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Galaxy evolution
The story of galaxy
evolution in full colour
Richard S Ellis, Roberto G
Abraham, Jarle Brinchmann and
Felipe Menanteau show how
modern telescopes producing
colourful images are pointing
the way to understanding
galaxy evolution.
E
dwin Hubble was a perceptive observer
who introduced the morphological classification scheme for galaxies still in use
today. His “tuning fork” diagram (figure 1)
differentiates ellipticals from unbarred and
barred spirals, further classified according to
the tightness and fine structure of their spiral
arms which appears to correlate with the fraction of light present in the central bulge. Intermediate to the spirals and ellipticals, Hubble
also introduced the enigmatic lenticular or S0
class. These share the disk-like structure of spirals with an inner nuclear bulge, but are devoid
of spiral arms and share the smoothness and
colours of the elliptical population. Over 90%
of nearby luminous galaxies can be categorized
within Hubble’s scheme, described in more
detail by van den Bergh (1998).
Whilst it is natural to attempt to classify
galaxies according to their appearance, does
such a scheme offer any insight into the physical origin of the different types? Hubble evidently thought so: he referred to the Sc through
Sdm types as “late type galaxies” and the
spheroidals (a collective term for the ellipticals
and S0s) as “early types”, imagining an evolutionary sequence along the tuning fork. Indeed,
the integrated colours of galaxies of different
types show a strong trend that can be
explained in terms of their current star formation rate. Blue late-type galaxies can be understood as ones that are particularly active in
forming stars, whereas the redder early types
are more quiescent. However, this inference
need not necessarily imply that one type
evolves into the other. Rather the sequence
might represent the extent to which the star
formation declines from its initial value. Beatrice Tinsley, a pioneer of galaxy evolution
models in the 1970s, analysed the optical-
2.10
Holy Grail of modern astronomy is understanding the origin of Edwin Hubble’s
morphological sequence of galaxy types. What made some collapsing gas
clouds turn into elegant spiral systems like our own Milky Way, whereas others
became smooth, featureless ellipticals? More fundamentally, does the taxonomic
scheme introduced by Hubble in the 1920s have any physical relevance? The
Space Telescope that bears Hubble’s name is providing answers to these and
other questions in the context of modern theories of structure formation.
A
infrared colours of a sample of local galaxies
and argued that continuously star-forming spirals would look much the same in the past,
whereas the presently red ellipticals would,
closer to their moment of birth, have been
intensely luminous and blue, possibly
detectable at large redshifts as young primaeval galaxies. By peering into the past,
astronomers might be able to directly test Tinsley’s evolutionary predictions and, most importantly, find examples of primaeval galaxies.
Structure formation and dark matter
Since the 1930s there had been suspicions that
a large fraction of the matter in the universe is
not in stellar or gaseous form. By the late
1970s and early 1980s, evidence for dark matter became overwhelming. Estimates of the
gravitating mass in various regions could be
contrasted with that responsible for the luminous component. Together with constraints on
the amount of dark matter that could be made
of baryons (protons, neutrons and related particles) from the abundances of light elements
formed in the Big Bang, it became clear that
much of the dark matter must be of a different,
non-baryonic form. (The dark matter picture is
described in more detail than below by Baugh
et al. 1996, Cole et al. 1994, Frenk et al. 1985
and White and Rees 1978.)
This deduction transformed our view of how
galaxies form. In order to explain the observed
large-scale structure and abundance of galaxies,
the cold dark-matter model posits that each visible galaxy is embedded in a much larger darkmatter halo of non-baryonic material, which
detached from the expanding cosmic plasma at
early times. The gravitational merging of these
halos drives the growth of structure and the
rate of cooling of hydrogen gas drawn into the
merging halos governs the history of assembly
and star formation in normal galaxies.
