B. L. Ziegler1 - D. Thomas2 - A. Böhm1 - R. Bender2,3 - A. Fritz1 - C. Maraston2
1 - Universitätssternwarte Göttingen, Geismarlandstraße 11,
37083 Göttingen, Germany
2 - Max-Planck-Institut für extraterrestrische Physik,
Giessenbachstraße, 85748 Garching, Germany
3 - Universitätssternwarte München, Scheinerstraße 1,
81679 München, Germany
Received 2 December 2004 / Accepted 7 December 2004
Abstract
We investigate in detail 13 early-type field galaxies with
0.2<z<0.7 drawn from the F ORS Deep Field. Since the majority
(9 galaxies) is at
,
we compare the field galaxies to 22
members of three rich clusters with z=0.37 to explore possible
variations caused by environmental effects. We exploit VLT/FORS
spectra (
)
and HST/ACS imaging to determine internal
kinematics, structures and stellar population parameters. From the
Faber-Jackson and Fundamental Plane scaling relations we deduce a
modest luminosity evolution in the B-band of 0.3-0.5 mag for both
samples.
We compare measured Lick absorption line
strengths (H
,
H
,
H
,
Mgb, and Fe 5335) with evolutionary stellar
population models to derive light-averaged ages, metallicities and the
element abundance ratios Mg/Fe. We find that these three stellar
parameters of the distant galaxies obey a scaling with velocity
dispersion (mass) which is consistent with that of local
nearby galaxies. In particular, the distribution of Mg/Fe ratios of
local galaxies is matched by the distant ones, and their derived mean
offset in age corresponds to the average lookback time. This
indicates that there was little chemical enrichment and no significant
star formation within the last
5 Gyr.
The calculated luminosity evolution of a simple stellar population model
for the derived galaxy ages and lookback times is in most cases
consistent with the mild brightening measured by the scaling relations.
Key words: galaxies: elliptical and lenticular, cD - galaxies: evolution - galaxies: abundances - galaxies: stellar content - galaxies: kinematics and dynamics - galaxies: distances and redshifts
With these observational quantitative measurements, it is now possible to perform robust tests of galaxy formation theories. The basic prediction of cold dark matter (CDM) models of hierarchically growing structure is the gradual increase of mass of galaxies. Connecting this mass assembly with the physics of star formation leads to further predictions for the evolution of the stellar populations of a galaxy. The combination of characteristic velocities and sizes could in principle constrain the mass of galaxies, but this transformation is degenerate. The main problem is the cutoff radius for the dark matter halo which must follow certain assumptions by considering various constraints which greatly affect the estimated total mass of a galaxy. Thus, observations can better be taken to investigate the luminosity and chemical evolution and the age distributions of the stellar populations.
For spheroidal (elliptical) galaxies, CDM simulations yield a dependence on environment (e.g. Baugh et al. 1996). Galaxies born in high-density regions that evolve into rich clusters are cutoff from external influences much earlier on average than those in the field. Two thirds of the field ellipticals should still be produced at z<1 (e.g. Kauffmann et al. 1997), whereas the last major merger between two large galaxies in high density environments ending up in a big elliptical is predicted to occur at somewhat higher redshifts (e.g. Kauffmann 1996; Cole et al. 2000). These predictions manifest themselves in measureable parameters like mean age, metallicity and element abundances on the one side and a specific luminosity and color evolution on the other side. Thus, field ellipticals, in particular the bright ones, should on average have younger global ages (e.g. Kauffmann & Charlot 1998) and more solar-like element abundance ratios (e.g. Thomas 1999) than cluster ellipticals.
The FJR & FP scaling relations (e.g. Bender et al. 1992),
the color-magnitude (e.g. Bower et al. 1992) and
Mg-
relations (e.g. Bender et al. 1993)
of local early-type galaxies are very tight indicating that the
bulk of their stars must have been formed at high redshifts (
).
Nevertheless, this homogeneity can be reconciled with the hierarchical
merging scenario (Kauffmann 1996).
Difficulties arise because of a probable degeneracy between age and
metallicity effects, in particular for the Mg-
relation
(e.g. Thomas et al. 2005; Kuntschner et al. 2002; Trager et al. 2000).
But the Fundamental Plane is very well suited to study the redshift
evolution of the luminosity (e.g. Bender et al. 1998) and
the mass/light ratios (e.g. Franx 1993).
Observations up to
were predominantly carried out for
(bright and massive) galaxies in clusters.
Here, consistent results were obtained of a very modest brightening and
slow decrease of M/L with z fully compatible with the assumption of a very
high formation redshift (e.g. van Dokkum et al. 1998; van Dokkum & Stanford 2003; Kelson et al. 2000).
On the other hand, findings for high-z field galaxies are more discrepant.
Some studies argue for a strong evolution (e.g. Gebhardt et al. 2003; Treu et al. 2002)
whereas other authors favor a behaviour very similar to cluster galaxies
(e.g. van Dokkum & Ellis 2003; van der Wel et al. 2004).
The differences might mainly come from the low number of observed
galaxies.
But the samples might also be affected by the so-called progenitor bias
(van Dokkum & Franx 2001) in the sense that the selection criteria for early-type galaxies
differ strongly among researchers.
