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!

The Bright Galaxies that HST Can Barely See

The large number of galaxies that are clearly detected in the deep HST/CANDELS images enable us to carry out very exciting studies that we regularly report in this blog. Today, instead we will focus on a special type of galaxies that are very faint in all the CANDELS images, but at least 40 times brighter at longer wavelengths, in the so-called mid-infrared regime. Until very recently, only a few isolated cases of these galaxies were known, but thanks to the depth of the CANDELS data, and making use of mid-infrared Spitzer Space Telescope images, we have discovered 25 such galaxies within a single CANDELS field.

Since its launch in 2003, the Spitzer Space Telescope has allowed us to study, in a systematic way, the infrared emission of galaxies at different cosmic times. With respect to previous infrared observatories, Spitzer represented a major step in infrared astronomy, which was possible thanks to the fast progress of infrared detector technology over the last three decades.

Four examples of sources that are bright in the Spitzer Space Telescope mid-infrared
images, but very faint in the HST/CANDELS images. The multi-wavelength analysis of these
sources indicates that they are very likely massive galaxies formed in the first two billion years
of cosmic time. Image credit: Caputi et al. (2012), ApJ, 750, L20.

At low redshifts, the Spitzer mid-infrared images trace the dust emission of star forming galaxies, which occurs after the dust is heated by the UV photons produced by the new stars. The UV photons that are the consequence of accretion of matter onto a galaxy's central black hole can have a similar effect, namely heating any surrounding dust and making it emit at mid-infrared wavelengths. But for high redshift galaxies, the mid-infrared emission seen in the Spitzer maps has a rather different origin: it directly traces the redshifted light of the galaxy oldest stars. The CANDELS images, in turn, show the redshifted light of a (high-redshift) galaxy's young stars.

By comparing the multi-wavelength emission of our 25 Spitzer-bright galaxies with theoretical galaxy spectral models, we determined that the vast majority of these sources are very likely at high redshifts (z>3), which means that we are observing the light that was emitted by these galaxies when the Universe was less than two billion years old. Actually, nowadays we know many z>3 galaxies, but the properties of our newly discovered 25 galaxies are very special: the fact that they are bright in the Spitzer images, but much fainter in the CANDELS maps, indicates that these objects should be among the oldest and most massive galaxies to be found at such early cosmic times.

Finding massive galaxies in the early Universe has important implications for galaxy formation theories, which need to explain how such objects could have formed so quickly and efficiently only a few billion years after the Big Bang. According to our most-accepted cosmological model, the Cold Dark Matter model, galaxies are embedded in dark matter halos, and grow with them hierarchically, through mergers, from small to larger units, through mergers. In such a scenario, one would expect that massive galaxies are the last to form. But different astronomical observations  conducted over the last decade indicate that the most massive galaxies that we see in the Universe today basically finished their growth when the Universe was less than a half of its present age (this is 8 billion years ago), while less massive galaxies continued forming a significant amount of stars later. This phenomenon is what extragalactic astronomers call 'galaxy downsizing.' So, searching for massive galaxies further back in time is very important for understanding when galaxies could assemble a large mass for the first time in the history of the Universe, and thus constraining galaxy formation models.

Another exciting aspect of our new, massive galaxy candidates at high redshifts is that they potentially constitute the 'tip of the iceberg' of a  larger galaxy population that still remains to be discovered. To fully understand the importance of such a galaxy population we will have to wait for the advent of the HST successor, the James Webb Space Telescope (JWST), which is due for launch in 2018.  The JWST will provide us with much deeper images than current telescopes, thanks to its large collecting area  -- seven times larger than the HST, and almost 60 times larger than Spitzer. With these deeper images,  we will be able to search for fainter analogues of our galaxies at higher redshifts. In the meanwhile, we are trying to follow up our galaxies at far infrared wavelengths with the Atacama Large Millimetre Array (ALMA), which is the only instrument that, currently, can independently confirm the nature of our sources.