The dark-matter picture can successfully
reproduce both the degree of fluctuations
observed in the microwave background and
correlations found in the local galaxy distribution. It predicts that dark-matter halos assemble hierarchically from smaller examples
according to their mutual gravitational attraction. However, in describing how galaxies
assemble, we are primarily concerned with the
baryonic component and the rate at which
pristine hydrogen gas converts to stars as well
as, ultimately, with dynamical and structural
details which would enable us to understand
and differentiate spirals from ellipticals and
bulges from spiral arms which themselves
appear in barred and unbarred form.
Although promising progress has been made
in simulating how galaxies might assemble in
the dark-matter picture, via calculations of
how gas cools and forms stars in assembling
dark-matter halos, we are a long way from a
detailed prediction of how Hubble’s sequence
emerged. For this reason we proceed along
phenomenological routes. One of the most
important issues we might hope to understand
is whether the morphologies we observe today
represent those established at birth, or whether
transformations can occur. In the former case,
galaxy morphologies would bear important
clues to physical conditions at the epoch of formation (as well, perhaps, as pinpointing that
epoch) whereas in the latter case, the abundances of the various types would constrain the
history of the transformation processes.
Before the development of the dark-matter
picture, most cosmologists believed that at least
some galaxies (e.g. the presently quiescent ellipticals) formed via a rapid “monolithic” collapse
April 2000 Vol 41
Galaxy evolution
(c)
Sa
SO1
SO2
SO3
Sb
0.0
Sc
SO3/Sa
(a)
initial parabolic
trajectories
E0
E3
E5
E7/SO1
1.5
SBO1/2
(b)
classical
SBO2/3
SBO3/SBa
SBa
SBb
SBc
hierarchical
merging gas
clouds
gas in
merging darkmatter halos
1.87
monolithic
collapse,
cooling and
star formation
slow
collapse,
cooling
governed by
feedback
3.00
feedback
removes
remaining gas
early disk
systems
spheroidal
galaxy
later merging
produces
spheroidals
spiral
at high redshift. Gas turning promptly into stars
at the time of collapse would explain many of
their observed properties. The high central stellar densities in local ellipticals would reflect the
mass density of the universe at those early
times, particularly if the protogalactic cloud collapsed by only a small amount. A modest collapse factor would also be consistent with the
small angular rotation observed; the spin-up
during collapse would be slight. By contrast,
spiral galaxies might represent systems that collapsed by significant amounts, with star formaApril 2000 Vol 41
tion delayed until the gas clouds had collided
many times and settled into a disk. Small
amounts of initial angular momentum in the
protogalaxy would amplify rotation during collapse, leading to rapidly rotating stellar systems
(figure 1). The key factors differentiating the
two classes are the collapse factor and the
degree to which energy released during collapse
is dissipated through gas cloud collisions. For
ellipticals, there is no dissipation. Stars form in
situ at the time of collapse, producing galaxies
with smooth shapes. For spirals, significant dis-
1a: The Hubble Sequence in the form of a
“tuning fork”, according to Hubble’s original
scheme. Spheroidal galaxies lie in the stem
of the fork and spirals and barred spirals in
each prong. 1b: In the classical picture,
galaxies evolve in isolation, retaining their
morphologies from birth. The hierarchical
picture supposes that galaxies assemble
gradually within merging dark matter halos.
1c: Numerical simulations by Joshua Barnes
(Univ. Hawaii) and others suggest ellipticals
might also form when gas-rich spirals
merge. The proportions of galaxies of
different morphologies at large look-back
times test the two contrasting pictures.
sipation implies large collapse factors, rapidlyrotating products and disk-like stellar distributions.
A central question is whether galaxies form
and evolve in isolation. Unlike stars in our own
galaxy, individual galaxies have large crosssections compared to their typical separations
and, consequently, collisions are expected to
occur. At early times, the number of galaxies
per unit volume was higher and so we can
expect mergers to have been more frequent.
