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1996MNRAS.281..487K
Mon. Not. R. Astron. Soc. 281, 487-492 (1996)
The age of elliptical galaxies and bulges in a merger model
Guinevere Kauffmann
Max-Planck-Institut fUr Extraterrestrische Physik, D-85740 Garching bei Munchen, Germany
Accepted 1996 January 29. Received 1995 September 20
ABSTRACT
The tightness of the observed colour-magnitude relation for elliptical galaxies has
often been cited as an argument against a model in which ellipticals form by the
merging of spiral discs. A common view is that merging would mix together stars of
disparate ages and produce a large scatter in colour. Here we use semi~analytic
models of galaxy formation in a fiat cold dark matter (CDM) universe to derive the
distribution of the mean ages and colours of the stars in cluster elliptical galaxies
formed by mergers. It is seen that most of the stars in cluster ellipticals form at
relatively high redshift (z> 1.9) and that the predicted scatter in the colourmagnitude relation falls within observational bounds. We conclude that the
apparent 'homogeneity' in the properties of the stellar, populations of cluster
ellipticals is not inconsistent with a merger scenario for the origin of these
systems.
The analysis is then extended to the stellar populations of field ellipticals and the
bulges of spiral galaxies. The mean stellar age of a bright field elliptical is on average
4 Gyr less than that of a cluster elliptical. The ages of bulges are found to correlate
strongly with the luminosity of the associated disc. The bulges of late-type spirals are
predicted to be older than the bulges of early-type spirals.
Finally, we derive predictions for the properties of elliptical galaxies in clusters at
high redshift. The number of recently merged objects increases with redshift and this
causes the scatter in the colour distributions to become progressively larger.
However, an rms scatter in rest-frame U - V colour greater than 0.1 is predicted only
at redshifts greater than 1.
Key words: galaxies: elliptical and lenticular, cD - galaxies: formation - galaxies:
fundamental parameters - galaxies: stellar content.
1 INTRODUCTION
There have long been two competing views on the formation history of the elliptical galaxies we see today. One is
that most of the stars in present-day galactic bulges and
ellipticals were produced during a relatively short, early
phase of intense star formation at high redshift. The second
view is that elliptical galaxies are relative latecomers, having
been produced as a result of the merging of disc galaxies
drawn together by gravity as their surrounding dark matter
haloes coalesced.
In recent years, the second view has come to enjoy
increasing popularity. Detailed numerical simulations have
shown that mergers between two spiral galaxies of comparable mass lead to the production of spheroidal merger remnants with physical characteristics such as density profiles,
gravitational radii, mean velocity dispersions and surface
brightnesses that are quite comparable to those of observed
ellipticals (see Barnes & Hernquist 1992 for a review). In
addition, observational evidence has begun to accumulate
which indicates that mergers and interactions are rather
common in the Universe, particularly at high redshift.
Hubble Space Telescope (HST) images indicate that, at redshifts ~ 0.4, 25-30 per cent of all galaxies show disturbed
morphologies (Driver et al. 1995). At lower redshifts,
almost all of the most luminous lRAS galaxies have turned
out to be interacting systems, leading to the conclusion that
the merging process may fuel very intense bursts of star
formation (see Joseph 1990 for a review). Schweizer &
Seitzer (1992) present evidence indicating that the UBV
colours of elliptical galaxies become systematically bluer as
the amount of fine structure in these galaxies increases. If
© 1996 RAS
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..487K
\
488 G. Kauffmann
fine structure is indeed a measure of the dynamical youth of
a galaxy, these observations suggest that many present-day
ellipticals have undergone a merger-induced starburst in the
past. Likewise, analyses of the stellar ages of elliptical
galaxies using absorption-line index strengths show that the
outer parts of elliptical galaxies are both older and more
metal-poor than the nuclei, consistent with the merger/starburst picture (Faber et al. 1995).