Ultraviolet Observations in CANDELS

The CANDELS project is primarily focused on observations at optical and near-infrared wavelengths, that is light that is received at the wavelengths that the human eye can see and a little bit longer. However, a fortunate trick of the Hubble Space Telescope (HST) orbit also allows us to observe one of the five CANDELS fields at ultraviolet (UV) wavelengths, that is slightly shorter wavelengths than the blue end of the rainbow. UV observations are especially interesting, because very massive, very hot stars emit light strongly at those wavelengths.

How We Get UV Observations

Optical HST image of the Hubble Deep Field (in gray scale) with
Far-ultraviolet observations over layed (in purple). The Far-ultraviolet
are wavelengths even further in the blue direction than the UV
observations planned for CANDELS. Most galaxies are not detected
in the far-UV, despite the image being very deep, because their light is
shifted redward by the Doppler shift ("redshift"). Image Credit: Harry Teplitz

HST orbits the Earth, which means that most of the time when it wants to point at an interesting position on the sky, it can only do so during a fraction of the orbit during which the Earth isn't in the way. There are a few places on the sky, though, for which HST can point continuously during the entire orbit. The GOODS-North field in CANDELS is one of these special places. This means that we have about twice as much time available to observe that field than usual. CANDELS uses some of the extra time to observe GOODS-North in the UV.

As with all CANDELS fields, GOODS-North is observed many times, building up the signal through repeated observations. The schedule of the repetitions is designed for the supernova search. In practice, this means that even though we got the first UV observations in the spring of 2012, it will be spring of 2013 before we have enough data to see most of the UV objects in the images.

Major Science Goals

The most massive, young, and hot stars emit light strongly in the UV. This makes UV observations particularly effective for studying galaxies that are forming many stars.  These data allow us to find these galaxies and to study how they formed.  

In a little more detail, we have three major goals for the UV observations:

1.  Finding and studying strongly star-forming galaxies:  

The most popular way to find distant star-forming galaxies is to look for a strong feature in the distribution of their light across the spectrum.  This technique, referred to as looking for the "Lyman break" or "dropouts", will be familiar to people who have read about the rest of the goals for CANDELS, because it is also used to find galaxies in the very distant Universe. The advantage of adding UV data to CANDELS is that it allows us to use the same method to find galaxies when the Universe was about 25% of its current age. And, by using the same technique to find them, we can compare those galaxies directly to the much more distant ones.
 

In practice, the "dropout" technique means that we look for galaxies that are bright in most of the CANDELS bands, but are much fainter in the UV.  That is, they "drop out" of the UV. This is an indication that the UV light that they emit has been absorbed by neutral hydrogen before it reaches HST.

Once we find these galaxies, we can ask many intersting questions about them: Do they tend to be big or small? Do they tend to have a lot of dust? Do they tend to be in groups together or are they isolated?  

2.  The build up of galaxy structure from sub-galactic clumps

Galaxies grow and develop structure over time. Through gravitational effects, possibly including merging with other galaxies, they become like the galaxies we see today. We know that they undergo periods of intense star formation, but we are still learning about how they form their distinctive structures like spiral arms.  

There is evidence that as many galaxies grow, they form small clusters of hot stars, which are often called "clumps", which then migrate together into larger structures. Measuring the number, size, and brightness of these clumps can help us understand how galaxies form their structure. It can also help us distinguish which kinds of galaxies form through the mergers of smaller galaxies, and which kinds form primarily by themselves.  

Clumps have been studied extensively in galaxies when the Universe was about 25% of its current age. UV observations, which will see the hot stars that make up the clumps, will allow us to study them in galaxies later in the histroy of the Universe, when it was around 50% of its current age.

3.  How does ionizing radiation escape from galaxies?

There was a period of time in the relatively early Universe, known as the "dark ages", when most of the electrons and protons in the Universe were together in Hydrogen atoms. When galaxies began to form, they emitted a lot of energetic photons (referred to as "ionizing radiation") which broke apart the Hydrogen atoms, an event called "Reionization." Exactly how this happened is one of the great mysteries of cosmology. In particular, we don't know how the photons got out of the galaxies that contained the hot stars that emitted them.