Numerical simulations suggest dramatic
2.11
Galaxy evolution
changes can occur in such circumstances. Take
a collision of two spiral galaxies as an example. In such a case, the delicate stellar disks are
destroyed, the circular stellar orbits are randomized and the hydrogen gas is either
expelled or converted into stars, leaving a
smooth elliptical remnant (figure 1). Close
examination of some (but not all) ellipticals
reveals tell-tale remnants of recent merging. In
order to prevent wholesale destruction of spirals by merging, dark-matter enthusiasts have
suggested that large galaxies must have assembled quite recently so that many ellipticals are,
in fact, the merger product of two spirals.
The rate of merging expected in the darkmatter models is sensitive to the details of the
cosmological model. If, for example, we admit
the presence of a non-zero cosmological constant as suggested by recent supernovae data,
then the rate of merging expected within the
redshift range probed by HST is modest. But if
we live in a high density universe with no cosmological constant, a great deal of recent
merging is expected.
In summary, the key difference between the
traditional and dark-matter pictures for the
origin of galaxy morphologies lies in the extent
to which galaxies may have interacted during
their history, as well as the extent to which it
may be possible to have early “monolithic”
collapse of large systems as opposed to more
gradual, hierarchical assembly.
Studies of galaxy evolution using HST
Following the insertion of correcting optics in
the instrumentation on board Hubble Space
Telescope (HST), it became clear to our group
at the Institute of Astronomy at Cambridge that
we could directly witness the history of galaxy
morphology by exploiting deep HST images
spanning a substantial range in look-back time.
Prior to HST, attempts had been made from the
best ground-based observatories to resolve and
classify distant galaxies but, even in the best
conditions, galaxies as close as redshifts of
0.3–0.5 (corresponding to “look-back times” of
3–5 billion years in a universe 12–15 billion
years old) were mostly blurs of light (figure 2).
In contrast, the best HST images permit reliable classifications to be made to epochs corresponding to redshifts of 1–2, some 60–70% of
the way back to the Big Bang. Early work with
HST concentrated on randomly chosen small
fields observed with the Wide Field Planetary
Camera 2 (WFPC-2) in parallel, while others
used primary instruments to observe specific
targets. A collection of about 100 such deep
images comprised the Medium Deep Survey
(http://www.stsci.edu), one of the early Key Projects of the telescope. The disadvantage of this
dataset was the absence of comprehensive redshift data for the fields in question. The field of
view of WFPC-2 is poorly matched to ground2.12
based multi-object spectrographs and thus a
second project, undertaken jointly with colleagues in France, Canada and Australia, consisted of producing a mosaic of WFPC-2 images
of redshift survey fields. A substantial boost
was later provided by the colour exposures
taken as part of the two Hubble Deep Field
campaigns (figure 2, http://www.stsci.edu).
Comparing morphological distributions
observed at different eras or faintness limits is
fraught with possible pitfalls. Faint, distant
galaxies are redshifted so that images taken in
a given photometric band refer to ones viewed
at shorter rest wavelengths where the light distribution in galaxies is governed by younger
stars which clump in associations. So a galaxy
may appear more patchy or irregular at faint
magnitudes merely as a result of this effect.
Likewise, lower surface brightness features
such as spiral disks will rapidly be dimmed
beyond detection at great distances and so certain classes of galaxies may appear more concentrated and dominated by their nuclear
regions than would otherwise be the case. At
first it was thought these effects would be insuperable, but in many fields HST has observed
the same galaxies in different wavelengths so
that the effects can be calibrated. Moreover, an
industry of detailed numerical simulations has
been developed, whereby multicolour images
of local galaxies are reconstructed so as to
mimic their appearance with HST at high redshift. To the accuracy required to compare the
morphological distributions with predictions,
the corrections for these effects can be adequately controlled (figure 2).
What then are the key questions that HST
data should address in attempting to piece
together a coherent picture for the origin of the
Hubble sequence of galaxy types?
The faint blue galaxy problem
A long-standing puzzle in extragalactic astronomy has been the origin of the excess blue,
star-forming galaxies seen in deep images.
