76 research outputs found
The Dependence of the Galaxy Luminosity Function on Environment
We present luminosity functions for galaxies in loose groups in the Las
Campanas Redshift Survey, differentiated by their environment (defined by the
line-of-sight velocity dispersion sigma of the host groups) and also by their
spectral type (emission or non-emission, defined by the equivalent width of the
3727-Angstrom [OII] line).
We find systematic variations in the Schechter parameters alpha and M* for
non-emission line galaxies over a range of 0 < sigma < 800 km/s. Alpha varies
from 0.20 to -0.91, indicating an increase in the steepness of the faint end
slope with increasing sigma. The accompanying variation in M* appears to be
accounted for by the intrinsic correlation with alpha and does not indicate a
significant physical variation in the bright end of the luminosity function.
For emission line galaxies, we find no significant systematic variation of the
luminosity function with the environment. Our results show that emission and
non-emission galaxies generally occupy two distinct regions in the alpha-M*
parameter space. From our luminosity functions, we derive the number ratios of
emission to non-emission galaxies as a function of environment and absolute
magnitude, showing that the relative abundance of non-emission line galaxies
generally increases for all magnitudes -23 < M_R < -17.5 towards high-sigma
environments, from ~80% to >90% at M_R = -22 and from ~10% to >50% at M_R = -18
(H_0 = 100 km s^{-1} Mpc^{-1} and q_0 = 0.5).Comment: 34 pages, 10 figures; accepted for publication in the Ap
Galaxy Luminosity Functions from Deep Spectroscopic Samples of Rich Clusters
Using a new spectroscopic sample and methods accounting for spectroscopic
sampling fractions that vary in magnitude and surface brightness, we present
R-band galaxy luminosity functions (GLFs) for six nearby galaxy clusters with
redshifts 4000 < cz < 20000 km/s and velocity dispersions 700 < sigma < 1250
km/s. In the case of the nearest cluster, Abell 1060, our sample extends to
M_R=-14 (7 magnitudes below M*), making this the deepest spectroscopic
determination of the cluster GLF to date. Our methods also yield composite GLFs
for cluster and field galaxies to M_R=-17 (M*+4), including the GLFs of
subsamples of star forming and quiescent galaxies. The composite GLFs are
consistent with Schechter functions (M*_R=-21.14^{+0.17}_{-0.17},
alpha=-1.21^{+0.08}_{-0.07} for the clusters, M*_R=-21.15^{+0.16}_{-0.16},
alpha=-1.28^{+0.12}_{-0.11} for the field). All six cluster samples are
individually consistent with the composite GLF down to their respective
absolute magnitude limits, but the GLF of the quiescent population in clusters
is not universal. There are also significant variations in the GLF of quiescent
galaxies between the field and clusters that can be described as a steepening
of the faint end slope. The overall GLF in clusters is consistent with that of
field galaxies, except for the most luminous tip, which is enhanced in clusters
versus the field. The star formation properties of giant galaxies are more
strongly correlated with the environment than those of fainter galaxies.Comment: 53 pages, 8 figures, 1 ASCII table; accepted for publication in Ap
Disentangling Morphology, Star Formation, Stellar Mass, and Environment in Galaxy Evolution
We present a study of the spectroscopic and photometric properties of
galaxies in six nearby clusters. We perform a partial correlation analysis on
our dataset to investigate whether the correlation between star formation rates
in galaxies and their environment is merely another aspect of correlations of
morphology, stellar mass, or mean stellar age with environment, or whether star
formation rates vary independently of these other correlations. We find a
residual correlation of ongoing star formation with environment, indicating
that even galaxies with similar morphologies, stellar masses, and mean stellar
ages have lower star formation rates in denser environments. Thus, the current
star formation gradient in clusters is not just another aspect of the
morphology-density, stellar mass-density, or mean stellar age-density
relations. Furthermore, the star formation gradient cannot be solely the result
of initial conditions, but must partly be due to subsequent evolution through a
mechanism (or mechanisms) sensitive to environment. Our results constitute a
true ``smoking gun'' pointing to the effect of environment on the later
evolution of galaxies.Comment: 31 pages, including 5 figures; accepted for publication in Ap
Can Early Type Galaxies Evolve from Fading the Disks of Late Types?