The fact remains, however, that the stellar populations of
present-day elliptical galaxies appear to be rather homogeneous. The colours, mass-to-light ratios and Mg II absorption strengths of ellipticals are correlated only with their
central velocity dispersions, and the scatter about these
relations appears to be surprisingly small. Bower, Lucey &
Ellis (1992) show that the intrinsic scatter about the U - V
and V - K colour-magnitude relations for early-type
galaxies in the Coma and Virgo clusters is less than 0.05
mag. This leads to the conclusion that elliptical galaxies
must have formed the bulk of their stars at redshifts greater
than 2, or else the observed homogeneity in the colours
would require a very precise synchronization of formation
epochs and star formation histories for these galaxies.
Likewise, Bender, Burstein & Faber (1993) find a very tight
relationship between the strength of the Mg II index at the
centre of ellipticals and their central velocity dispersions,
(1. Galaxies show a mean scatter of only 0.025 mag in
Mg II, versus a full range of nearly 0.35 mag in Mg II. This
observed scatter sets an upper limit of 15 per cent on the
rms variation of both age and metallicity at fixed (1 for bright
ellipticals.
The tightness of these limits calls into question the tenability of the merger model, as merging might be expected to
mix together stellar populations of quite disparate ages
(Renzini 1995). In addition, a starburst accompanying the
merging event would increase the amount of light contributed by young stars. However, detailed predictions of the
resulting scatter in quantities such as colours and Mg II
strengths have not yet been made for any realistic theory of
elliptical galaxy formation via merging. In this paper, we
present results calculated for a b = 1.5 COM universe using
semi-analytic techniques. In this model, elliptical galaxies
are produced when two progenitor galaxies of roughly equal
mass merge with each other. Spiral galaxies are formed in a
two-step process: two galaxies of equal mass must first
merge with each other to form the bulge component, then
gas from the surrounding dark matter halo cools and settles
to form a new disc. It has already been demonstrated in
Kauffmann (1995) that this model provides a good fit both
to the properties of cluster ellipticals seen today, and to the
evolution in the colours of cluster galaxies at higher redshift.
Models of this type also naturally result in a higher ratio of
elliptical to spiral galaxies in clusters than in the field
(Kauffmann, White & Guiderdoni 1993, hereafter KWG).
This is because the dark matter haloes of cluster galaxies are
assumed to be disrupted and stripped by tidal heating and
encounters with other galaxies in the cluster potential. As a
result, gas can no longer accrete and form a new disc. The
results of the analysis in this paper show that, although
elliptical formation is far from a synchronized process in this
model, merging in clusters does in fact take place at early
enough epochs for the scatter in the colours and mean
stellar ages to be in good agreement with observations.
We also extend the analysis to the stellar populations of
field elliptical galaxies and the bulges of spirals. The models
predict that bright elliptical galaxies in low-density environments are formed by much more recent merging events than
cluster ellipticals and consequently contain younger stars.
The colours of field ellipticals are bluer on average and
show more scatter. The models also predict that the ages of
spiral bulges should correlate strongly with the luminosity of
the associated disc. Since discs are accreted gradually over a
Hubble time, late-type spirals must form their bulges at
higher redshift than earlier types. It is possible that this
correlation may be hidden if substantial inflow from the disc
to the bulge is able to take place, for example as a result of
bar formation.
Finally, we make some predictions for the scatter in the
colours of cluster elliptical galaxies at high redshift. Morphological classification of galaxies at z ~ 0.6-0.7 is now
possible using HST, and the hope is that the colours of
galaxies observed at an epoch when the Universe was a
fraction of its present age will place strong constraints on
cosmological models.
2 SEMI-ANALYTIC MODELS OF
ELLIPTICAL GALAXY FORMATION
The semi-analytic models we employ are described in detail
in KWG and Kauffmann & White (1993). Application of
the model to the evolution of the galaxy population in clusters at high redshift is discussed in Kauffmann (1995). Here
we present a brief summary of that paper.
An algorithm based on an extension of the PressSchechter theory due to Bower (1991) and Bond et al.
(1991) is used to generate Monte Carlo realizations of the
merging paths of dark matter haloes from high redshift until
the present. This algorithm allows all the progenitors of a
present-day object, such as a cluster, to be traced back to
arbitrarily early times. Dark matter haloes are modelled as
truncated isothermal spheres and it is assumed that, as the
halo forms, the gas relaxes to a distribution that exactly
parallels that of the dark matter. Gas then cools and condenses on to a central galaxy at the core of each halo. Star
formation and feedback processes take place as described in
KWG. In practice, star formation in central galaxies takes
place at a roughly constant rate of a few solar masses per
year, in agreement with the rates derived by Kennicutt
(1983) for normal spiral galaxies.