Since the late 1970s it has been known that
there are many more such systems than expected from our knowledge of the locally observed
population. One of the first results from the
Cambridge HST surveys was the identification
of a strong increase with redshift in the fraction of morphologically peculiar galaxies. The
integrated luminosity density of such systems
appears to account for almost all of the rise
with redshift seen in deep surveys (figure 3).
What can have happened to these star-forming
peculiars so that they were so numerous 5 billion years ago but virtually absent by the present
epoch? Two hypotheses are popular. The first
suggests that the peculiars are transformed via
mergers or by other means into regular ellipticals and spirals. This “recent-merger” view is
consistent with the predictions of hierarchical
models. The second and more interesting idea is
that the distant irregulars are being seen during
an unusually active period in their activity, perhaps even at the moment of their formation;
starved of further infalling gas, these galaxies
subsequently fade to low surface brightness systems which are difficult to detect today.
As discussed above, initially there was some
concern that the abundant population of faint
peculiar galaxies represented misclassified spirals or spirals rendered peculiar by a short-term
burst of star formation. Infrared data recently
acquired with UKIRT and the HST NICMOS
camera demonstrates that these optically peculiar systems appear irregular at longer wavelengths as well and, importantly, that their
established stellar mass is quite significant.
Although blue light could fade quite quickly if
the star formation rate stopped abruptly, their
infrared luminosities would hardly change.
Inevitably therefore, the intermediate redshift
irregulars must be represented in the census of
infrared-luminous galaxies which today are
dominated by regular systems. It seems, therefore, that this remarkable population must have
transformed into spirals and ellipticals.
Mergers and field ellipticals
The growing evidence that mergers shaped
some fraction of the Hubble sequence raises
the question of whether we can quantify the
rate of merging observed at faint limits and
reconcile this with that expected in hierarchical
pictures. Together with our colleagues in
France, Canada and Australia, the Cambridge
team has examined many deep fields taken
with HST and conducted a census of those systems likely to merge. Around each galaxy we
can, knowing its redshift, search for companions to a fixed luminosity limit and within
20 kiloparsecs. Projected line-of-sight pairs can
be removed statistically. This study revealed,
for the first time, a dramatic rise with redshift
in the number of likely associated pairs (figure
3). Broadly speaking, the number of close
pairs, corrected for known biases, increases
with redshift z as (1+ z)3. However, in order to
convert the rate of physically associated pairs
into the true rate at which galaxies are merging, we need to know precisely the time it takes
for a pair to coalesce. Whilst merging is clearly occurring, and at a rate that steeply increases with redshift, without some dynamical data
on close pairs, it is difficult to make direct
comparisons with hierarchical models.
We decided to consider a second approach.
Figure 4 shows the internal colour distributions
for a selection of field elliptical galaxies of
known redshift in both Hubble Deep Fields. A
surprising fraction have significantly bluer
cores, suggesting recently arrived younger stars.
A similar study applied to ellipticals in dense,
rich clusters finds far fewer examples. This difApril 2000 Vol 41
Galaxy evolution
NGC 5689
NGC 5965
NGC 7537
(b)
WFPC2/NICMOS
ground
(a) NGC 5838
SO
(c)
300
Sa
NGC 2715
Sbc
NGC 2715
(synthetic z=0.7 HDF image)
40
200
20
100
0
100
200
300
NGC 2715
(synthetic z=1.0 HDF image)
40
0
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NGC 2715
(synthetic z=1.3 HDF image)
40
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2: The superlative image quality of Hubble Space Telescope (HST).
2a: This contrasts a distant spiral galaxy observed with the WFPC2 on HST
and with one of the best ground-based telescopes.
2b: The two Hubble Deep Field images are particularly powerful and have
enabled our group to extend morphological studies to the distribution and
physical origin of colours of internal components of galaxies at high redshift.