Many stellar population studies of the age and metallicity distribution have
been carried out for local galaxies.
The majority of cluster ellipticals have very old mean ages
(10 Gyr), high metallicities (
)
and are enhanced in Mg over Fe compared to the solar ratio
(
)
(e.g. Thomas et al. 2005; Mehlert et al. 2003; Poggianti et al. 2001; Eisenstein et al. 2003; Trager et al. 2000). There are
trends between these parameters and the luminosity with less luminous
galaxies having a wider spread and being skewed to younger ages. A
more diverse behaviour is found for S0 galaxies in clusters. At least
two families can be distinguished with one being very similar to the
ellipticals and the other having younger luminosity-weighted ages
(
5 Gyr) indicating recent star formation activity. For
field early-type galaxies, Kuntschner et al. (2002) find that they are younger
(by 2-3 Gyr) but more metal-rich (by
0.2 dex) than cluster
galaxies exhibiting similar super-solar Mg/Fe ratios. In their recent
study of field and cluster galaxies compiling high-quality data from
several sources, Thomas et al. (2005) show that mean ages, metallicities,
and
/Fe ratios correlate with galaxy mass (
). Both
zero-point and slope of the
/Fe-
relation are
independent of the environmental density, while the ages of objects in
low density environments appear systematically lower accompanied by
slightly higher metallicities.
Little work has been done yet investigating ages and metallicities
from absorption lines in distant galaxies because the high
S/N needed for such an analysis is achieved only with long exposures
even with large telescopes and because individual lines can be severely
corrupted by terrestrial spectral features in the red wavelength
regime. But models of galaxy formation can be constrained much
more effectively by measurements at higher redshifts because differences among
the predictions are much larger. Jones et al. (2000), for example, use
combinations of H
with metal lines measured in co-added spectra of
several E and S0 galaxies in clusters at
z=0, 0.3, 0.5,
respectively. They claim that there is no significant difference in
the age-metallicity distribution between these two galaxy types at
any redshift. Kelson et al. (2001) presented a study of the evolution of
the H
and H
Balmer absorption in galaxies in four clusters up to
z=0.8. Assuming that the same relation between these indices and
velocity dispersion holds at all redshifts, they derived a modest
evolution in the zero-point as expected for a passive evolution of old
stellar populations and consistent with the evolution of M/Ldetermined from an FP analysis. Recently, Eisenstein et al. (2003) examined
22 000 bright early-type galaxies from the SDSS averaging the spectra
within bins of luminosity, environment, and redshift (0<z<0.5) to
produce high-S/N mean spectra. They confirmed that these L*galaxies have mainly old quiescent stellar populations with high
metallicities and an excess of
- (Mg-) elements with respect
to the solar value independent of environment or redshift.
In this paper, we analyse the properties of galaxies at redshifts
both in clusters and in the field. In Sect. 2 our
observations and data analysis is described. The Faber-Jackson
relation and the Fundamental Plane are used to investigate the
evolution in luminosity in Sect. 3. Measured absorption line
strengths (Sect. 5) are compared in Sect. 6 to stellar population
models that explicitely take into account variations in the abundance
of elements to explore the chemical enrichment histories of the
galaxies. A summary and discussion is presented in Sect. 7.
Throughout the paper, we adopt the "concordance'' cosmology with
,
,
and
H0 = 70 km s-1 Mpc-1(e.g. Tonry et al. 2003).
This yields a distance luminosity of 104.75 for the Coma cluster which is used as a
local reference sample.
The spectroscopic observations of our sample of field ellipticals were performed in parallel to those of late-type galaxies in the F ORS Deep Field that form the basis of our analysis of the evolution of the Tully-Fisher relation (Böhm et al. 2004; Ziegler et al. 2002). The main differences are that the early-type galaxy candidates were placed onto slitlets independent of their position angles and were observed with more than one MOS setup in many cases to increase the respective exposure time. All in all, nine different setups with the FORS1&2 instruments at the VLT were acquired in three different observing runs with a total integration time of 2.5 h for each setup. Typically, each setup contained 2-4 elliptical candidates.
Object selection was based on the deep UBgRI photometry of the F ORS
Deep Field (FDF), which is described e.g. in Heidt et al. (2003).
Candidates were chosen according to their spectrophotometric type and their
extended structureless appearance.
Main criterium was the apparent brightness ()
in order to
ensure sufficient S/N in the absorption lines for a robust determination
of line strengths and velocity dispersions.
Photometric redshifts were restricted to
of objects with an
early model SED type (for a description of the photometric redshift
performance see e.g. Bender et al. 2001).
Although the two-dimensional distribution of FDF objects indicated a cluster of
galaxies with
and our primary goal was to target
field ellipticals, we did not preselect against such candidates.
After the spectral analysis, we could confirm that 15 out of
32 observed different elliptical galaxy candidates are most probably members
of a cluster.
Taking our measurements of the radial velocities of these galaxies, the lower
limit for the velocity dispersion of the cluster is
km s-1.
Since the cluster center was not observed but only the outer parts, the true
is probably larger.
At z=0.33, the Mg 5170 absorption feature is unfortunately corrupted due
to terrestrial absorption (the B band)
making it impossible to accurately determine the internal velocity dispersion
of a galaxy (see below).
