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Conselice: Galaxy evolution A new view of S galaxy evolution Christopher Conselice looks back at galaxies in the early universe and, thanks to technological advances such as the Hubble Space Telescope, can compare their evolution with the development of galaxies in the local universe today. 1: M81, a spiral galaxy similar to the Milky Way. The spiral arms comprise young, bluish, hot stars formed in the past few hundred million years, while the central bulge contains older, redder stars. (NASA, ESA and The Hubble Heritage Team [STScI/AURA]) ome of the oldest and most profound questions humans have ever asked include: “Where do we come from?” and “How did the universe we see around us originate?” The idea of what defines the universe has constantly changed through time, and until the early 20th century the universe was thought by astronomers to consist of what we now call the Milky Way galaxy. However, since then we have learned that the universe is a universe of galaxies, and that galaxies in a practical and real sense define our universe. Understanding galaxies is therefore the way in which we connect the universe as a whole to our own existence, because we of course live in a galaxy ourselves. When galaxies were first recognized as distinct objects they were used as cosmological probes, either through their brightness or from the brightness of stars within them. It was only later that the study of galaxy evolution began, by trying to answer the questions of how galaxies first formed and how they have evolved through time (see, for example, Spinrad 2006). Originally, it was thought that galaxies, like our own Milky Way, formed in a similar way to stars – by a gravitational collapse of gas early in the universe’s history. Turning this collapsing gas into stars over a rapid period of time was thought to be the method in which galaxies form (Eggen et al. 1962). While this mode of thinking about galaxy formation is now outdated, as I will describe below, there is evidence that this process is potentially partially correct. Taking our own galaxy, we see stars and features which can indeed fit this “monolithic” collapse model, and any idea about galaxy formation must account for the fact that there are old stars in every nearby galaxy that has been studied in detail. Part of the reasoning behind the idea that galaxies form in a collapse is due to the fact that in the nearby universe nearly all galaxies have a shape or morphology which is either a disc galaxy or an elliptical (figures 1 and 2), both of which are relatively stable structures and therefore could have been in their present form for a very long time, on the order of billions of years. Hubble looks deep 2: The immense elliptical galaxy M87, showing a smooth, yellow population of older stars and a myriad of star-like globular clusters. A population of old stars is a characteristic of this type of galaxy, with many of them formed more than 10 billion years ago. (NASA, ESA and the Hubble Heritage Team [STScI/AURA]) A&G • August 2011 • Vol. 52 The Hubble Space Telescope (HST) challenged this view when it began to take images of the distant universe in the mid-1990s. When HST first took deep images of distant galaxies it was obvious that fainter, and presumably more distant galaxies, look and are quite different from the galaxies we see in today’s universe. Understanding what this means, and how we can understand the physics of galaxy formation from these objects, have been major goals in astronomy for the past 15 years. These galaxies, as imaged by the HST, appear 4.31 Conselice: Galaxy evolution to be very distorted and irregular in appearance. It took some time, however, before the distances to these galaxies were measured using their spectra – showing that these distorted galaxies were much more common in the past than they are today. However, a large fraction of what we know about the formation of galaxies we see in the modern universe originates from observations of these distant galaxies. A major aspect of this understanding comes from interpreting the distribution of the stars in these distant galaxies. This article recounts how this understanding of galaxy structure reveals the way in which galaxies in the nearby universe formed. astronomy. However, because of the effects of the expanding universe, the light from these galaxies is redshifted, and thus what we are actually observing from these distant galaxies is their rest-frame ultraviolet light. The problem with this is that the ultraviolet region of their spectrum is dominated by light coming from young, bright, massive stars that make up only a small fraction of the mass of galaxies. And what we know about galaxies in the nearby universe originates mostly from observations at the rest-frame optical light where the mass of a galaxy is better represented. Because of the redshifting of galaxy light it is very difficult to study galaxies in the distant universe in How do we find the first galaxies? the same way we do nearby galaxies. This redshift, however, allows us to identify In all of astronomy, and in science as a whole for that matter, understanding the biases and distant galaxies not only due to the shifting of errors in our methods are critical for making spectral features, most commonly emission progress. Often we can be fooled into believ- lines from Lyman-alpha at 1216 Å, but also ing we are observing one thing, but are in fact using features such as the Lyman-break. This observing something completely different. This is a break – an extreme drop in flux at waveis particularly the case when trying to study lengths bluer than a particular