2c: This shows how distant galaxies viewed in redshifted filters may be
rendered unfamiliar by subtle effects including surface brightness dimming
and wavelength shifts caused by the cosmic expansion. At Cambridge we
have pioneered detailed simulations to quantify the possible biases.
1020
(b)
(a)
z=1.355
z=0.958
luminosity density (W Hz–1 Mpc–3)
0
Sb
1019
E/SO
spirals
peculiars
1018
0.0
0.2
0.4
0.8
0.6
1.0
redshift
0.3
(c)
0.021(1+z)3.4
z=0.952
z=0.498
3a: HST reveals that most faint blue galaxies have irregular forms and groundbased spectra reveal that many are actively forming stars. 3b: By comparing their
luminosities to those of their more regular counterparts at various redshifts, their
contribution to the luminous output of the universe is found to be rapidly declining.
What, then, is happening to them? They might merge into more regular systems.
3c shows how, by counting projected pairs within a fixed physical scale, a merger
fraction can be calculated at various redshifts. At redshifts of order 1, the fraction
of merging systems is much more than 8 times higher than it is locally, so the
excess of faint blue galaxies is probably a manifestation of galaxy merging.
April 2000 Vol 41
merger fraction
0.2
0.10
0.08
0.06
0.04
0.03
visual classification
pair count, I<22.5 ground based selected
pair count, I<22.6 HST selected correction
patton et al. (1997)
0.02
1
(1 + z)
2
2.13
Galaxy evolution
ference is expected if clustered regions represent
peaks in the early density field whose assembly
history was accelerated. In other words, one
would have to go to much earlier times in clustered regions to find similar examples.
It is natural to ask whether the decline in the
number of blue irregulars is consistent with the
frequency with which we see blue clumps in the
emerging population of field ellipticals. Recently we have begun to examine the HDF ellipticals illustrated in figure 4 to determine the rate
at which stellar mass is arriving as a function of
redshift. This can then be directly compared
with the predictions of hierarchical merging as
well as the possibly associated demise in the
population of faint, blue peculiars.
We can consider this question in terms of
population statistics as well as through detailed
studies of individual galaxies. Returning to the
question of the infrared luminosity distributions which constrain the assembled stellar
masses in various types of galaxies, at the most
fundamental level if blue light from irregulars is
being reapportioned in field ellipticals, we can
expect the infrared luminosity density of the
former to decline with time at the expense of a
rise in that of the latter. Reassuringly, with
some uncertainties, this transformation is
indeed seen (figure 4). In the more detailed
study, we can ask whether the mass associated
with the “blue lumps” in the field ellipticals is
consistent with an appropriate growth rate.
Unfortunately, powerful though such a constraint would be, to convert the blue light seen
in the HDF ellipticals into a mass arrival rate
we need more astrophysical information. Are
we catching the blue light during the most
active stage of star formation (in which case the
mass involved could be quite modest) or, more
likely, is there some duty cycle of activity in
which case we will be observing galaxies at various stages in an extended process. Given that
many examples would be seen away from their
(presumed brief) most active moment, the mass
arrival rate could be more significant (figure 4).
In order to disentangle these possibilities,
spectroscopic diagnostics are necessary to constrain the ages and metallicities of the young
stars involved. Such programmes are ideally
suited to integral field spectroscopy with the
new generation of telescopes, including Gemini.
Bars and bulges in normal spirals
Any self-respecting theory of galaxy formation
must also explain the more detailed structures
seen within galaxies, most notably the emergence of spiral arms, the significance of the
bulge or spheroidal component of galaxies and
stellar bars. Modelling the internal characteristics of galaxies in the context of dark-matter
theories is a less mature field, but several key
predictions have emerged from theorists in
recent years and many of these are directly
2.14
testable with HST datasets.