For that reason we do not include these cluster galaxies in our analysis and,
from hence forward, when we speak of FDF galaxies only the field ellipticals
are meant.
The elliptical candidates were spread out among the spiral candidates for
each setup placing them on a region of the CCD corresponding to observed
wavelengths where either the Mgb-feature (
Å) or
the G-band (
Å) should have been visible according
to the photometric redshifts.
As is described in more detail in Böhm et al. (2004) the MOS spectroscopy was
performed using grism 600R with order separation filter
GG435 with FORS2 (September and October 2000) and FORS1 (October
2001) at the VLT.
With slit widths of 1
,
a spectral resolution of
was
achieved, the spatial scale was 0.2
/pixel, and seeing conditions
were sufficient (varying between 0.4
and 0.9
FWHM) to meet
the Nyquist theorem.
Image reduction followed the usual steps of bias subtraction, flat fielding,
sky subtraction and wavelength calibration. Because some spectra exhibited
spatial distortions, the images of each individual slitlet were extracted
from the full frame after bias subtraction allowing the typical
two-dimensional image reduction of long-slit spectroscopy. All images of
one respective slitlet including the science, flatfield and arclamp frames
were rectified in exactly the same manner with the procedure described in
Jäger et al. (2004) to ensure correct wavelength calibration.
The spectral rows of an object were averaged using a Horne-based
(Horne 1986) algorithm as implemented in the image analysis software
MIDAS
(see e.g. Ziegler et al. 2001).
Finally, the one-dimensional spectra of the respective exposures were
summed.
In those cases where a galaxy was observed in two or three different MOS
setups, the wavelength ranges covered did not match each other exactly
leading to varying final count rates at different wavelengths.
The internal velocity dispersions
and the radial velocities v
of the galaxies were determined applying the Fourier
Correlation Quotient (FCQ) method with an updated version of Bender (1990).
For each galaxy,
and v were measured using 3-4 different
kinematic standard stars.
Deviations from star to star ranged from 2 to 20 km s-1.
Since only one good template star (SAO 162947) was available that was
observed with FORS like the galaxies,
three more K giant stars (SAO 32042, SAO 80333 & SAO 98087) were used
which had been observed with MOSCA at the 3.5 m-telescope at the Calar Alto
observatory
and have sufficiently resolved spectra (
km s-1 around
Mgb).
FCQ was run on either a "red'' part of a galaxy spectrum (ideally covering
H
- Fe 5335, but sometimes only Mgb or only H
could solely be used)
or a
"blue'' part (around the G-band), in some cases (5 out of 13) in both regimes.
In order to determine the instrumental resolution of the galaxy spectra, the
widths of 3-5 emission lines of the respective arc spectrum used for the
wavelength calibration were measured at wavelengths corresponding to Mgb and
the G-band separately.
Typical instrumental resolutions are
km s-1.
To provide the "correct'' template for the respective instrumental
broadening, each stellar
spectrum was artificially smeared out to
before its usage in FCQ.
In the Appendix, we tabulate the error-weighted averages of
and
v (corrected for the radial velocities of the template stars) and their
respective errors for 13 FDF field galaxies that had sufficient S/N.
We give only either the values derived from the blue or the red spectral
part depending on their quality, since only these values are used for the
analysis below.
The signal-to-noise of a galaxy's continuum was determined with FCQ,
too, which was calibrated with Monte-Carlo simulations using the stellar
spectra.
Absorption line strengths as defined in the Lick system (Worthey & Ottaviani 1997; Trager et al. 1998)
were measured as described in Ziegler et al. (2001).
To this purpose, the galaxy spectra had been artificially broadened first to
match the instrumental resolution of the stellar Lick/IDS spectra
(
km s-1 around Mgb) before the equivalent
widths were calculated taking the redshifts into account.
The measured values were then corrected for the decrease in strength caused by
the broadening due to the galaxies' internal velocity dispersions.
The correction functions for the different absorption lines were determined
by simulating this effect with the stellar spectra.
Lick indices of the galaxies can be badly affected if either the central
bandpass or those that sample the pseudo-continuum of a line are redshifted
into the region where telluric emission lines are so strong that the residual
spectrum after sky subtraction is too noisy.
A line gets totally corrupted if it falls onto the telluric absorption
features of the A- and B-band.
Since the FDF galaxies have varying redshifts different lines are affected
in each case.
Therefore, the quality of each measurement was checked individually and we
give quality flags along with the line strengths and their (statistical)
errors in the Appendix.
The presented values are still not directly comparable to the central values
of local galaxies that were measured on the Lick system.
The reason for this is that the distant galaxies are apparently so small that
the slitlet of width 1
covered a large fraction of the light
distribution, typically 1-2 half-light radii
.
The observed line strengths are, therefore, only luminosity-weighted average
values, which are not equivalent to the central values if a radial gradient
across the galaxy exists.
To take this effect into account, the measured values were corrected based on
the logarithmic gradients as they were determined by Mehlert et al. (2003)
with high-S/N spectra of Coma cluster galaxies (see also Jørgensen et al. 1995).
The aperture of the 13 FDF galaxies were determined individually considering
both the slitwidth and the number of rows that were averaged by the Horne
extraction algorithm.
