the Universe peaked, but that is also marks a key phase in morphological evolution, where galaxies undergo a dramatic transformation from disk to eventually bulge-dominated systems.  

The redshift evolution of the morphological fractions in our galaxy sample, after binning into redshift bins of width z = 0:5, using three alternative cuts in morphological classification.

In previous CANDELS posts the issue of what triggers the switching off of star-formation in galaxies to make them passive has been discussed, and it has been suggested that while the presence of a prominent bulge may best correlate with passivity, some passive galaxies with significant disks have also been observed. By utilising our decomposition of objects into their separate bulge and disk components we  directly addressed this question by including star-formation activity for the objects in our sample. When we did this, we found that the majority of star-forming galaxies are disks, and passive galaxies are bulges, but interestingly, a significant fraction (~40%) of passive galaxies have disk-dominated morphologies, i.e. where less than 50% of the total light from the galaxy is contained in the bulge, where the advantage of our decomposition technique allows us to explicitly assess how much of the overall light from the system is associated to the different components. Moreover, we also found that some of our passive galaxies appear to be pure disks. As discussed in previous posts, this suggests that while the traditional star-formation quenching (i.e., shutting down) mechanism of major mergers may indeed be important for some massive galaxies, there may also be additional physical processes which can quench star-formation in a galaxy but leave a massive disk intact.

The next step in our work is to extend our analysis to the CANDELS-COSMOS field to allow greater area coverage, and we are currently implementing a new technique to extend our decomposition of bulge and disk components to the other 3 CANDELS bands (F125W, F814W and F606W) in order to provide photometry for separate components to conduct individual SED fitting, with the aim of generating separate stellar mass and age estimates for the different components. This will add an extra dimension to our morphology decompositions and shed new light on the properties of high redshift massive galaxies.

The Winding Road of Becoming an Astronomer

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

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

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

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

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

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

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

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

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

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

How Do Galaxies Grow?

The beauty of CANDELS is that it allows us to study how galaxies grow over an unprecedented dynamic range in mass and time. A particular focus of CANDELS is studying Cosmic Noon, the epoch from redshifts around 4 to 1, when the universe was about 2 to 6 billion years old. During Cosmic Noon, the universe was forming stars at more than ten times its current rate, making it the most active period of galaxy growth in cosmic history. Clearly, if we want to know how galaxies grow, studying this epoch is pretty important.

One of most influential new observations that has emerged from this epoch is a remarkably simple and tight relation between a galaxy's star formation rate and it's stellar mass: The two are linearly related!  In other words, larger galaxies form stars faster. This relation, now referred to as the "galaxy main sequence", might not sound terribly surprising, but it gives us a crucially important clue: It implies that galaxy growth is a smooth process. This is far from obvious when you look at a CANDELS image, because the universe looks like it is filled with a bunch lumps that we call galaxies! Moreover, we see that galaxy mergers are much more common during Cosmic Noon than today. Mergers are known to temporarily boost star formation by large factors (called a starburst) without increasing the stellar mass (as) much.  If this was the dominant process for galaxy growth, the galaxy main sequence would not be tight, it would show a huge scatter! The fact that we see a tight relation can be used to show that merger-induced starbursts likely contribute only about 10% of star formation during Cosmic Noon.

Simulations of galaxy formation can help us interpret what we see in the real Universe. This movie from a simulation shows only the stars (although the simulation itself also includes gas and dark matter) -- bluer stars are younger, redder are older. The volume depicted is roughly 50 million light years on a side, with face-on (left) and edge-on (right) views. We see how galaxies start out small and young arranged along the filamentary Cosmic Web, and merge together into larger, older galaxies -- this is called hierarchical structure formation. If stars were all that were visible (like in CANDELS images), one might infer that galaxies grow mostly by merging. But simulations suggest that this is only the easily-visible tip of the iceberg, and that relatively smooth gas inflows are actually the driver of galaxy growth (see the movie below).