As we have seen, the gradual cooling of gas
into dark-matter halos leads naturally to disklike systems, a fraction of which may later merge
to form ellipticals. If these disks assemble too
early they may lose their dynamical rotation in
the inner regions, depositing angular momentum
into their more extensive dark halos. Moreover,
early formation of spirals may lead to an overproduction of present-day ellipticals since merging is much easier at high redshift. For both reasons, theorists prefer to delay gas cooling until
quite recently and hope observers will find evidence for strong evolution in the numbers and
sizes of spiral disks to redshifts of order unity.
Examples of “grand design spirals”, those
with magnificent sweeping arms, can be found
to redshifts beyond 1 and straightforward
count analyses as a function of apparent magnitude and redshift detect no obvious decline in
the numbers of large disk galaxies to limits
where they can be recognized (figure 5). If anything, there are more disk galaxies at faint limits than expected. This appears to be because,
statistically, individual spiral disks have higher
surface brightnesses at high redshift with bluer
rest-frame colours as expected if their star formation at that time was more vigorous.
Dynamical data obtained by Nicole Vogt (also
at the Institute of Astronomy) and her colleagues using the 10 metre Keck telescope, can
be used to track possible evolution in the relationship between total mass (as defined by the
stellar rotation in the disk) and luminosity (the
so-called Tully-Fisher relation). Again there is
no striking evolution in this relationship out to
redshifts of 1. Whilst the samples are still small,
these simple observational tests suggest that the
bulk of the disk galaxy population was already
in place with its present-day properties at z =1.
However, recently we have been examining
some of the finer details. A test within reach is
to try and identify possible changes in the sizes
of disk galaxies. The light distribution in nearby disk galaxies follows an exponential dependence on radius and the scale length of this
exponential fall-off can be determined by fitting
profiles to the images of the larger systems out
to redshifts of 1. Early work with our Canadian and French colleagues indicated no substantial decline in the number of large disks over
this redshift range. However, that study was
based on only a handful of galaxies. A more
precise analysis using a larger sample, based on
the infrared luminosity of a galaxy as a robust
measure of its integrated stellar mass, shows a
modest decline in the degree of concentration
of disk systems per unit mass as a function of
redshift. Such analyses are giving the first quantitative estimates of the growth rate of stellar
disks. These spirals grow inside large dark-matter halos and by combining the rotational
velocity studies by Vogt and her colleagues and
the stellar masses derived using infrared luminosities, we can also obtain direct estimates of
how large a fraction of the total mass of a halo
ends up in galactic stars. This fraction is of
great interest in distinguishing between theoretical models. The dependence of this ratio on the
mass of the galaxy, such as will soon be observationally possible by extending these comparisons to larger samples, will give important
insight into the process of galaxy assembly.
What about the central bulges of spiral galaxies? These could form in various ways. Most
astronomers believe they represent cores which
suffered early collapse at high redshift, i.e.
miniature primordial galaxies around which
subsequent assembly occurs. However, they
could also represent evidence for minor mergers, e.g. as miniature ellipticals in a different
sense, resulting from the coalescence of unequal
mass progenitors. Whereas two equal mass spirals would catastrophically destroy their
respective disks in a head-on collision, forming
a smooth elliptical product, in a less extreme
collision one spiral might survive and adopt an
elliptical-like bulge. In this respect the darkmatter picture predicts a complex interplay
between types. For example, a small elliptical
(or bulge) might form from two spirals or irregulars and then grow via accretion of gas into a
spiral. Regardless of these continuing transformations, in hierarchical case one would expect
to find most bulges statistically older than ellipticals since the latter can only be formed from
the merger of near-equal mass systems.
Fortunately, bulges can be isolated in the
HDF spirals to quite high redshift (figure 5)
and thus it is possible to examine some of these
predictions. Comparing the colours of bulges
and ellipticals at the same redshift is a particularly revealing experiment since, regardless of
the particular dark-matter model, a central
prediction is that ellipticals should have
formed more recently and be slightly bluer
than bulges. In fact, we find a significant
colour dispersion for bulges with many much
bluer at intermediate redshifts than their elliptical counterparts (figure 5). This suggests a
more recent origin for at least some bulges
unless older structures are perhaps being rejuvenated by accretion of cool gas.