limit – in the the first galaxies in the universe – because the spectra of distant galaxies such that at wavefirst step in studying these systems is simply to lengths lower than 912 Å, light from the galaxy locate them reliably. is completely absorbed by hydrogen gas. The This is particularly important when studying reason for this is that the amount of energy distant galaxies, simply because distant galax- associated with photons at wavelengths less ies are fainter than nearby galaxies, and thus it than 912 Å is enough to ionize hydrogen. Thus, is possible, perhaps even likely, that we this light is easily absorbed by the copious are missing a significant fraction of amount of gas within the galaxy and the population of galaxies in the within the intergalactic medium Faint distant universe. between these distant galaxies galaxies are not The reason for this is simple: spirals or ellipticals, and our own. By searching for when we examine distant galthis break, we can find candibut look as if axies, those that we are able to date distant galaxies through they are ‘traindetect are not representative of imaging – we look for galaxies wrecks’ the ancestors of the galaxies we that are bright at one filter, but find in the local universe, but are that essentially disappear using just the brightest few that are there. a filter just a bit bluer. This is the These galaxies are very bright and thus traditional way to find distant galaxies easy to see because they are undergoing intense and is being used today to find the most distant star formation, a common process in the early galaxies in the universe. universe. However, there are also galaxies that We are also now able to measure the amount are more “quiescent” or “passive” – not actively of mass in a galaxy which is in the form of forming stars – and these galaxies are much stars – called the stellar mass – and this has harder to find. Astronomers are still not sure if become the standard way to trace galaxy evoluthey have found all of these passive systems in tion through time. The reason for this is that a the early universe. The reason that this is impor- galaxy can not easily lose stellar mass – it can tant is that we need a way to connect the galax- gain stellar mass through various processes, but ies that we see in the distant universe with those it cannot lower its stellar mass over time. The that we see today, which requires a “complete” stellar mass is also the luminous mass because it is the stars that create the light that we see. sample of galaxies at high redshift. One way to find galaxies is to examine the The relationship between the luminosity of a amount of light, or luminosity, originating from galaxy and its brightness depends on the starthese early galaxies, and compare it to the distri- formation history of a galaxy. Galaxies with bution today. Ideally this would work, but there recent star formation have copious bright, but are a few things that go against this approach short lived, O and B stars, which give the galfor connecting galaxies at different distances axy a large luminosity for a given amount of using the amount of light they emit. One issue is mass. Older galaxies will not have these bright that until recently we had no choice but to view stars, as they die after a relatively short period distant galaxies in the observed optical light of time – a few hundred million years – and thus – the traditional wavelength of observational these galaxies will have a lower luminosity per ‘‘ ’’ 4.32 unit stellar mass than a galaxy with a recent star-formation event. Galaxies furthermore have two other types of mass: the amount of mass in gas, such as hydrogen, which can form into stars, and therefore can increase or decrease with time; and the dark matter mass. The dark matter mass can be up to ten times the stellar mass, based on observations in the nearby universe. These two types of masses – gaseous and dark – are, however, difficult to measure over time, unlike stellar mass, which can be measured for even the most distant galaxies. As mentioned above, because distant galaxies are faint, it is hard to study them in any detail. But we can study the most massive galaxies because these systems are much brighter than galaxies of the same mass today, as a result of their having younger stellar populations; thus they are also easier to find. In addition, technology allows us to examine these systems in the same way we can study nearby galaxies. Much of this advance is due to the HST, as well as to the large 8–10 m telescopes on the ground, that allow us to study distant galaxies in the kind of detail needed to determine how galaxy evolution occurred. Galaxy formation and the HST In the 20 years the HST has been active, it has revolutionized almost all areas of astronomy. Perhaps the area where it has had its most profound impact is within the study of distant galaxies. The reason for this is that when observed from the ground, only the closest and largest galaxies are resolved. That is, we can study the internal structures of these systems for only the nearest galaxies using ground-based telescopes. The reason for this is that the blurring of light resulting from passage through the Earth’s atmosphere puts a natural limit on the resolution of distant objects. Resolving galaxies is important if you want to study anything about the system beyond simply examining the amount of light coming from it. For example, rotation curves, morphology, structures such as bars, star-forming regions, and even individual stars can only be found in galaxies that are resolved. Hubble allows us to resolve distant galaxies to a level which allows us to compare them with