This tight galaxy main sequence has a surprising corollary: If we only see the "lumps", while galaxies grow smoothly, this must mean that we don't see most of the matter that drives galaxy growth!  So where is this vast, unseen reservoir of fuel? Most believe that it is gas in the intergalactic medium, the vast expanses of space surrounding galaxies. This gas is very difficult to detect (which is why the space between galaxies looks so "empty"), but there is lots of observational evidence from the so-called Lyman alpha forest that the vast majority of atoms in the Universe live in the intergalactic medium, not in galaxies.  

Despite theoretical expectations, nobody has conclusively observed this intergalactic gas "in the act" of falling into galaxies, so at present this fueling mode remains a bit of theorist's fancy. Indeed, this is a major focus for next-generation telescopes such as JWST, GMT, and TMT, whose increased sensitivity may be able to directly detect inflow. Nonetheless, CANDELS is already providing indirect constraints by providing the best measurements to-date of the galaxy main sequence.

This simulation movie shows the gas rather than the stars. The large left panel shows the velocity field, in which one can see gas streaming down three converging filaments into a galaxy. The volume shown is roughly 7 million light years on a side. The "Density" panel shows the gas density -- little bright knots are where galaxies would form, and one would see stars, but the remainder of the gas is very difficult to detect. Other panels show gas temperature, metallicity, and metal-line absorption. The Cosmic Web is full of gas, providing relatively smooth fuel for galaxy growth. At around redshift z=3, one can even see an outflow that ejects gas from the galaxy (which is at the center of the panel) perpendicular to the inflow filaments.

A second remarkable new addition to the landscape of galaxy formation is that matter doesn't just flow in to galaxies, it also flows out. In fact, a lot of it flows out -- observations during Cosmic Noon suggest that the rate of mass flowing out from galaxies likely exceeds the rate forming into stars! Expelling this much gas at hundreds of km/s as observed requires enormous amounts of power, comparable to the power from all the supernovae going off in the galaxy. This seems rather surprising because galaxies appear to be rather serene, isolated object, as opposed to roiling pots of explosive energy that drive huge galactic outflows. But again, that may just be because we can't see the gas!

These new observational ingredients seen in galaxies during Cosmic Noon have caused CANDELS theorists to rethink some fundamental ideas about galaxy formation. How do we put all these insights together into a full story for how galaxies grow? One emergent paradigm is that galaxy growth proceeds via a balance between inflows and outflows. Material is brought in by gravity from the intergalactic medium, some of it gets turned into stars, and the rest is expelled in an outflow. This process is continuous and ongoing, resulting in a fairly smooth rate of growth. This is now colloquially referred to as the baryon cycle.

A helpful analogy for this is a factory. What does a factory do? It takes in raw materials, produces a product, and emits some waste. It does so fairly continuously, modulo small fluctuations in supply and demand. Analogously, a galaxy takes in gas accreted from the intergalactic medium, turns them into stars, and ejects outflows. This fluctuates as the inflow varies (after all, the universe does have some lumps!), but overall is a fairly steady. Hence in the baryon cycle view, galaxies are like gas processing factories, continually churning out stars from gas, at a rate that is mostly smooth with mild variations.

The outflows then represent the factory's waste. In fact, astronomers even use the word "pollution" to describe this ejecta. But in this case, astronomers mean something very specific -- pollution here refers to the fact that outflows carry out the by-products of star formation, which are the heavy elements ("metals") synthesized in the cores of stars. You  have probably heard that we are all made of stardust, that carbon, oxygen, and indeed all elements other than hydrogen and helium are produced by nuclear fusion inside stars. When stars explode in supernovae that drive these outflows, they pollute the outflowing material with these metals. The metals therefore provide an excellent tracer for where outflows have reached. Remarkably, quasar absorption line observations indicate that they have reached millions of light years away from galaxies!