At a subsequent redshift, a halo will have merged with a
number of others, forming a new halo of larger mass. All gas
that has not already cooled is assumed to be shock heated to
the virial temperature of this new halo. This hot gas then
cools on to the central galaxy of the new halo, which is
identified with the central galaxy of its largest progenitor. The
central galaxies of the other progenitors become satellite
galaxies, which are able to merge with the central galaxy on
a dynamical friction time-scale. If a merger takes place
between two galaxies of roughly comparable mass, the
merger remnant is labelled as an 'elliptical' and all cold gas
is transformed instantaneously into stars in a 'starburst'.
Note that the infall of new gas on to satellite galaxies is
not allowed, and star formation will continue in such objects
only until their existing cold gas reservoirs are exhausted.
Thus the epoch at which a galaxy is accreted by a larger halo
© 1996 RAS, MNRAS 281, 487-492
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..487K
Ages of elliptical galaxies in a merger model 489
delineates the transition between active star formation in
the galaxy and passive evolution of its stellar population.
The elliptical merger remnants in clusters hence redden as
their stellar populations age. Central galaxy merger remnants in the 'field' are able to accrete new gas in the form of
a disc to form a 'spiral' galaxy consisting of both a spheroidal
bulge and a disc component. As demonstrated in KWG, this
model is able to account for the observed numbers and
luminosity distributions of galaxies of different morphologies, both in clusters and in lower-density environments.
The spectrophotometric models of Bruzual & Charlot
(1993) are used to translate the predictions of the models
into observed quantities such as magnitudes and colours,
which may be compared directly with the observational
data.
In the present paper, we focus on the ages and colours of
the stars of the merger remnants as predicted by the model.
The cosmological initial conditions are a b = 1.5 CDM
universe with 0=1 and Ho=50 km S-I Mpc- I . Although
this model does not match the COBE measurements of the
amplitude of the microwave background fluctuations on
degree scales, it has become the de facto standard model
used in galaxy formation studies, since it provides a reasonable fit to the observed small-scale clustering properties of
galaxies.
3 THE AGES AND COLOURS OF
ELLIPTICAL GALAXIES
Fig. 1 shows the V luminosity-weighted mean stellar age of
the galaxies in a cluster of 1015 Mo' The symbols in the plot
represent the morphological type of the galaxy: large filled
circles correspond to ellipticals, open squares to SOs, small
filled circles to spirals and pinwheels to ellipticals that have
been formed by a recent merging event ( < 1.5 Gyr ago).
The classification into morphological type is based on the B-
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Simien & de Vaucouleurs (1986). As can be seen, the stellar
populations of spiral galaxies span a very wide range in age,
but those of early-type galaxies are confined to ages
between 8 and 12.5 Gyr. For our adopted cosmological
parameters, this means that the bulk of stars in elliptical and
SO galaxies were formed at redshifts exceeding 1.9.
The age distribution of early-type galaxies found in haloes
of mass 1013 Mo is plotted in Fig. 2(a). These galaxies either
occur in small groups, or have no companion of comparable
luminosity, and thus can be regarded as 'field' ellipticals. In
order to get a sizeable sample of these objects, it was necessary to combine the populations of 100 such low-mass
haloes. As can be seen, the field elliptical population splits
up into two distinct classes. The faint field ellipticals have
stellar ages comparable to those of early-type galaxies in
clusters. There is also, however, a population of bright field
ellipticals that have much younger stellar populations
(mean ages between 5 and 8 Gyr). These bright galaxies are
all central galaxies and thus are still in the process of accreting cold gas. They are also mostly recent mergers. The fact
that we see these galaxies as ellipticals is simply because
they have not yet had the time to accrete a new disc following their last big merging event. Fig. 2(b) shows the age
distribution of early-type galaxies in massive groups
(1014 Mo)' There is some tendency for the most luminous
group members to be recent mergers, but the distribution as
a whole is rather similar to that of the cluster ellipticals in
Fig. 1.