 
In order to figure out how this ionization radiation escapes, we need to be able to observe it. The best way to do this is to look at galaxies that are similar to those which caused Reionization, but are a little closer so that we can study them. 

The CANDELS UV observations will allow us to make some of the best measurements ever taken of ionizing radiation escaping from galaxies.

The Next Steps

CANDELS is currently taking UV images, with a single pass of the GOODS-North field once every couple of months. It will take about a year to build up enough of these images to reach the sensitivity needed for the science goals described above.  We are eagerly waiting for next year, when we can begin to look at these exciting data and see what we can learn about strongly star-forming galaxies, the build up of galaxy structure, and how ionizing radiation escapes from galaxies. Stay tuned!

How Old Are Galaxies?

"How old are you?" A simple question, frequently asked of children, but rarely asked of an adult in polite company. When we ask the question, we are asking for the number of years that have elapsed since you were born. But how old are you, really? There were nine months of cellular development before birth, so for some purposes, maybe we should include that. Also, the average age of cells in your skin is less than two weeks, your stomach cells are typically less than 5 days old, and your blood cells are less than 4 months old. The average age of all the cells in your body is less than 10 years. So how old are you, really?

Okay, so for humans, if you want to get persnickety, maybe age isn't such a well-defined concept, but still, time since birth is generally a pretty useful definition.

When did galaxies form?

Two nearby galaxies. At the center is an elliptical galaxy,
which is basically a ball of old stars. A spiral galaxy, similar to
Milky Way,  appears in the upper right. Spiral galaxies are
still forming stars today. Do these galaxies have the same age?
This turns out to be a tricky question to answer.
Image from the Hubble Space Telescope.

When does the clock start for galaxies? When I was in graduate school, there was a concept of the epoch of galaxy formation, when big galaxies started forming. This idea was largely driven by the observation that nearby elliptical galaxies - and the central bulges of spiral galaxies - are filled with old stars that are not organized into thin, rotating disks.  It seemed likely that this was because galaxies formed their stars before the gas had a chance to settle into a thin disk.

If galaxies formed fast, then they should have been very bright when they were forming all those stars. So bright that once telescopes were equipped with modern CCD detectors in the late 1970's, it should have been possible to see them at large distances. No one could find them. Astronomers found lots of little faint blue galaxies, but these were smaller galaxies, that were much closer to us than expected.

At about the same time that observers weren't finding the epoch of galaxy formation, theorists were developing the idea that galaxies formed hierarchically, centered on the densest patches of dark matter in the early universe.  These early galaxies started forming their first stars when they had acquired only a tiny fraction of their present day material. They subsequently grew larger as more gas fell in at later times, and when they merged with other galaxies. This hierarchical theory simultaneously explains why we couldn't find the epoch of galaxy formation and why galaxies cluster together on large scales.

The implication is that galaxies started out with very few stars, and may have taken a long time to reach their peak star-formation rate. That's indeed what we infer today when we estimate the average number of stars formed per unit volume in the universe. This cosmic star-formation rate peaked when the universe was about 3.5 billion years old, even though we have now found galaxies that existed when the universe was less than a billion years old.

Inferring galaxy ages from colors

Even though the hierarchical models have been several decades and we have known for more than a decade that the globally-averaged cosmic star-formation rate started out much lower than the peak rate, it has taken a while for astronomers to realize that when they try to infer the age of an individual galaxy from its colors, the traditional assumptions are probabably incorrect. The traditional assumption has been that the star-formation starts off high and drops off exponentially. This was motivated by the idea that a galaxy starts with a fixed reservoir of gas and its star formation must keep dropping as that gas is used up. These are known as "tau models" in the jargon (not this kind of tau model!), because the Greek letter tau is usually used to designate the timescale for star-formation to reach roughly half of its initial peak.  Recent studies have shown that analyzing galaxy colors using tau models can give very misleading estimates of star-formation rates and ages, and that adopting different models can improve the estimates. See, for example, papers by my fellow bloggers Janine Pforr, Stijn Wuyts and my former student Joshua Lee.