The baryon cycle scenario can straightforwardly explain why the cosmic star formation rate is higher by a factor of 10 during Cosmic Noon compared to today -- it's because the universe was smaller and denser, the hence the expected gravitational inflow rate is higher by exactly that amount back then! It straightforwardly explains the tight relation between star formation rate and stellar mass, with the scatter around the mean trend reflecting the "lumpiness" of accretion. It is attractive in its simplicity and is well-situated within hierarchical structure formation models.

But none of that necessarily makes the baryon cycle scenario correct. Indeed, it is far from the only way to explain these data. The increased merger rate during Cosmic Noon suggests that the lumpiness plays a key role, and could drive the increased star formation rates. Moreover, galaxies around Cosmic Noon tend to look more disturbed, suggesting that even if the inflow is smooth over long timescales, on short timescales it may be quite chaotic and stochastic. Finally, the baryon cycle idea centrally invokes two phenomena -- inflows and outflows -- that have at best only recently been detected, and have yet to be well-characterized.  Hence while from a theoretical standpoint it is a simple and attractive model, from an observational standpoint there is much work to be done to test the baryon cycle scenario.

The unparalleled dynamic range of CANDELS, probing massive galaxies down to dwarfs at Cosmic Noon, will measure galaxy scaling relations like the main sequence with unprecedented accuracy, and relate it to many other interesting galaxy properties such as morphologies, colors, environment, and structural parameters. CANDELS theorists are working hard to take these observations and interpret them to see if they are consistent with the baryon cycle scenario. As scientists always love to subvert the dominant paradigm, observers and theorists alike are looking for ways in which the baryon cycle fails, or at least is an incomplete description of what's going on (which it almost certainly is). Through such investigations, we hope to gain a more detailed understanding how these gas factories process their raw material into stars during the most active phase of universe, Cosmic Noon.

WFC3 Early Release Science (ERS) Observations

The GOODS (North and South) fields are two major fields observed by the CANDELS program. The GOODS-N field will be fully covered by CANDELS near-IR observations, while only ~70% of the GOODS-S field area will be observed by CANDELS near-IR observations. The main reason is that the remaining ~30% of the area in the GOODS-S field was already observed by the Wide Field Camera 3 (WFC3) Early Release Science (ERS) observations. Today, I will summarize these ERS observations.

The multiwavelength 10-band color image of the entire ERS mosaic in the GOODS-South field combines a broad range of colors, from the ultraviolet, through visible light, and into the near infrared. Such a detailed multi-color view of the universe has never before been assembled in such a combination of color, clarity, accuracy, and depth. Credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski, R. O'Connell, P. McCarthy, N. Hathi, R. Ryan, H. Yan, and A. Koekemoer.

The Scientific Oversight Committee (SOC) was constituted in 1998 to oversee the science definition and the technical development of the WFC3. In recognition of their effort during 11 years of WFC3 development, the STScI director allocated ~210 orbits (~300 hours) of the HST time to the SOC to do challenging science programs designed to test key capabilities of the WFC3. To this effect, the SOC distributed these orbits equally between two major programs, to investigate (1) nearby galaxies and (2) distant galaxies. The combination of these two programs is referred to as the 'WFC3 Early Release Science (ERS) Program." I will only talk about the 'distant galaxies' program, as it is most relevant to the CANDELS program. The 'distant galaxies' program was allocated 104 orbits (~150 hours).

I got involved in this program through my PhD advisor, Rogier Windhorst, who is a member of the SOC. Myself, Seth Cohen, Russell Ryan, and few SOC members were involved in planning this 'distant galaxies' program. The GOODS-S field was chosen because of the availability of extensive ancillary data in this field. The primary goal of the ERS program was to showcase capabilities of the WFC3, so we planned 3 sets of observations: ultraviolet imaging (total 40 orbits), near-infrared imaging (total 60 orbits), and near-infrared grism (low resolution spectroscopy, total 4 orbits). The ultraviolet and near infrared imaging covers the full ERS field (~45 arcmin²), while the grism observations cover only ~1/10th of the ERS field.