nearby galaxies as imaged with ground-based telescopes. The smallest distant galaxies are just at the resolution limit of the cameras available on the HST. Between 1994 and 1997 the HST revealed that distant galaxies were very different from nearby galaxies. A major aspect of these studies was due to campaigns of deep Hubble imaging, now known as the “Hubble Deep Field” and later the “Hubble Ultra Deep Field” (figure 3a). These were several imaging surveys taken with the Wide-Field Camera-2 (WFC2) and the Advance Camera for Surveys (ACS) of single A&G • August 2011 • Vol. 52 Conselice: Galaxy evolution (a) (b) 3 (a): The Hubble Ultra Deep Field, taken by the Advanced Camera for Surveys on the Hubble telescope, represents a narrow, deep view of the cosmos. The smallest, bluest galaxies existed in the first billion years of the universe. The nearest galaxies – the larger, brighter, well-defined spirals and ellipticals – thrived in the past six billion years or so. In this image, blue and green correspond to visible light, for example in hot, young, blue stars and Sun-like stars in the discs of galaxies. Red represents near-infrared light, such as the glow of galaxies composed of old stars or those shrouded in dust. (b) A close up of some of the complex, irregular “train-wreck” galaxies captured in the image. (NASA, ESA, S Beckwith [STScI] and the HUDF Team) pointings over the course of many days. What these imaging studies showed was that structurally, faint galaxies looked “peculiar”. They are not spirals or ellipticals, but look as if they are “train-wrecks” or galaxies that were in some obvious formation mode (figure 3b). This formation clearly produces the distorted structures we see, which gravitationally are not in equilibrium. When we measure the amount of mass in the form of stars in these distant galaxies, we find that they contain a similar amount of stellar mass as the most massive galaxies in today’s universe. Because galaxies can only gain mass, and not lose it over time, these distant galaxies must somehow evolve into the massive galaxies that we see in today’s universe, which are mostly elliptical in appearance. Evolving these distorted galaxies into the smooth ellipticals we see today is possible simply as a result of gravitational relaxation over a relatively quick timescale. However, the question remains as to how and why these galaxies look the way they do in the distant universe; specifically, we need to understand how they got distorted in the first place. Furthermore, kinematic studies of these galaxies show that they often do not have an ordered A&G • August 2011 • Vol. 52 internal motion similar to nearby disc/ellipticals – further evidence for their violent origin. There are other clues we must consider. One is that these galaxies are undergoing intense star formation with on the order of a few hundred solar masses of new stars formed per year, compared with just a few solar masses each year forming in our own galaxy today. Galaxies need to contain stars for us to see them, and these stars must form at some point. We now know that this process does not happen quickly in the early universe, but is extended over time. In fact, observations show that active star formation spans the history of the universe and is still taking place in many galaxies today. The star-formation history is such that it ramps up from the start of the universe and peaks at around 2–3 billion years after the Big Bang, then gradually declines. Today the star-formation rate is roughly a tenth of what it was at its peak. Understanding why the star-formation rate is so high in the early universe is an important question – as is why it later declines. Another major clue, still not totally understood, is that galaxies in the distant universe are much smaller, by up to an order of a magnitude, and have mass densities around 50 times higher, than similar mass galaxies we see today. Furthermore, on average, the masses of these early galaxies are also smaller than galaxies of similar luminosity in today’s universe. These are two further observables that show galaxies are evolving significantly over time. Is all this evolution produced by one or many processes? Are they interrelated, or are they independently produced? These are questions astronomers are still trying to understand. However, all of this observational evidence does not allow us to determine exactly how galaxy formation occurs. We can see galaxy formation in progress in terms of stars building up and an increase in size and mass, but these do not reveal the mechanisms by which galaxies are forming. In fact, galaxies can build up their stellar mass in several ways which can relate to the initial formation, or to subsequent formation episodes. One of the major processes that theorists believe drive the formation of galaxies is the merging of two existing galaxies. This is a natural prediction which relates to the structure and make up of the universe and especially to the nature of dark matter. As is well known, astronomers now believe that most of the material in 4.33 Conselice: Galaxy evolution the universe is in a “dark” form that cannot be period. The critical aspect of this, however, is detected directly. The nature of this dark mat- that very little to no additional mass is added to ter, however, has a direct and perhaps primary a galaxy once it forms in the very early universe. role in the formation history of galaxies. This was the dominant idea for how galaxies This dark matter, furthermore, is thought to formed up until the 1980s. make up the bulk of the matter within galaxies. The other idea, based on Cold Dark Matter This has many implications beyond