Bulges could also form from instabilities that
arise naturally in stellar disks. Numerical simulations suggest a stellar disk can become unstable and form an inner bar which, ultimately,
itself evolves into a bulge-like structure. Bars
are hard to see reliably in faint galaxies; their
visibility depends on the angular size and orientation of the host galaxy as well as the wavelength of observation. Notwithstanding these
difficulties, we recently demonstrated (with
Michael Merrifield at the University of Nottingham) that HST should be able to recognize
some barred galaxies beyond a redshift of 0.6,
April 2000 Vol 41
Galaxy evolution
(a)
(b)
1
2
lookback time (billions of years)
6
3
4
5
4: The assembly history of
field ellipticals, once
thought uniform in shape
and colour.
(a) Distant ellipticals in
less dense regions show
greater variety in internal
colours, e.g. blue cores.
(b) The decline in mass
density over time in
irregulars is mirrored by a
marginal increase in that
associated with ellipticals.
Dots and the shaded
region represent quiescent
elliptical galaxies, while
the coloured lines
represent the colour tracks
of galaxies following a
starburst.
7
scatter in internal cloud
0
0
0.5
1.0
redshift
(a) 120
(b)
40
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pixel
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log 1 + z
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(d) 2.5
(c)
2.0
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z=1.146
z=0.642
bulge V–I
80
1.0
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B–V
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z=0.457
z=0.321
0
0
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photometric redshift
1.0
5: The detailed evolution of features in spiral galaxies.
(a) A montage of those in the Hubble Deep Field oriented to reveal both their disk (spiral arm) and bulge (nuclear) components. (b) For flux-limited samples, the
redshift distribution indicates that the bulk of the spiral population existed at redshift 1. (c) and (d) Photometric decomposition suggests a gradual decline in
star formation rate with time. Bulges, classically the earliest structures around which spiral disks assembled, have a much larger range of colours than would
be expected in this case. The inner regions of spiral galaxies might be rejuvenated by inwardly-falling gas as the disk assembles around them.
April 2000 Vol 41
2.15
Galaxy evolution
(a)
telescope
focal surface
(b)
spectrograph
input
pupil
imagery
lenslets
spectrograph
output
spectrum
detector
datacube
lenslets
and
fibres
fibres
slit
g
c
1
image
slicer
(c)
fibre feed from Gemini
cryogenic
camera
insulated instrument
enclosure
–205°C
(68 K)
fibre slit
H
steering
mirror
J
masks
grating
Schmidt
plate
whereas in fact we had a hard time finding any.
Although bars may be transient features, the
absence of any examples beyond a modest redshift makes it difficult to believe they represent
a frequent route to the production of bulges
which are common at all redshifts.
Where next – resolved spectroscopy?
Until recently, studies of faint galaxies have concentrated on the statistics of their integrated
properties. Morphologists like ourselves have
been criticized for dealing with visual features
that may not have physical significance. But as
we have seen, galaxy formation is most likely to
be a continuous process characterized by some
or all of merging, gaseous infall, dynamical
instabilities and star formation timescales. It is
important to move away from broad-brush pictures dominated by simpler “counting tests”
and tackle the detailed astrophysics necessary to
understand the present diverse population.
This hoped-for change in philosophy coincides with a revolution in ground-based capability from the new generation of actively controlled, large optical and infrared telescopes
equipped with spectrographs capable of undertaking detailed internal studies of galaxies such
as those displayed in this article. The Keck telescope has already shown the way via the first
rotation curves discussed earlier. Resolved
spectroscopy can be combined with HST multicolour data to reveal star formation histories
and dynamical, chemical and excitation prop2.16
–55°C
(218 K)
2
3
4
mirrors
l
1
2
slit
3
4
6: The availability of resolved multicolour images from HST arrives at a time of
great excitement in the development of ground-based facilities.