Star-formation histories of galaxies from a hierarchical model (blue), compared to the best-fit tau models that were inferred from the galaxies' colors. You can see that the tau models are not at all representative of the true star-formation histories. But the problem is that the blue curves are models too. For real galaxies, we don't know what the true answer is. What we need is simple models that work well for a wider variety of possible star-forming histories than the tau models. From Lee et al. 2009.

 

A new definition of age?

 

Back when it seemed reasonable to use tau models, it seemed reasonable to designate the galaxy age as the elapsed time since it formed its first stars. This would be equivalent to galaxy birth. But now that we know that galaxies formed slowly, it's much less obvious what we should use as an age. On the one hand, the "chemical evolution" of a galaxy - the build-up of the heavy elements created in stars - is highly influenced by the first few stars. Once the gas in a galaxy contains heavy elements, it's ability to cool to form stars is dramatically altered. So we are interested in when galaxies formed their first stars. But on the other hand, most of the stars in a typical galaxy didn't form until billions of years later.

When using galaxy colors to estimate ages, the simplest approach is to estimate when the galaxy had formed half of its present stellar mass. This is probably the most reliable estimate of age that we can make if we have nothing but the galaxies' colors to work with, and no particular preconceptions about their star-formation histories.  But this is like starting the clock when the galaxy was a teenager. It's also a peculiar definition in the sense that a galaxy's age will not increase linearly with time. In fact, under this definition, if a galaxy doubles its stellar mass every 100 million years, then it will always be 100 million years old. If it then suddenly doubles its mass in only 10 million years, it will become only 10 million years old. This can be a bit confusing.

Another possibility is to adopt a different set of models that might be more representative of the true star-formation histories of galaxies than than the tau models. We can then define age as the time since t=0 in those models, as was done for the tau models. Unfortunately, there is currently no consensus on what to use as an alternative model. This is a topic of debate and discussion at galaxy evolution conferences. Maybe this will settle out and we will all agree on how to set our clocks. Or maybe the consensus will end up being that age is not a useful concept for galaxies. It's too soon to tell.

So the question "how old is this galaxy?" turns out to be more subtle than we might have thought. Scientific progress is a continual process of unveiling our ignorance. Each advance in our understanding leaves us still ignorant, but ignorant at a much deeper level.

Meet Steve Rodney

From time to time we'll bring you a biographical post introducing one of the astronomers writing for the CANDELS blog.  This week, we introduce Steve Rodney.  You can find his earlier posts here.

View of Hanauma Bay on Oahu.

Currently I'm a postdoctoral researcher and a Hubble fellow at the Johns Hopkins University (JHU) in Baltimore, Maryland. Before landing here on the shores of the Chesapeake, I was a graduate student at the University of Hawaii Institute for Astronomy. Several of my classmates from Hawaii are also part of the CANDELS team: Dale Kocevski, Liz McGrath, and Jeyhan Kartaltepe were all Hawaii grad students while I was there. We're now scattered across the country, but team meetings give us the occasional opportunity to reminisce about shave ice, moonlight surfing, and Andy's sandwich shop. 

Me and the kids in Baltimore.

JHU is a great place to be, and I am very lucky to be working here with some fantastic people… but unfortunately living in Baltimore is a source of some significant environmental stress for me. Not because of the drugs and crime of Baltimore (excellent dramatizations notwithstanding, Baltimore is actually a great city to live in). No, I'm referring to the deep emotional trauma that comes from growing up a fan of the hapless Cleveland Browns, and now watching my 4-year old daughter wearing her Ravens jersey to pre-school. (If you're not an NFL fan, then think of the Ravens vs Browns as Manchester United vs Liverpool, but imagine Liverpool is hopelessly inept over many decades. If you're not a soccer fan either, then... well, nevermind). I can only hope that my daughter will eventually outgrow this phase of moral depravity.