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Figure 1. The V-luminosity weighted mean stellar ages of the
galaxies in a cluster of mass 1015 Mo' Large filled circles represent
ellipticals, open squares are SOs, small filled circles are spirals and
three-pronged pinwheels are elliptical galaxies that were formed by
a major merger in the past 1.5 Gyr.
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Figure 2. The mean ages of the early-type galaxies in (a) haloes of
mass lO13Mo and (b) haloes of mass 1014 M o ' Symbols are as in
Fig. 1.
© 1996 RAS, MNRAS 281, 487-492
© Royal Astronomical Society
e
Provided by the NASA Astrophysics Data System
1996MNRAS.281..487K
G. Kauffmann
490
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Fig. 3 is a histogram of the fraction of the total V-band
light of a cluster elliptical or spiral galaxy contribution by
stars that form at a given redshift. The solid line represents
the average contribution, while the filled squares and circles
represent the maximum and minimum contributions
respectively. The difference between the average star formation history of a cluster elliptical and that of a spiral is
illustrated clearly in this diagram. A large fraction of the Vband light of spirals comes from stars that formed in the
past 5 Gyr, whereas on average this contribution is negligible
in ellipticals. It should be noted, however, that a small subset of elliptical galaxies does exhibit substantial star formation at late epochs. The dotted line in the top panels in Fig.
3 illustrates the epoch at which elliptical galaxies typically
underwent their last major merging event. As can be seen,
most ellipticals were formed by a merger that occurred
between 5 and 10 Gyr ago.
It is also apparent from this diagram that a substantial
fraction of the stars in an elliptical galaxy may have been
formed before the merging event that actually caused the
transformation in morphology. Star formation may take
place at a constant rate in the two spiral progenitors of an
elliptical for several gigayears before they are able to merge.
It is thus important to distinguish exactly what is meant by
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Figure 3. The fraction of the total V-band light of cluster elliptical
or spiral galaxies contributed by stars that form at a given epoch.
The solid line is the average star formation history. The squares
and circles represent, respectively, the maximum and minimum
contributions to the total light for any galaxy at that epoch. The
dotted line is a histogram of the percentage of elliptical galaxies
that experienced their last major merger at a given epoch.
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Figure 4. The distribution in U - Vand V - K colour of early-type
galaxies in a cluster of 1015 M 0 • Filled circles are ellipticals and
open squares are SOs.
'the formation epoch of ellipticals'. It can be defined either
as the epoch when the bulk of the stars were formed or as
the epoch when the merging event took place.
Finally, in Fig. 4, we have used the evolutionary synthesis
models of Bruzual & Charlot (1993) to generate a scatterplot of the U - Vand V - K colours of early-type galaxies in
clusters. In U - V, the average colour of an elliptical is 1.37
with an rms standard deviation, (1, of 0.034. In U - K, the
mean is 3.01 and (1 is 0.045. The rms is calculated using the
'median absolute difference' technique so that the results
may be compared directly with the data of Bower et al.
(1992). If SO galaxies are, included, the rms increases to
~0.07. Although the mean colours are roughly comparable
to the average colours in the data of Bower et al., there is no
correlation between the colour and the magnitude of the
galaxy, in contradiction with the observations, which show
strong reddening in the stellar populations of elliptical
galaxies with increasing luminosity. As shown in Fig. 1,
cluster elliptical galaxies of all luminosities have roughly the
same mean stellar age, so any trend in colour would have to
result from metallicity differences between galaxies of different masses. We have neglected chemical enrichment in
this analysis. The rms scatter in the colour of the model
ellipticals is well within the upper limit for the intrinsic
scatter in the colour of Coma and Virgo ellipticals quoted
by Bower et al. (1992). The inclusion of the SO popUlation
increases the scatter by a factor of 2. It is interesting that
Bower et al. also found that the removal of SO galaxies
© 1996 RAS, MNRAS 281, 487-492
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..487K
Ages of elliptical galaxies in a merger model 491
caused the scatter to decrease in their data, but not by as
much as a factor of 2. However, it is clear that, in the model,
the scatter quoted for the early-type population will depend
on the precise value of the disc-to-bulge ratio at which a
galaxy is labelled as being 'early-type'.