The same correction procedure was applied to the velocity dispersions, too.
Structural parameters as well as the total brightness of a galaxy were
measured on our HST/ACS images of the FDF.
With a mosaic of four pointings, almost the entire field of the FDF was imaged
through the F814W filter by the ACS/WFC with a total exposure time of 2360 s
per quadrant.
Analysis was performed on the pipeline-reduced images with an additional
cosmic ray rejection procedure.
The two-dimensional surface brightness distribution of a galaxy was fitted
using the GALFIT package of Peng et al. (2002) to determine total light and
half-light radius.
Four different fitting functions (pure de Vaucouleurs, pure Sersic with
variable exponent (n=1-4), and these two profiles in combination with an
exponential disk component) were applied.
Comparing the residual images and reduced -quality values, the best
fit to the observed light distribution of a galaxy was assessed,
which was in most cases the combination of a Sersic plus disk component.
To test the robustness of the profile analysis, 2D-surface brightness fits
were also performed using the GIM2D package of Simard et al. (2002).
Total magnitudes determined with the two methods agree well within the errors
as well as with those values derived with the SExtractor package (Bertin & Arnouts 1996)
applied on the groundbased I band images.
The magnitudes were calibrated onto the Vega system with a zeropoint of
25.478 and a color term of
0.042 (V-I) + 0.012 (V-I)2
(M. Sirianni, ESA STScI, priv. com.).
Aperture colors (2
diameter) were taken from our groundbased
photometry.
K-corrections were calculated by convolving respective filter transmission
curves with the redshifted SED of the elliptical template galaxy from
Kinney et al. (1996).
We checked for internal consistency by transforming observed F814W-magnitudes
into rest-frame Johnson-B as well as observed F ORS filter magnitudes
that best match the redshifted B-band (g for 0.2<z<0.3 and R for
0.3<z<0.7).
The derived
values agree well with each other (differences are
smaller than errors).
Structural parameters, total magnitudes as well as the distance luminosities for the cosmology we use are given in the Appendix.
To explore possible differences in galaxy evolution due to
environment, we investigate here in addition to the field galaxies
also early-type galaxies in clusters. Since the majority of the FDF
galaxies have redshifts around
,
we take
galaxies from three clusters at
:
Abell 370,
CL 0949+4409, MS 1512.4+3647. The image reduction and data
analysis was already published in Ziegler & Bender (1997); Bender et al. (1998); Saglia et al. (2000). In
contrast to Ziegler & Bender (1997), we here aperture correct line indices and
the velocity dispersions using the logarithmic gradients to be fully
consistent with the field ellipticals. All other corrections and the
derivation of the Lick line indices were performed as described above.
Half-light radii and total luminosities were determined on HST/WFPC2
images using the method described by Saglia et al. (1997). Combining de
Vaucouleurs and exponential surface brightness profile fits with their
procedure is equivalent to the GALFIT measurements used for the FDF
galaxies, so that the field and cluster samples can be directly
compared to each other here. Absolute magnitudes and pysical sizes
were re-calculated for the cosmology adopted here.
As the first of the scaling relations that combine the kinematics
(representing both luminous and dark mass) with the stellar populations
(baryons only) of galaxies we investigate the Faber-Jackson relation
(FJR, Faber & Jackson 1976). Here, the absolute B magnitude and the
velocity dispersion
of local cluster galaxies follow a tight
relation. In Fig. 1, we compare the distant galaxies to
the local sample of ellipticals in the Virgo and Coma clusters from
Dressler et al. (1987). The straight line is a principal component fit to
the nearby galaxies (Ziegler & Bender 1997). For this comparison, we use the
luminosities of the distant galaxies as derived from the ground-based
photometry in order to have a somewhat larger sample (trends are the
same when restricting to space-based magnitudes). The FDF data points
match the distribution of the distant cluster galaxies very well.
Both distant samples are tight and clearly offset to larger
luminosities with respect to the local FJR. At low
a few
galaxies with rather large offsets do exist in both samples.
Excluding these outliers, the 17 distant cluster galaxies are brighter
on average for given
by
with a standard deviation of 0.25. This general brightening is
consistent with expectations from passive evolution models if a formation
redshift of
is assumed.
For a simple stellar population the predicted B-band evolution of such an
object between z=0 (age 12 Gyr) and z=0.4 (age 8 Gyr)
is
(using models by Maraston (2005) for two times solar metallicity
and Salpeter initial mass function).
A similar offset is displayed by the ten FDF ellipticals:
with standard deviation of 0.27.
![]() |
Figure 1: Faber-Jackson relation between the central velocity dispersion and the absolute restframe B magnitude. Local early-type galaxies in the Virgo (small triangles) and Coma (small squares) clusters are from Dressler et al. (1987), the straight line indicates their principal components fit (Ziegler & Bender 1997). Both, ellipticals in distant clusters (large triangles) and in the field (large circles) follow a similar distribution, which is offset to larger luminosities. |
Open with DEXTER |
In Fig. 2, we present the Fundamental Plane (FP) in the
restframe B band. The edge-on projection is chosen in a way that
the distant-dependent parameter
(effective or half-light radius)
is plotted on the x-axis, whereas the distant-independent parameters
and
(effective surface brightness) are
displayed along the y-axis. The local sample which defines the FP
comprises both elliptical and S0 galaxies in the Coma cluster from
Dressler et al. (1987). Total luminosities and half-light radii of these
galaxies were determined by Saglia et al. (1993) in exactly the same manner
as our distant cluster galaxies by a simultaneous de Vaucouleurs and
exponential fit to the respective surface brightness profile. The
straight line indicates the principal components fit from
Bender et al. (1998).