simply the cosmology, is that galaxies form through the idea that the light we see from galaxies is just a mergers explained above. However, finding small part of the mass in these systems. The rea- evidence for this formation is very difficult and son is that the nature of the dark matter particle only with the HST were astronomers able to determines how galaxies will form. Simply put, show that galaxies indeed gain a large fraction if the dark matter particle were moving quickly of their mass through the merger process. at near the speed of light, such as a neutrino, A primary method is through examining then the first structures in the universe would the structures of galaxies and how they have be large. These large structures would then con- evolved through time. As mentioned above, dense to form smaller systems as the universe early Hubble observations showed that galaxies aged. This would imply that structures such as were very distorted compared with today. Disgalaxy clusters would form first, the most mas- torted structures can, however, arise from sevsive galaxies would form next, and then small eral causes. Clumpy star formation can produce galaxies would form last. structures like the ones seen in distant galaxies, However, because there is significant evidence as can the smashing together of already existing that dark matter is cold, meaning the dark mat- galaxies (figure 4). Uncovering which processes ter particle is moving slowly with respect to the are occurring to produce the observed distorted speed of light, then the first structures would be galaxies will reveal how galaxies are forming. There is, however, one key difference: within small and the evolution of the universe would occur such that large structures build up from mergers, the older stellar populations are in the merging of these smaller ones. That is, the a non-equilibrium distribution, as well as the universe’s history is one of building up larger younger stars that may be forming. For mature and larger galaxies and galaxy clusters through galaxies with ongoing star formation, the older the assembly of already existing systems. So the stellar populations are not distorted but symfirst galaxies should be dominated by smaller, metrical, as equilibrium in a stellar system is lower mass systems compared with galreached relatively quickly. axies in today’s universe. Starting in 1995, I and others A developed tools to determine During the early days of examtypical massive ining galaxies at high redshift whether a galaxy is undergoing with the HST, there was some galaxy has had four a major merger based on its circumstantial evidence that to five major mergers structure. The primary way we did this is through using the picture of “bottom-up” in the past 10 billion formation was in fact correct. the distribution of light within years A hierarchical formation histhese distant galaxies resolved tory not only predicts that the by Hubble, and analysing them earliest galaxies have typically low in a quantitative way. That is, we masses, but also that the most massive were among the first to examine the structures in the universe – galaxy clusters – are quantitative structures of galaxies rather than not built up until late in the universe’s history. use visual estimates of morphology as had been This is observed: massive galaxy clusters are done for decades previously. very rarely seen before the universe was half The main method that is used to do this is its current age. Before this time, there are very through indices which measure and quantify few candidate galaxy clusters, and within the the structures of galaxies. One of these indices first few billion years of the universe’s his- is the asymmetry index, which is a measure of tory there are no known structures as large as how asymmetric a galaxy is. It is calculated by gravitationally bound galaxy clusters. This all rotating a galaxy by 180° from its centre and demonstrates a bottom-up formation history for then subtracting this rotated image from the large structures in the universe. However, find- original image. The residuals of this process ing direct evidence for this process in forming reveal whether the galaxy has any asymmetthe galaxies themselves is more difficult. ric light and, most importantly, quantifies how much. Galaxies with a high residual asymmetry Dept of Mergers and Acquisitions are typically galaxies undergoing mergers (ConThis leads to the question of how galaxies form. selice 2003). Thus this method can be used, There are currently two major ideas. The first is along with similar parameters, to determine in simply that galaxies form like stars – through a quantitative way the history of galaxy merging a collapse of gas. This gas collapses and is con- within the universe. verted into stars over some very rapid timeThis new process allows us to quantify the ‘‘ ’’ 4.34 structural properties of galaxies, and it was shown that these structural properties correlate well with physical properties of the galaxies themselves, such as the mass, the star formation rate, and the degree to which assembly is going on through the merger process. One of our primary goals was to use our new system of measuring galaxy structure to determine the merger history of galaxies. To calibrate this system we examined nearby merging galaxies as well as simulations of the merger process. From this, we were able to find a region of galaxy structure parameter space which uniquely identified galaxies undergoing merging. However, using these structural parameters requires that we have the right kind of data with which to analyse distant galaxies. As mentioned above, galaxies can look very