4 THE BULGES OF SPIRAL GALAXIES IN
THE FIELD
As has been discussed, central galaxy merger remnants in
the field are able to accrete new gas, form a disc and become
a 'spiral' galaxy consisting of both a bulge component and a
disc component. Fig. 5 shows the distribution of bright field
galaxies as a function of M(bulge) -M(tot), the difference
between the B-band magnitude of the bulge and the total Bband magnitude of the galaxy. Early-type galaxies have
M (bulge) - M (tot) < 1 (Simien & de Vaucouleurs 1986). In
Fig. 6, we plot the V-luminosity weighted mean stellar ages
of the bulges as a function ofM(bulge) -M(tot). There is a
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Figure 5. The distribution of 'field' galaxies as a function of the
difference in the B-band magnitude of the bulge and the B-band
magnitude of the whole galaxy.
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strong correlation between bulge age and bulge-to-disc
ratio. As was already noted, early-type galaxies in the field
have young bulges, with ages ranging between 5 and 8 Gyr.
Late-type galaxies have bulges as old as 12 Gyr. It would be
extremely interesting to investigate whether correlations of
this type are found for real spiral galaxies. If not, it may be
an indication that bulge formation and disc formation do
not follow the simple two-step process assumed here.
Indeed, there are indications that bar instabilities in discs
may fuel nuclear starbursts in many galaxies (see for
example papers by Kenney and collaborators). If bar-driven
star formation accounts for a substantial fraction of the total
mass of stars in the bulge, the correlation shown in Fig. 5
may be completely wrong.
5 ELLIPTICALS IN CLUSTERS AT HIGH
RED SHIFT
Present-day elliptical galaxies contain the oldest stars in the
Universe. The repair of HST permits the direct identification of elliptical and SO galaxies at significant cosmic depth.
The hope is, therefore, that studies of the stellar populations of ellipticals at early epochs will place strong constraints on cosmology, in much the same way that studies of
the ages of globular clusters have ruled out high values of
the Hubble constant.
Ellis (1995) has measured the rest-frame U - V colours of
elliptical galaxies in the cluster 0016 + 16 at z = 0.54 and
finds an rms scatter of 0.07. In Fig. 7, model predictions for
the U - V colours of cluster ellipticals are shown ata series
of redshifts. Recent mergers are distinguished from 'ordinary' ellipticals by large open circles in the plot. There are
two trends that should be noted. First, the 'red envelope' of
the distribution shifts bluewards with increasing redshift.
The magnitude of this shift is consistent with what one
would expect from the passive evolution of a 12-Gyr-old
burst, as given by the models of Bruzual & Charlot (1993).
Secondly, the scatter in the colour distribution increases
with redshift and most of this increase arises from a growing
population of galaxies which have merged in the 1.5 Gyr
prior to the epoch of observation. Note, however, that the
scatter is not dramatically large until the redshifts are in
excess of 1. This may be understood as follows. As pointed
out by Charlot & Silk (1994), the colours of stars formed in
a short burst evolve very rapidly for the first 3-4 Gyr. After
that, the evolution slows down and the colours redden only
gradually over time. So long as most of the stars in the
elliptical population of a cluster are formed 3-4 Gyr prior
to the time the cluster is observed, no great conspiracy is
required to produce a small scatter in the colours. At redshifts greater than 1, this is no longer possible (assuming
Q = 1 and Ho = 50 Ian S-1 Mpc- 1) and the scatter becomes
very large. Spectroscopic age indicators may turn out to be
more sensitive tests of cosmology.
6 DISCUSSION AND CONCLUSIONS
3
M(bulge)-M(tot)
Figure 6. The V-luminosity weighted mean stellar age of the bulge
component of a field galaxy is plotted as a function of the magnitude difference between the bulge and the galaxy as a whole.
Two major conclusions follow from this analysis. One is that
star formation in cluster ellipticals formed by mergers
occurs at high enough redshifts for the predicted scatter in
the colour-magnitude relation of these systems to fall
within observational bounds. That ellipticals in clusters
© 1996 RAS, MNRAS 281, 487-492
© Royal Astronomical Society • Provided by the NASA Astrophysics Data System
1996MNRAS.281..487K
492
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elliptical galaxies. What this paper has demonstrated is that
this process can work in detail, and leads to a population of
cluster ellipticals that matches the observations rather
well.