The distant galaxy samples displayed here are reduced by two (FDF) and
six (clusters) objects with respect to the FJR analysis, because these
galaxies are not visible on the respective HST images. Not counting
the galaxy with the smallest and largest radius, the distant cluster
members are on average brighter by
with standard deviation of 0.19. This assumes that the slope
of the FP does not change within the
4 Gyr lookback-time. This
is equivalent to assuming that the structural parameters
and
do not change within that time and that only the luminosity
of the stellar populations evolves. The derived value of the global
luminosity increase is then again compatible with a pure passive
evolution of the bulk of the stars that have already old ages
(
8 Gyr) at the time of observations.
The sample of distant field galaxies lacks the large ellipticals and
cD galaxies present in clusters of galaxies. On the contrary, the
field sample encompasses some smaller galaxies. These objects display
the largest luminosity offsets from the local FP, but lie still within
from the mean offset of the cluster galaxies. Excluding
the two galaxies with "positive'' evolution, the average brightening
of the remaining nine FDF galaxies is
with standard deviation of 0.28.
The one outlying galaxy (object FDF 6336) has a very elongated and
distorted appearance on the HST image. Its very low Sersic index may
indicate the presence of a disk but the disk/bulge ratio is very
small. In the Kormendy diagram (relating
to
)
it is also
an outlier with either too faint a surface brightness or too small a
half-light radius. The same behaviour is exhibited by object
FDF 7796, which may have an overestimated
.
The combined
effect makes this galaxy look "normal'' in the FP. But its
appearance is again very elongated and distorted on the ACS image and
it may be associated with a companion galaxy. The galaxies which are
likely to be lenticular galaxies judging from their appearance do
follow the same distribution in the FP as the ellipticals without any
prominent disk.
![]() |
Figure 2: The Fundamental Plane in B seen edge-on. The local relation is a principal components fit to elliptical and S0 galaxies in Coma. Ellipticals in distant clusters are indicated by large triangles, distant field ellipticals by large circles. A typical errorbar is displayed in the upper left corner. |
Open with DEXTER |
![]() |
Figure 3:
Mgb-![]() ![]() |
Open with DEXTER |
Another scaling relation that combines the kinematics to the stellar
population is the Mgb-
relation (Bender et al. 1993).
In the following sections and figures we compare the distant galaxies
to the recent compilation of 124 early-type local galaxies by
Thomas et al. (2005) that contains both morphological classes E and S0 and
encompasses different environments from the field to clusters. They
combined galaxies with high S/N measurements of absorption line
strengths and velocity dispersions selected from
Beuing et al. (2002); González-González (1993); Mehlert et al. (2003) and Mendes de Oliveira et al. (2005). This compilation
has the advantage for us that very similar methods to our procedures
were used for the derivation of indices and
utilizing the
same correction functions. Furthermore, model parameters of the
stellar populations (see Sect. 6) are based on the same
code. Thus, possible systematic biases between the distant and local
samples are greatly reduced.
In Fig. 3, we show the Mgb-
relation of the local
sample with the distant galaxies overplotted. We linearily fit the
total local sample by a bisector, since differences between the
various local subpopulations are significantly smaller than the
average offset displayed by both distant samples. For fixed
,
Mgb values are smaller with a mean offset of -0.8 Å in both
cases. This can be interpreted as evolution in age and metallicity
(Bender et al. 1996). To further explore this, we will compare below the
measured equivalent widths of absorption features to evolutionary
models of stellar populations in so-called line diagnostic diagrams
(Sect. 5). In Sect. 6, we derive model ages and
metallicities and explore their dependence on mass
.
For galaxies with
km s-1, the relation of the distant
samples is as tight as for the local comparison galaxies. Smaller
galaxies display a larger scatter which is compatible with the result
by Thomas et al. (2005) that low-mass galaxies have more extended star
formation histories.
Ziegler & Bender (1997) speculated whether some of the cluster galaxies with low
Mgb for their
might be post-starburst E+A galaxies, where
a recent (
1.5 Gyr ago) starburst diluted the Mgb feature.
Those E+A galaxies are thought to be intermediate between spirals
and ellipticals in the sense that two spirals are transformed into an
early-type galaxy through merging. Although some local
representatives of this galaxy type still have discernible disks,
their surface brightness profiles are already bulge dominated
(Yang et al. 2004) and their kinematics pressure supported
(Norton et al. 2001). One of the characteristics of E+A galaxies is their
relatively strong absorption in the higher Balmer lines like H
compared to quiescent ellipticals. Unfortunately, we cannot address
this issue for the cluster galaxies, because our spectra do not cover
this "blue'' part. Also, the [O III] 5007 line, if present in
emission often used as indicator for ongoing star formation or for
correcting the H
absorption for possible emission fill-in, falls
into the atmospheric B band at redshifts around 0.4 and,
therefore, cannot be used for this investigation.