Multiwavelength postage stamps of objects with interesting morphological structure
in the 10-band ERS color images of the GOODS-South field: from left
to right, high signal-to-noise detections of ERS galaxies resembling the main
cosmological parameters w, λ and ρ respectively. These images illustrate the rich and
unique morphological information available in the 10-band panchromatic ERS data set.
Credit: Rogier Windhorst and WFC3 SOC

These observations are being used for various science goals, including identifying and understanding passive galaxies, star-forming galaxies at intermediate redshifts, high redshift galaxies, emission-line galaxies, and galaxy morphology/mergers at various redshifts. These data are also useful to identify faint red stars in our own galaxy. Details of ERS observations are in Windhorst et al. 2011. The intermediate depth ERS observations are excellent complementary data to the CANDELS wide shallow and the CANDELS deep observations. CANDELS team is generating a large near-infrared mosaic in the GOODS-S field, which will include ERS as well as CANDELS datasets in a single image. Keep coming back for more results from these datasets.

Meet Christina Williams

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

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

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

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

Fieldwork on the sea ice near Barrow, AK

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

ASTE site, in the Atacama Desert, Chile

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

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

Climbing Pigeon Spire, Bugaboo Provincial Park,
British Columbia

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

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

Studying Galaxy Evolution with CANDELS Observations of High Redshift Clusters

Galaxy clusters and superclusters represent the largest bound structures in the universe. Because of this they are key probes of the large scale physics driving the expansion of the universe and the coalescence of matter which ultimately spawns the galaxies we observe. Studies of galaxy clusters also played an important role in the discovery of dark matter, the bullet cluster ultimately being a smoking gun. The topic of this blog post, however, is using galaxy clusters as laboratories for studying galaxy evolution.

By the early 1980's astronomers were finding strong evidence that the morphology of galaxies is correlated with their environment (the density of galaxies in the vicinity). Galaxies observed in high density environments are far more likely to be massive, elliptical, and passively evolving (little or no ongoing star formation), but the physical origins of this relationship are still not entirely known. It is likely related to interactions between the galaxies and the cluster and/or interactions between the cluster galaxies themselves. Galaxy-galaxy interactions include galaxy mergers which can significantly disrupt and randomize the orbits of their stars, growing and transforming the galaxy in the process.

Top: false colour image of the cluster combining
images from Subaru and the Spitzer Space telescope.
Bottom: Detection image for the cluster showing
the overdensity of galaxies at the redshift of the cluster.

Recently a technique has been developed for discovering galaxy clusters at high redshift which exploits the fact that as you observe galaxies at increasing distances certain spectral features redshift into redder photometric bands. Using data from large scale, deep photometric surveys it is possible to measure the density of galaxies which have similar colors measured in various bands. However, because these discoveries are based on photometry, the certainty you have that clusters you find are not just chance alignments of unassociated galaxies is relatively low (more on this unfortunate fact can be found in this previous post). Nevertheless, this technique was used by CANDELS team member Casey Papovich and collaborators to discover a galaxy cluster at a redshift of 1.62, meaning light we detect from these objects was emitted when the universe was less than a third of its current age!

Since its discovery, this cluster has been extensively studied. Spectroscopic observations were undertaken in order to confirm the narrow range in redshifts for potential cluster members. Despite difficulties in observing strong spectral features at this redshift (due to overlap with strong night sky emission), redshifts for 11 galaxies were obtained and found to lie close to the expected value for this cluster. X-ray observations were also taken using NASA's Chandra X-ray Observatory finding that the x-ray emission from this cluster is dominated by a single point source with weak extended emission. This, along with the spatial distribution of cluster members, supports a hypothesis in which this is a young cluster still in the process of forming (it has also been dubbed a "proto-cluster").