One of the conversations that will come up for most any astronomer from time to time is the discussion of "Why astronomy?" Sometimes this is about personal choices: "Why did you choose to become an astronomer instead of a chemist or a doctor?" For myself, astronomy has always been a study that I was drawn to because of the stories. As a kid, I read the legends of Greek mythology, and was fascinated to find them echoed on the sky in the constellations. I loved to trace the stories of ancient heroes, gods and monsters that were painted across the heavens. Later, the study of physics opened up for me the fascinating stories within the stars, from the birth of a star incubated in a dusty envelope of gas, to the fragile beauty of stellar death throes. Studying astronomy takes these wonderful images and unfolds them to reveal the complex puzzles and deep mysteries of the universe. For me personally, this was the hook that drew me in. 

Cherry blossoms and the Jefferson
Memorial in Washington, D.C.

Sometimes the "why astronomy" question is framed more broadly: "OK, maybe astronomy is great for you, but what practical use does it serve in our society?" A glance at recent Nobel Prizes can illustrate this very legitimate question. Astronomers won a Nobel prize in 2011 for discovering Dark Energy. Exotic and fascinating, but also intangible and wholly disconnected from everyday human lives. In contrast, the recently announced 2012 Nobel prize in chemistry recognizes advances in the understanding of G-protein-coupled receptors (GCPRs). This work is fundamentally connected to drugs used in the treatment of a wide range of ailments, from common allergies and high blood pressure to breast cancer and schizophrenia. One might look at these two fields and legitimately question whether we as a society should be investing so much (both in dollars and in human capital) in the study of distant stars, when there are real problems with real people that can be addressed with other avenues of scientific exploration. 

I think this is an important question, and a conversation that we astronomers and fans of astronomy should have more often and more publicly. Especially in a time of tight budgets for research funding and significant skepticism about the value of science in general, we all should have a coherent argument for why basic research is important. For me, the answer is that the pursuit of understanding is a fundamental quality of humanity. What separates us from other species on this planet is our ability to consider the universe, to seek a deeper understanding of how it works and what is our place within the grand cosmos. In a way, astronomy is a bit like poetry, art and music. It is a discipline that provides its own reward by enriching our lives. We astronomers should never forget that it is a special privilege to be able to devote ourselves to this task, and that we have a responsibility to share what we learn with the world around us. 

Sunrise over Maui, viewed from Makapu'u point on Oahu.

How to Measure the Star Formation Rates of Galaxies

One important question that astronomers try to understand is when in the history of the Universe were the stars in galaxies formed. In order to address this question one can either measure the amount of mass in stars in each galaxy and analyze this as a function of time or age of the Universe. Or one can measure the on-going rate of star formation as a function of the age of the Universe. In this post I would like to focus on star formation rates and how astronomers can measure them. 

The current rate at which a galaxy is forming stars describes just how many new stars there have been born in a limited time period. When stars (or a stellar population) form from a molecular cloud one can (theoretically) count the number of stars with a particular mass and do this for a range of masses, meaning one (theoretically) knows the distribution of stars as a function of stellar mass. This is called the stellar initial mass function. Within this newly formed population of stars the most massive stars are the hottest and burn through their fuel the quickest which means they will live the shortest lives. The hottest stars are also the brightest. So if we can measure how many of the brightest stars there are we can determine the total number of stars that formed using the initial mass function.

Astronomers measure star formation rates with the help of a variety of wavelength ranges from the X-ray to the radio. All of these so-called star formation indicators probe the most massive stars. 

Pillars of creation, a site of star formation in the Milky Way,
credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)

Young, massive stars shine particularly bright in the ultra-violet wavelength range of the electro-magnetic spectrum. Naturally, this is the first wavelength range to consider for a measurement on the number of massive stars. Unfortunately, because stars form within and from clouds of gas and dust, the light that they emit is at least partially absorbed by the gas and dust around them, we say the star light is attenuated. Consequently, any emission we still measure from the UV light, reflects only a portion of the stars that have been formed. 