(a) Large 8–10 m class telescopes such as the UK’s Gemini, ESO’s VLT and the
now well-established pair of Keck telescopes, promise to complement these
images with resolved spectroscopy (not possible with the smaller 2.5 m HST).
(b) Innovative techniques based on Integral Field Units have been developed to
feed light from a 2-D image (such as a faint resolved galaxy) so that an
independent spectrum can be obtained for different components.
(c) The most immediate version of these devices will be contained in the
Cambridge Infrared Panoramic Survey Spectrograph (CIRPASS) nearing
completion at the Institute of Astronomy and destined for early use on GeminiNorth in the second half of 2000. (From Allington-Smith with permission.)
86 cm
primary mirror
erties for suitable sub-components. The full
panoply of techniques used so successfully on
local galaxies can be used, within constraints
defined by telescope time and photon limits, on
intermediate redshift populations.
In practical terms, 2-D spectroscopy can be
achieved via mapping with a long slit or, more
economically, via an integral field unit – a
device which redistributes an array of spatial
elements so that the spectrum of each element
can be assembled conveniently on a 2-D detector (figure 6). Both the optical and nearinfrared spectrographs on the Gemini telescopes (respectively GMOS and CIRPASS –
http://www.ast.cam.ac.uk/) will offer such a
capability (see also Allington-Smith et al.
Allington-Smith et al., Mountain et al., Parry
et al.). Integral field spectroscopy will be extraordinarily demanding in telescope time
because, unlike conventional spectroscopy, the
high signal to noise essential for detailed astrophysical studies must be obtained for each subcomponent of the galaxy. Nonetheless, the
rewards will be great.
Resolved spectroscopy will complete a historical trend of increasing detail in the study of
faint galaxies. The faint blue population was
first identified in the 1970s using photography
at the prime focus of 4 m telescopes. Subsequently, redshifts were added using multiobject spectrographs in the 1980s. In the 1990s
HST’s exquisite image quality brought resolved
images, multicolour photometry and galaxy
morphologies. In each case it has taken several
years for observers (and theorists) to understand how to adapt to each new dimension of
data. Significantly, however, each new dimension has transformed the field. We are poised
now, with Gemini, VLT, Keck and Subaru, to
witness a further transformation via the
deployment of integral field spectrographs. ●
Richard S Ellis, Roberto G Abraham, Jarle Brinchmann and Felipe Menanteau, Institute of Astronomy, Madingley Road Cambridge CB3 0HE.
Further reading
Allington-Smith J R et al. 1997 Proc. SPIE 2871 1284–2294.
Allington-Smith J R et al. 1998 Proc. SPIE 3355 196–205.
Baugh C M et al. 1996 MNRAS 283 1361.
Cole S et al. 1994 MNRAS 271 781.
Frenk C S et al. 1985 Nature 317 595.
Mountain M et al. 1997 Proc. SPIE 2871 15–23.
van den Bergh S 1998 Galaxy Morphology and Classification
Cambridge University Press.
White S D M and Rees M J 1978 MNRAS 183 341.
● The work of the Cambridge group has appeared in the following
recent publications:
Abraham R G et al. 1996 MNRAS 279 L47–L52.
Abraham R G et al. 1999 MNRAS 303 641–658.
Abraham R G et al. 1999 MNRAS 308 569–576.
Brinchmann J et al. 1998 ApJ 499 112.
Brinchmann J 1999 The use of infrared luminosities to probe the
history of mass assembly of galaxies PhD thesis, University of
Cambridge.
Ellis R S and Abraham R G 199, in preparation The properties of
bulges in galaxies in the HDF.
Glazebrook K et al. 1995 MNRAS 275 L19–L22.
Menanteau F et al. 1999 MNRAS 309 208–220.
Menanteau F et aI. 1999 in preparation Internal colour dispersion in
ellipticals in the HDF.
April 2000 Vol 41