In 2011, I joined the team studying this cluster led by Casey Papovich, just as the first Hubble imaging from the CANDELS project was being released. The depth and resolution of these images finally made it possible to study the structures and morphologies of these galaxies. Because the large field of view of these images allows cluster and field populations to be examined in a single uniform data set, CANDELS imaging is ideal for studies of environmental trends. Using a code called GALFIT, I was able to measure sizes of these galaxies, as well as a parameter which measures how compact these galaxies are: the Sersic index. A general rule of thumb is disky galaxies have Sersic indices around 1 while spheroidal galaxies have Sersic indices between 2.5 and 4.

Hubble Space Telescope coverage from CANDELS. The cluster sits at the edge of a much larger field allowing us to construct a large comparison sample of non-cluster galaxies from a single uniform dataset.

In the first paper published using the results of these fits, we looked primarily at the sizes of passive galaxies. We first found that, at this redshift, in both the cluster and the field, passive galaxies have smaller sizes than similar galaxies in the local universe. This means that these galaxies must evolve in size over the subsequent ~10 billion years. Second, in the cluster there is a dearth of compact passive galaxies, possibly indicating that the galaxies within the cluster are experiencing a accelerated evolution compared with those in the field. This accelerated evolution could be related to an enhanced rate of mergers in the higher density environments. Evidence supporting this scenario was found in a study by Jennifer Lotz who shows that members of this cluster are far more likely to have very close neighboring galaxies than members of the field population (this study will be the topic of an upcoming blog post).

We extended the analysis of our first paper by folding in the structure of these galaxies using the Sersic index. If this cluster were to follow a trend similar to the morphology density relation we see locally, one would expect passive cluster galaxies to be more compact (and therefor have larger Sersic indices). When we looked at this, however, we found just the opposite! The passive galaxies in the cluster were found to have more disk like structures and larger sizes (when compared with other passive galaxies in our sample), attributes which are more characteristic of galaxies which are actively forming stars. The spatial distribution of these galaxies was also found to be very interesting: the structural differences we observe appear to be driven by passive galaxies in the outskirts of the cluster. Passive galaxies in the cluster core and the field appear to make up a more uniform sample of compact spheroids.

So what's going on here? One possibility is that these galaxies were actively star forming, disk galaxies which were recently accreted into the cluster environment. During the process of accretion, interactions with the cluster itself stopped their star formation. This could be caused by gas (which fuels star formation) being stripped from the galaxy or being heated and thus preventing it from collapsing into stars. This heating could possibly be caused by intergalactic shock waves that are propagated as a cluster is in the process of collapsing. These shock waves are formed when the galaxies being rapidly pulled inwards meet the ambient medium surrounding the cluster. While these shocks are largely theoretical at this point, there has been recent evidence in their favour in the nearby Coma cluster.

One caveat to this study is that there is a small number of cluster galaxies which are bright enough for us to observe and many of our distance measures have a large uncertainty. Because of this, our results are at the edge of statistical significance. However, similar results were found by Ruth Grützbauch and collaborators for a more highly evolved cluster at a slightly lower redshift. Their cluster, XMMU J2235.3-2557 at a redshift of 1.39, is found to have what they describe as a "quenching radius". Any galaxy within this distance from the cluster is extremely passive, but as you move further out the star formation rate increases. A picture is emerging in which galaxy clusters influence the star formation of galaxies at extremely large distances from the cluster core, much further out than previous estimates. To confirm this result will require more similar studies of high redshift clusters, so stay tuned!

What is an Observing Proposal?

Have you ever wondered what astronomers have to do to get to go on observing runs to telescopes? You might think that there are a lot of telescopes and thus astronomers can go observing whenever they want to or you might think that they observe every night. Actually, time on telescopes is in very high demand and astronomers have to compete with each other for every night of telescope time they get. In order to enter this process, astronomers must write a document called an observing proposal. This is the typical process for almost all telescopes, including big and small facilities, facilities run by a single University, those run by groups of Universities, and national facilities. Space-based telescopes (like Hubble!) also follow this procedure.