In the case of the FDF galaxies, we could measure H
for seven
objects. But only three (FDF 1161, 4285, 7459) of the four FDF
galaxies (& FDF 6439) with seemingly too low an Mgb for their
have H
measurements. The other four FDF objects with
known H
(FDF 5011, 6307, 6336, 7796) are distributed in the
Mgb-
plane like the other ones, although one of them
(FDF 7796) has rather strong H
.
Only one of the low-Mgb objects
(FDF 7459) has very strong H
absorption which would be in
compliance with a post-starburst model.
Absorption line strengths can be taken as measurements of the average
age and metallicity of a stellar population. Within the Lick/IDS
system, H
usually is taken as an indicator of age, whereas a
combination of Mgb, Fe 5270, and Fe 5335 yields information on the
mean metallicity [Z/H] (e.g. Faber et al. 1985; Trager et al. 1998). Since a
galaxy's spectrum is averaged over the slit aperture, any possible
radial gradient of the lines is smeared out and the measurements
represent luminosity-weighted mean values only. The first effect is
counterbalanced by correcting the observed line strength (see
Sect. 2) according to aperture size assuming the validity
of the locally determined radial gradients at higher redshifts. The
age and metallicity of a stellar population can be deduced by
comparing different indices in so-called line diagnostic diagrams to a
model grid. Minimizing the differences between observed and model
values, best fitting values for age and metallicity can be determined.
Before we investigate the fitted model parameters in
Sect. 6, we show the actual data in Figs. 4 and 5. As mentioned before, important absorption lines of a
galaxy can be redshifted into the regime of strong terrestrial
emission and absorption features, so that it is not possible to
measure them with sufficient accuracy. For our distant galaxies at
one or more of the four traditional indices are
affected. In order not to greatly reduce the samples, we do not
combine Fe 5270 and Fe 5335 to a mean Fe index
nor do we add Mgb to form [MgFe].
To investigate qualitatively age and metallicity effects, we plot in
Fig. 4 only H
versus Mgb with a model grid from Thomas et al. (2003)
that takes into
account the enhancement of Mg over Fe observed in most elliptical galaxies
by fixing [Mg/Fe] to 0.3.
The local cluster galaxies are mainly located in a region between solar and
thrice solar [Z/H] and old ages of 5-15 Gyrs (Thomas et al. 2005).
The error-weighted mean values for the local cluster galaxies are
1.76 Å for H
and 4.67 Å for Mgb.
The distant cluster ellipticals show a wide spread in Mgb with accordingly
sub- to supersolar metallicities.
The error-weighted mean values are 1.74 Å for H
and 4.28 Å for Mgb.
The statistics of the distant field ellipticals suffers from the low number
of eight galaxies.
Three galaxies have intermediate
populations similar to the mean of the distant cluster sample.
Two FDF galaxies have such low H
that they fall below the "oldest'' model
line (15 Gyr) and seem to have subsolar [Z/H],
whereas three other ones are compatible with ages of 2-3 Gyrs
with
[Z/H] assuming simple stellar populations (SSPs).
![]() |
Figure 4:
The same samples (and symbols) as in Fig. 3, displaying
H![]() ![]() ![]() |
Open with DEXTER |
To look for any possible enhancement of Mg over Fe with respect to the solar
abundance ratio
,
we plot in Fig. 5 the Fe 5335 index versus Mgb.
Local galaxies are located between
[Mg/Fe].
The distant objects scatter within the same range independent from being in
a cluster or field environment.
![]() |
Figure 5:
The same samples (and symbols) as in Fig. 3, displaying
Fe 5335 vs. Mgb to investigate the relative enrichment of Fe and Mg.
The lines connect model values from Thomas et al. (2003) for a constant age of 10 Gyr.
Solid lines correspond to
![]() |
Open with DEXTER |
We now discuss the model parameters for the galaxies. For each
individual galaxy best fitting model parameters were determined by a
minimization algorithm as explained in detail in Thomas et al. (2005). The
measured (
and aperture corrected) line strengths were
compared iteratively to predicted model line strengths where the
[Mg/Fe] ratio, metallicity [Z/H], and age was varied. Observed
strengths of Mgb and Fe 5335 were used in combination whenever their
measurement was trustworthy. These lines were simultanously fitted
together with one of the Balmer indices H
,
H
,
or H
(when
available). With that procedure we took into account the dependence
of these lines on the Mg/Fe ratio, which is in particular important
for the higher order Balmer lines (Thomas et al. 2004).
![]() |
Figure 6:
The samples (and symbols) as in Fig. 3
except that the local sample is now restricted to Category 1 (ages older
than ![]() ![]() |
Open with DEXTER |
In Fig. 6, we investigate the scaling relations of the
resulting stellar parameters with velocity dispersion found for local
early-type galaxies (Thomas et al. 2005). Ages, metallicities and
/Fe ratios correlate with
and therefore stellar mass.
For the comparison, we now only take the 86 galaxies from the local sample
classified as being old (
5 Gyr),
since the presented scaling relations were
derived for this object class (Category 1) only.
The local cluster and field samples differ slightly in the zeropoint
of the linear fits for ages and for metallicities. In
Fig. 6, we overplot the derived stellar population
parameters of the distant galaxies. For this comparison, we
add the respective look-back time to the age of a
galaxy.