Now there are 2 choices, one can either try to correct for the dust attenuation or try to find the "missing" parts of the UV light. Let's start with the first, correction for dust. Here, again, you have several options. First, you can measure the slope of the galaxy spectrum in the UV and compare it to the slope that one would theoretically expect a spectrum to have. The difference between these two values is the amount by which the spectrum was attenuated (reddened). On the other hand you can fit theoretical galaxy spectra to the entire spectral energy distribution of the galaxy and gain a value of the amount of dust that reddened the galaxy from the best fit. However, there are several reasons why a galaxy's spectral energy distribution can have this shape and dust is only one of them. You can check this previous post to learn more about this issue. Finally, you can measure the strength of spectral lines, in particular hydrogen recombination lines such as Hydrogen alpha and Hydrogen beta, and forbidden oxygen lines ([OII] and [OIII]). These spectral emission lines occur because the most massive and hot stars heat and ionize the gas in their vicinity. When the ionization states change, light at particular wavelengths is emitted, which we then observe as emission lines. This line emission is also affected by dust attenuation.

If you went down the other route, you would need to find the missing light. Well, this UV light was absorbed by the dust around the newly formed stars. Consequently, the dust heats up and then re-radiates the light in the infrared portion of the electro-magnetic spectrum. Clearly, this measurement only provides you with the part of the UV light that was absorbed, so you measure the bit that you missed in the UV. So the combination of the uncorrected UV and IR star formation rate measurement gives you the total star formation rate in the galaxy. 

Unfortunately, the measurement of the IR emission also has disadvantages. For example, dust can also be heated somewhat by evolved, older stellar populations. Furthermore, dust emits the reprocessed light at a range of wavelengths depending on the size of the dust grains. In order to account for all emission a wide wavelength range in the IR needs to be covered and if the spectral energy distribution is too sparsely sampled in this region some portion of the emission might be missed. Moreover, at high redshift we are only able to detect the most luminous IR galaxies, but not those that are more like the Milky Way because they are too faint in the infrared.

I also mentioned at the beginning that X-ray and radio emission can be used to determine star formation rates. However, these are more uncertain as active galactic nuclei often dominate the emission at these wavelengths, hence what we measure does not all come from stars.

Astronomers try to combine various measurements for star formation rates and try to cross-correlate and calibrate the different ways to measure them. However, depending on the redshift of the galaxies in question, not all ways of measurement are possible or even accessible to us. In future posts on CANDELS science you will find out which ways CANDELS members use to get a handle on the star formation rates and what they have learnt from the measurements.

Galaxy Zoo meets CANDELS

As we discussed in this previous post, classifying a galaxy into morphological categories can tell us a lot about its structure. We often want to have classifications of large numbers of galaxies in order to  compare various properties (such as color, mass, star formation rates, etc.) with morphology. However, visual classifications can be very time consuming and classifying large samples (thousands of galaxies or more) can be a daunting prospect for any individual. In 2007 two astronomers, Kevin Schawinski and Chris Lintott, had a unique idea for how to deal with this problem - involve the general public in classifying galaxies - and Galaxy Zoo was born.

For the original Galaxy Zoo project, over one hundred thousand volunteers signed up to classify nearly one million galaxies from the Sloan Digital Sky Survey (SDSS). These volunteers determined whether each of the galaxies was a spiral or an elliptical and if it was a spiral whether it was rotating clockwise, counter-clockwise, or viewed edge on. Galaxy mergers and image artifacts were also options that the classifiers could select. Not only did these citizen scientists quickly take to classifying galaxies, they had fun and learned a lot about galaxies in the process. The Galaxy Zoo webpage hosts a forum where volunteers can post about interesting objects they find and discuss their classifications. One of the exciting aspects of having all of these galaxies looked at individually was the ability to identify rare and unique objects that had not been seen before, such as Hanny's Voowerp. These classifications have provided an incredible data set for Galaxy Zoo scientists and a number of publications have resulted from this tremendous effort.

The Galaxy Zoo project was further expanded with the start of Galaxy Zoo 2, which included a much more detailed look at a subset of galaxies, and Galaxy Zoo Hubble, which asks volunteers to classify galaxies imaged with the Hubble Space Telescope in a number of deep fields. Last month, Galaxy Zoo relaunched in its latest incarnation and now includes reprocessed SDSS images along with HST images from CANDELS. These new images have been discussed in great detail on the Galaxy Zoo blog. This is a unique and exciting project for CANDELS because now galaxies at high redshift with near-infrared data will be classified alongside SDSS galaxies by many people to produce a fantastic data set of classifications.