Image of Kitt Peak National Observatory, managed by NOAO. Image credit: Michael L. Weasner

In order to apply for telescope time, the first step is to come up with a good idea. Astronomers often have multiple projects going on at once and we are always thinking of new ideas and questions and ways to improve upon what we know. Once we have an idea we have to decide what telescope (and instrument) would be the most suited to accomplishing the science goals we have in mind. The instrument needed can often be more important than the telescope itself. Is the goal imaging or spectroscopy? Is there a particular wavelength range or filter needed? Do we need to target a single object or small patch of sky or are we surveying a large area? All of these factors go into selecting the best telescope-instrument pair. This selection can also depend on the University or country that the astronomer is at. Different Universities have access to different Observatories depending on funding, instrument development, and various other partnerships. There are also various facilities run on a national level -- for example NOAO in the US or ESO in Europe.

The next step is the bulk of the work: writing the actual proposal. A typical observing proposal has several components. The main one is called the Scientific Justification -- basically, describe why the science project you want to do is interesting. This is the place where an astronomer has to really sell their idea and convince others that answering this particular question is very important and must be done. Often, those reviewing proposals do not work in the particular specialty of the proposer so a good proposal is one that can be understood by any astronomer, not just experts on that particular topic. We must also clearly lay out the strategy of the science project here: how will the observations that we are proposing for answer this important question? What kind of data will be taken and how will this data be used to solve the problem presented? Often astronomers are limited to only a couple of pages of text so it can take a lot of work to say everything you want to say succinctly.

Another typical component of an observing proposal is a technical section. This is where an astronomer must go into detail about the instrument and telescope they are proposing to use and say why this particular combination is well suited. They must clearly demonstrate how much data they need and how much time this will take overall. Since observing time is a precious commodity, any time request must be clearly justified - if you say you will need two nights to accomplish your goals then you must show that two nights are really needed and one night would not be enough. Often, this portion is reviewed by people who are experts with the given instrument and who understand how well the instrument will perform.

It is important that a proposal be very well written! In fact, the ability to write well is a very important job skill for astronomers in general. An astronomer must be clear and concise in their proposal. If there is confusion about the goals or how they are going to address a particular problem, this could negatively impact the proposal's chance of success. It is also very useful to include informative graphics that illustrate the science goals and method presented. As they say, a picture is worth a thousand words, and a clear well-thought out figure can really strengthen a proposal. Finally, this might seem obvious but it is very important that all of the rules be followed! A proposal cannot be longer than the given limit and cannot be written in too small of a font or with tiny margins. This might seem picky, but when a person has to review a lot of proposals they all need to be easy to read. At last, the proposal is complete and can be submitted (and must be on time!). At this stage, the astronomer can sit back, relax, and start thinking about their next big idea.

But the process has only just started on the receiving end. Most observing proposals are then evaluated by a committee of peers (other astronomers, either from the specific institute that runs that particular telescope, or selected from all over). Every proposal is read by the entire committee and the committee gets together to discuss each one over the course of a few days. It's not always easy to pick out the best proposals to award time to. Often there are more excellent proposals than there are nights to be awarded. Intense discussions about the merits of each proposal results in a ranked list and time is given to those proposals at the very top. Every one that does not get their proposal accepted must try again next time.

This may sound like a lot of work, and it is, but whenever a proposal is accepted and we get the opportunity to observe and collect new data, it is all worth it. It is a great feeling to know that your peers have found your ideas worthy of supporting! This is a process that we go through once or twice a year for each telescope we would like to collect data with. One of the major observing seasons of the year just finished this past September. Luckily there is a little bit of a break before the next major deadline in February (for HST). This break is needed so that we can work on analyzing all of the data from the previous year!