For all three distributions, the distant samples nicely match the
local ones. The individual data points scatter around the local fit
lines, but not significantly larger than the local ones, and follow
the same trends. In particular, the local
/Fe-
relation is also obeyed by the distant galaxies. This is in
compliance with the derived ages that are younger only corresponding
to their look-back times. From this we can conclude that for the
majority of the local galaxies no significant star formation episodes
and no further chemical enrichment has taken place within the
last
5 Gyr. Slight differences (which are not significant) in
[Z/H]-
between the local and distant samples may be
attributed to the fact, that the aperture correction in the case of
the distant galaxies was done onto the nominal aperture whereas the
local galaxies were measured within 1/10th of
.
Since age and
/Fe have zero radial gradients in local galaxies
(Mehlert et al. 2003), only our [Z/H] determinations could be affected in
the sense that they are systematically slightly too low.
We have investigated the kinematic, photometric, structural and stellar population parameters of a sample of field galaxies with mean redshift of 0.4. The galaxies were drawn from the F ORS Deep Field on the basis of their early-type SED, i.e. their optical and near-infrared colours. It turned out that they all have indeed absorption-line spectra without any emission lines, i.e. without obvious indications of ongoing star formation. Their structure is mostly dominated by a bulge component, but a few galaxies clearly have disks resembling local lenticular S0 galaxies.
To learn about any possible evolution specific to the low-density
environment, we compare the FDF sample to galaxies in three rich
clusters at similar redshifts and to local samples both with field and
cluster galaxies. We first explored the luminosity evolution via
the Faber-Jackson and Fundamental Plane relations. Assuming that the
structure (size) and mass (velocity dispersion) of elliptical galaxies
do not change with time, we find consistently from both scaling
relations a very modest evolution in brightness that can be modeled by
passive evolution of a simple stellar population (SSP). The average
increase of the B luminosities by 0.3-0.5 mag is found for both
the distant field and cluster galaxies and can be explained by a high
formation redshift (
)
of their stars.
This result is consistent with those studies that also find a weak
luminosity evolution and little difference between field and cluster
ellipticals. In their FP study of 18 field early-type galaxies with
,
van Dokkum et al. (2001) also find that they are
brighter in B by 0.4 mag in general, which can be modeled with an
SSP only slightly younger than in the case of their cluster galaxies.
A similar conclusion was reached by Rusin et al. (2003), who measured an
increase by 0.5 mag at z=0.4. In contrast, other groups like
Treu et al. (2002) or Gebhardt et al. (2003) find that the evolution for field
galaxies is stronger and compatible with a significantly younger mean age
for their stellar populations than in the case of cluster galaxies.
Restricting their samples to galaxies around z=0.4,
amounts to 0.7-0.8 mag. The differences among the various studies
become even more prominent at higher z, where the latter authors
derive a much larger brightening for the field galaxies in comparison
to cluster members. The rather small differences at
may
arise from the still quite small numbers of observed galaxies per
sample. These small samples may therefore also be more affected by the
particular method to select the targets. Our FDF galaxies are, for
example, redder by 0.7 mag on average in observed (V-I) color than
the Gebhardt et al. sample, which has
for 5
ellipticals at 0.3<z<0.5, while both our field and cluster galaxies
have
.
This indicates that our field galaxies probably predominantly trace the
upper envelope of the age-
distribution.
Compatibility with SSP models of passive evolution is also found for
the majority of the FDF galaxies in the Mgb-
relation.
Their mean offset from the local relation is with -0.8 Å identical to the one for the distant cluster galaxies.
The local slope is followed by the distant galaxies as well
with a tendency for a slight steepening at the low-mass end.
This indicates that age cannot be the
principal driver of the main part of the relationship. Otherwise a
significant steepening also at the high-mass end would be detected, as
the Mgb index of younger objects,
that also have on average lower
according to the age-
correlation, would evolve
faster to lower index values with increasing redshift. This result is
in line with the conclusion drawn by several local studies
(e.g. Thomas et al. 2005; Kuntschner et al. 2002; Trager et al. 2000)
that the Mgb-
relation is driven by metallicity rather than by age.
To explore in
more detail the distribution in metallicity, chemical enrichment and
mean ages of the stellar systems, we compare diagnostic absorption
lines from the Lick system to SSP models. To this purpose we
simultaneously fit measured Mg, Fe, and Balmer indices by varying
model ages, [Z/H] metallicities, and [/Fe] ratios. We find a
broad range in these model parameters for both distant samples, field
and clusters. But their distributions are not random because all
three stellar population parameters scale with velocity dispersion,
and, therefore, to some degree with total mass, like their local
counterparts.
A passive evolution of cluster galaxies from z=0.5 to 0 was also
favoured by Jones et al. (2000) investigating the distribution of H
with
metal lines measured in co-added spectra. They detect no significant
difference between morphologically classified S0 and E galaxies
arguing also that the majority of S0 cluster members do not show any
evidence of recent star formation activity as expected in some
scenarios of galaxy transformation. Two of our FDF galaxies
(FDF 6336 & 7796) have extraordinary H
absorption leading to
young model ages under the assumption of a single stellar population.