The second major conclusion is that bright elliptical
galaxies in the field are intrinsically different from cluster
ellipticals. These galaxies have undergone recent merging
and star formation. It is only because we 'catch them at the
right time' that we observe them as elliptical systems. In a
few gigayears, they will have grown new discs and turned
back into spirals. There are some observational indications
that field ellipticals may be younger (De Carvalho & Djorgovski 1992), but these need to be studied in more detail.
We also predict that the ages of the bulges of spiral galaxies
in the· field should correlate with the luminosities of the
associated discs. Galaxies with higher bulge-to-disc ratios
should have younger bulges. If this correlation is not found,
it may indicate that bulge formation is an ongoing process.
Finally, we derive predictions for the colour distributions
of elliptical galaxies in clusters at high redshift. The scatter
is predicted to increase very substantially at redshifts
greater than 1.
~ 0.5
0
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ACKNOWLEDGMENTS
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I thank Ralph Bender and Steve Zepf for helpful discussions and Richard Ellis for pointing out the high-redshift
data.
REFERENCES
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Figure 7. Rest-frame U - V colour-magnitude relations for elliptical galaxies in clusters of 10'5 M0 observed at z=0.6, 0.9 and 1.5.
Large open circles denote ellipticals that have been formed by a
major merging event in the past 1.5 Gyr.
form at high redshift should come as no surprise. For a
random Gaussian initial density field, it is well-known that
the redshift of collapse of density peaks on galaxy scales is
boosted by the presence of the surrounding, large-scale
overdensity destined to collapse to form the cluster at z = 0
(Bardeen et al. 1986). Galaxies forming in peaks collapsing
at z > 2 quickly merge together within groups and their star
formation is truncated when their gas is used up. Thus, in
the hierarchical clustering picture, galaxies that form at high
redshifts are in some sense pre-selected to end up as cluster
Bardeen J. M., Bond J. R., Kaiser N., Szalay AS., 1986, ApJ, 304,
15
Barnes J. E., Hernquist L., 1992, ARA&A, 30, 705
Bender R., Burstein D., Faber S. M., 1993, ApJ, 411, 153
Bond J. R., Cole S., Efstathiou G., Kaiser N., 1991, ApJ, 379,
440
Bower R., 1991, MNRAS, 248, 332
Bower R., Lucey J. R., Ellis R. S., 1992, MNRAS, 254, 601
Bruzual G., Charlot S., 1993, ApJ, 405, 538
Charlot S., Silk J., 1994, ApJ, 432, 453
De Carvalho R. R., Djorgovski S., 1992, ApJ, 389, 149
Driver S. P., Windhorst R. A, Ostrander E. J., Keel W. C.,
Griffiths R. E., Ratnatunga K. u., 1995, ApJ, 449, L23
Ellis R. S., 1996, in Ostriker J. P., Bahcall J. N., eds, Unsolved
Problems in Astrophysics. Princeton Univ. Press, Princeton NJ,
in press
Faber S. M., Trager S. c., Gonzalez J. J., Worthey G., 1995, in Van
der Kruit P. G., Gilmore G., eds, Proc. IAU Symp. 164, Stellar
Populations. K1uwer, Dordrecht, p.249
Joseph R. D., 1990, in Wielen R., ed., Dynamics and Interactions of
Galaxies. Springer, New York, p.132
Kauffmann G., 1995, MNRAS, 274, 161
Kauffmann G., 1996, MNRAS, 281, 673 (this issue)
Kauffmann G., White S. D. M., 1993, MNRAS, 261, 921
Kauffmann G., White S. D. M., Guiderdoni B., 1993, MNRAS,
264, 201 (KWG)
Kennicutt R. C., 1983, ApJ, 272, 54
Renzini A, 1995, in Van der Kruit P. G., Gilmore G., eds, Proc.
IAU Symp. 164, Stellar Populations. K1uwer, Dordrecht,
p.325
Schweizer F., Seitzer P., 1992, AJ, 104, 1039
Simien F., de Vaucouleurs G., 1986, ApJ, 302, 564
© 1996 RAS, MNRAS 281, 487-492
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