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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thinking about galaxy formation is now outdated, as I will describe below, there is evidence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
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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
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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
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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
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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
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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
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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).
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