different, particularly distant ones, when examining their structures in “rest-frame” optical and ultraviolet light. So what we need is an instrument that allows us to probe the rest-frame optical light originating from these distant systems. New instrument This was possible due to a new near-infrared instrument on the HST – the Near Infrared Camera and Multi-Object Spectrograph (NICMOS) – which allowed astronomers for the first time to examine galaxies in the first half of the universe in terms of the bulk of their stellar mass, as opposed to simply examining the location of their star formation as revealed by the ultraviolet. Using this camera to take deep exposures of the distant universe, we were able to examine forming galaxies in terms of their bulk structure for the first time. What we found was quite surprising – a large fraction of massive galaxies, similar to the mass of the Milky Way, are undergoing mergers a few billion years after the Big Bang. For the most massive galaxies, the fraction is as high as 30%. In comparison, the fraction of similar mass galaxies today which are undergoing a merger is around 1%. Between the peak merger fraction, which occurs a few billion years after the Big Bang, and today, we can clearly see the merger history decline, and decline quite rapidly for these massive galaxies. What does this mean? If we know that half of all massive galaxies are merging, this tells us that merging is important in the galaxy formation process, but it does not tell us the total role of merging in creating galaxies. To understand this, we need to use the fraction of galaxies merging at a snapshot of time to derive the merger rate – that is, how often galaxies of a given mass merge through time. This requires that we understand the timescale for the mergers we see. To understand the importance of these time scales, imagine that the mergers we see occur very quickly. Then the fact that we see roughly half of all galaxies at a given time merging would A&G • August 2011 • Vol. 52 Conselice: Galaxy evolution 4: An assortment of colliding galaxies at various stages. In the nearby universe around one in every 100 galaxies is in the act of colliding, but mergers were more common long ago when galaxies were closer together. (NASA, ESA, the Hubble Heritage [STScI/AURA]-ESA/Hubble Collaboration, and A Evans [Univ. of Virginia, Charlottesville/NRAO/Stony Brook Univ.]) imply that mergers were very common. However, if the timescale for these mergers was long – say on the scale of the age of the universe – then the systems we see merging would remain in the same merger for a very long time and merging overall would not be a major process in galaxy formation. The merger fraction and process needs to be normalised by this timescale to understand the role of merging in forming galaxies. What we were able to determine from both simulations of mergers, and from the mergers themselves, is that the timescale for merging is roughly half a billion years, on average. This timescale then lets us determine the role of merging within galaxy formation. From this, we are able to show that a typical massive galaxy has experienced roughly four to five major mergers in the past 10 billion years. This will increase the masses of galaxies we see a few billion years after the Big Bang by several times their original mass. While this in itself is interesting, it is easy to show that this merging will allow the small, lower mass systems we see in the early universe to become the more massive systems that we A&G • August 2011 • Vol. 52 see today, i.e. the bulk of galaxy formation for some galaxies is occurring through the merger process. This merger process is also potentially the trigger for the intense star formation seen in the early universe, as well as driving gas into the centres of galaxies inducing the formation of the central black holes in these systems. While mergers can account for the bulk of galaxy formation for some massive galaxies late in their history, there are still galaxy formation modes that we are just now starting to investigate. These include the accretion onto galaxies of gas which is later converted into stars, as well as more minor mergers which can also build up galactic masses. Understanding the relative role of these processes, as well as observing the very first galaxies, will be the focus of galaxy studies over the next ten years using new telescopes and instrumentation, many of which are in the planning stages. One important project that will address the issue of galaxy formation and merging is the CANDELS survey being carried out with the new near-infrared camera on the HST, known as the Wide-Field Camera-3 (WFC3). This is a sig- nificant improvement over the NICMOS camera used in the earliest work on distant galaxies. Furthermore, ground-based adaptive optics are now becoming an alternative and powerful way to examine the structures of distant galaxies, and this will continue into the future. Looking further ahead, missions such as Euclid and WFIRST will perform near-infrared imaging across large portions of the sky and for the first time large amounts of resolved imaging will be available for examining galaxy evolution processes. The James Webb Space Telescope will furthermore allow us to examine the first galaxies in the universe, and ultimately we may be able to examine the formation mechanisms for the very first galaxies in just a few years. ● Christopher J Conselice is Professor of Astrophysics at the University of Nottingham, UK. [email protected] References Conselice C 2003 Ap. J. Supp 147 1. Eggen O J et al. 1962 Ap. J. 136 748. Spinrad H 2006 Galaxy Formation and Evolution (Springer). 4.35