Their strengths may also be explained in a scenario where these
galaxies had experienced a short intense star burst in their recent
past involving only a small fraction of their mass. The ACS images
reveal a significant disk component in these two galaxies, which may
point to a very early spiral morphology. In that case a low-level
continous star formation might be expected, but we do not detect any
emission lines.
Kelson et al. (2001) investigated the higher-order Balmer lines H
and
H
of early-type galaxies in three clusters at z=0.3,0.6 and 0.8
claiming that there is a correlation with
with small scatter.
Interpreting the offsets in the zeropoint for the three redshifts as
caused by an evolution in the mean age of the stellar systems, they
derive formation redshifts of
for SSP models assuming
co-evality for all galaxies. Since we only have four FDF galaxies
with high-quality measurements of the two indices (and none in the
cluster sample), we cannot confirm or exclude a correlation with
.
But the assumption that all galaxies have roughly the same
age must be rejected for our cluster galaxies.
![]() |
Figure 7: The Fundamental Plane as in Fig. 2 but restricted to those galaxies for which we could derive light-averaged model ages of their stellar population. The open symbols represent the data "corrected'' for the predicted brightening for each galaxy and nicely match the distribution of the local galaxies. |
Open with DEXTER |
We also investigated whether our findings derived from the Fundamental
Plane and the absorption line diagnosis with respect to luminosity evolution
are compatible with each other quantitatively.
We determine the model brightening of our galaxies from the predicted
restframe B magnitudes for the respective ages and lookback times
taking into account the derived [Z/H] metallicities and [/Fe] ratios
again using SSP models by Maraston (2005).
In Fig. 7, we make this comparison by subtracting the predicted
luminosity evolution from the observed data points.
The evolution-corrected distant FP falls onto the local relation and nicely
matches the distribution of the local galaxies.
The one outlier is galaxy FDF 7796 for which we speculated above
that it has undergone a recent starburst, thereby causing the light-averaged
age to be very low.
In summary, we conclude from our study that the evolution of the
stellar populations of the majority of early-type galaxies within the
last 3-6 Gyr is compatible with the scenario of passive evolution.
Since the /Fe-
and age-
relations have not
changed since z=0.4, no significant star formation and chemical
enrichment has occured since that time. To investigate certain issues
in more detail, like any possible difference between low- and
high-density environments, new studies with more objects are required
with high S/N measurements allowing the investigation of several
aspects (FP & line diagnostics) ideally out to higher redshifts.
For this, we are currently analyzing another sample of
distant absorption-line galaxies drawn from the William-Herschel Deep
Field for which we also have VLT spectra and HST/ACS imaging with a
similar quality as for the FDF galaxies presented here.
Acknowledgements
We thank the anonymous referee for a very quick response. We acknowledge fruitful discussions with all FDF consortium members, in particular with Dr. A. Gabasch, M. Pannella, Dr. R. P. Saglia, Dr. S. Seitz (all Munich & Garching) and Dr. K. Jäger and B. Gerken (Göttingen). We thank J. Fliri and A. Riffeser (Sternwarte Munich) for applying their cosmic ray removal algorithm to the HST/ACS images. We are grateful to Drs. J. Heidt, D. Mehlert and S. Noll (Landessternwarte Heidelberg) for performing the FDF spectroscopic observations and thank ESO and the Paranal staff and the STScI and ST-ECF staff for efficient support. We also thank the PI of the FORS project, Prof. I. Appenzeller (Heidelberg) and Prof. K. J. Fricke (Göttingen) for providing guaranteed time for this project. This work has been supported by the Volkswagen Foundation (I/76 520) and the BMBF/DLR (50 OR 0301) and made use of the ADS, astro-ph, and CDS data bases.
Table A.1:
Basic parameters of the FORS Deep Field galaxies.
1. ID from Heidt et al. (2003), 2. redshift, 3. luminosity distance, 4.
lookback time,
5. signal/noise,
6. velocity dispersion corrected for aperture (see Sect. 2),
7. its error,
8.-11. FORS total magnitudes (SExtractor's MAG_BEST),
12. absolute restframe B magnitude,
3., 4., and 12. for
,
,
H0 = 70 km s-1 Mpc-1.
Table A.2: Line strengths of fully corrected (see Sect. 2) Lick indices of the FORS Deep Field galaxies together with their errors and (by-eye) quality flags. Units are Å. Quality flags: 0: satisfactory, 1: a bit noisy, 2: in region of strong sky line, 6: affected by telluric B band, 7: affected by telluric A band, 8: affected by end of spectrum, 9: not visible.
Table A.3:
Fully corrected Fe 5335 Lick index and SSP model ages,
metallicities [Z/H], and Mg/Fe ratios [/Fe] of the FORS Deep Field
galaxies.
Average errors in
(age), [Z/H], and [
/Fe] are
0.18 dex, 0.2 dex, and 0.15 dex, respectively.
In Col. 8 it is indicated, which Balmer line was used
to derive the model age.
The last four columns are based on the HST/ACS images:
ACS total I magnitude (GALFIT),
mean surface brightness in B within
corrected
for cosmological dimming, and half-light radius (GALFIT).
Physical units for
,
,
H0 = 70 km s-1 Mpc-1.