A sampling of colorized CANDELS galaxies that are in the newly relaunched Galaxy Zoo

We worked together closely with the Galaxy Zoo team to produce images for the website. Astronomers are used to analyzing images taken with a specific filter, or one very narrow portion of the spectrum. As such, these images are scientifically very useful, but we must look at images taken in different filters in order to study various galaxy properties. The beautiful color astronomical images that you are probably used to seeing combine several of these filters together. Since CANDELS images are taken in the near-infrared, which is not visible to the human eye, visible colors are assigned to the different near-infrared filters. These images are thus false-colored, but these colors represent real physical properties. The pictures above highlight what some of these CANDELS galaxies look like in color as they are being classified by volunteers.

Since the success of Galaxy Zoo, a number of other Citizen Science projects have begun. Collectively, these projects are a part of the Zooniverse and include things such as finding planets around other stars, studying the surface of the moon, and investigating the history of the Earth's climate. There are a number of interesting projects that anyone out there can contribute to. We hope you explore some of these while you are exploring Galaxy Zoo and looking at CANDELS galaxies!

Astronomer of the Month: Nimish Hathi

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 Nimish Hathi.

Tell us a little about yourself!

My name is Nimish Hathi. I am a Postdoctoral Research Associate at Carnegie Observatories in Pasadena, CA. I am from India and have taken a long road to reach here. I did my Bachelor’s and Master’s in Physics/Electronics from Gujarat University in India. After my Masters, I went to Australia and attended the University of Queensland (UQ) in Brisbane for my second Masters, but this time in Astronomy. I spent a few months at the University of Western Australia in Perth before coming to the USA for my PhD. I completed my PhD at Arizona State University (ASU) in 2008.

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

My specific area of research is extragalactic astronomy. I study distant galaxies to understand their intrinsic properties and how these properties evolve with time. I am a co- investigator in the CANDELS team. I am involved in various working groups including galaxy morphologies, high redshift galaxies, and ultraviolet observations of lower redshift galaxies.

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

Challenge. Wow! While growing up in India, I did not have much exposure to astronomy and I got interested in astronomy only while taking postgraduate courses at UQ in Australia. So at a much older age than most other astronomers :-)

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

As an international student, I/we face different kinds of obstacles. These could range from language and communication to social environment to living away from families (I visit my family every ~3 years) to immigration issues (e.g., the process to get visas for study, work or permanent residency can be time consuming and very frustrating) to financial restrictions, etc. How do I overcome those? Using the `hang in there’ motto. Keep going day-by-day and learn to deal with (most of) them.

Who has been your biggest scientific role model and why? 

No one in specific but I always look up to Indian scientists (e.g., Ramanujan, C.V. Raman, Chandrasekhar) who have risen from humble origins to scientific glory.

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

Special, something different from normal (profession). I like what I do but it is also little frustrating at times when I think about the future. My favorite aspect is to see the reaction of the people (specially in India) when I say I am an astronomer because still it is not the obvious or common profession in India. It is mostly one of awe.

 

What motivates you in your research? 

In day-to-day research we try to explore and understand unknown, new things and that keeps me going. Even if we don’t solve or understand everything, we learn and develop new important skills.

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

Considering my very limited ground-based observing experience, I have to say the Magellan Telescopes in Chile.

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

In the future, I would love to continue doing astronomy research, but I am also open to teaching and other job options. I will know soon when I start applying for a new job later this year.

If you could have any astronomy related wish, what would it be? 

Two wishes:-) 

I wish for better funding for current and future astronomy projects to continue our excellent research and make way for new discoveries.
 
I wish for more opportunities for permanent jobs to attract and retain young astronomers.

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

The observational breakthroughs we have made in the last 10-20 years e.g., evidence for the big bang, existence of black holes, searching highest redshift galaxies, finding planets around other stars and many more.

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

I think the general public should know that astronomers not only gaze at the night sky with curiosity but they also develop techniques and technology which can be used in many other fields.

Enjoy the night sky :-)