We examine whether early-type galaxies in clusters may have evolved from
later types by the fading of their disks (e.g., as a result of ram-pressure
stripping or strangulation) or by enhancement of the bulge luminosity (e.g.,
due to tidal interactions and mergers). For this purpose, we compare the bulge
and disk luminosities of early- and late-type galaxies and of galaxies at
different radial distances from the cluster center. We find that, in order for
early-type galaxies, including S0s, to have evolved from late-type galaxies,
their bulge luminosities must have been physically enhanced. Disk fading models
cannot explain the differences observed. We then show that galaxy bulges are
systematically brighter at small projected distances from the cluster center,
while disk luminosities are uncorrelated with cluster-centric distance. Our
results suggest that bulge enhancement, not disk fading, distinguishes early
from late types and is thus at least partially responsible for the
morphology-environment relation of bright cluster galaxies.Comment: 20 pages, including 5 figures, accepted for publication in Ap
The U-band Galaxy Luminosity Function of Nearby Clusters
Despite the great potential of the U-band galaxy luminosity function (GLF) to
constrain the history of star formation in clusters, to clarify the question of
variations of the GLF across filter bands, to provide a baseline for
comparisons to high-redshift studies of the cluster GLF, and to estimate the
contribution of bound systems of galaxies to the extragalactic near-UV
background, determinations have so far been hampered by the generally low
efficiency of detectors in the U-band and by the difficulty of constructing
both deep and wide surveys. In this paper, we present U-band GLFs of three
nearby, rich clusters to a limit of M_U=-17.5 (M*_U+2). Our analysis is based
on a combination of separate spectroscopic and R-band and U-band photometric
surveys. For this purpose, we have developed a new maximum-likelihood algorithm
for calculating the luminosity function that is particularly useful for
reconstructing the galaxy distribution function in multi-dimensional spaces
(e.g., the number of galaxies as a simultaneous function of luminosity in
different filter bands, surface brightness, star formation rate, morphology,
etc.), because it requires no prior assumptions as to the shape of the
distribution function.
The composite luminosity function can be described by a Schechter function
with characteristic magnitude M*_U=-19.82+/-0.27 and faint end slope
alpha_U=-1.09+/-0.18. The total U-band GLF is slightly steeper than the R-band
GLF, indicating that cluster galaxies are bluer at fainter magnitudes.
Quiescent galaxies dominate the cumulative U-band flux for M_U<-14. The
contribution of galaxies in nearby clusters to the U-band extragalactic
background is <1% Gyr^-1 for clusters of masses ~3*10^14 to 2*10^15 M_solar.Comment: 44 pages, 11 figures, accepted for publication in Ap
The Environmental Dependence of the Infrared Luminosity and Stellar Mass Functions
We investigate the dependence of the galaxy infrared luminosity function (LF)
and the associated stellar mass function (SMF) on environment and spectral type
using photometry from the Two Micron All Sky Survey and redshifts from the Las
Campanas Redshift Survey for galaxies brighter than M_J<-19+5 log h. In the
field environment, galaxies with emission lines have LFs with much steeper
faint end slopes (alpha_J=-1.39) than galaxies without emission lines
(alpha_J=-0.59). In the cluster environment, however, even the non-emission
line galaxies have a steep faint-end LF (alpha_J=-1.22). There is also a
significant (95%) difference between the overall cluster and field LFs, . All of these variations are more
pronounced in the SMFs, which we compute by relating the strength of the 4000 A
break in the optical spectra to a mass-to-light ratio.Comment: 14 pages, 13 figures, emulateapj style ApJ, post-referee. Very minor
changes, mostly typographical in natur
What Fraction of Gravitational Lens Galaxies Lie in Groups?
We predict how the observed variations in galaxy populations with environment
affect the number and properties of gravitational lenses in different
environments. Two trends dominate: lensing strongly favors early-type galaxies,
which tend to lie in dense environments, but dense environments tend to have a
larger ratio of dwarf to giant galaxies than the field. The two effects nearly
cancel, and the distribution of environments for lens and non-lens galaxies are
not substantially different (lens galaxies are slightly less likely than
non-lens galaxies to lie in groups and clusters). We predict that about 20% of
lens galaxies are in bound groups (defined as systems with a line-of-sight
velocity dispersion sigma in the range 200 < sigma < 500 km/s), and another
roughly 3% are in rich clusters (sigma > 500 km/s). Therefore at least roughly
25% of lenses are likely to have environments that significantly perturb the
lensing potential. If such perturbations do not significantly increase the
image separation, we predict that lenses in groups have a mean image separation
that is about 0.2'' smaller than that for lenses in the field and estimate that
20-40 lenses in groups are required to test this prediction with significance.
The tail of the distribution of image separations is already illuminating.
Although lensing by galactic potential wells should rarely produce lenses with
image separations theta >~ 6'', two such lenses are seen among 49 known lenses,
suggesting that environmental perturbations of the lensing potential can be
significant. Further comparison of theory and data will offer a direct probe of
the dark halos of galaxies and groups and reveal the extent to which they
affect lensing estimates of cosmological parameters.Comment: 32 pages, 6 embedded figures; accepted for publication in Ap
The Faint End of the Luminosity Function and Low Surface Brightness Galaxies
SHELS (Smithsonian Hectospec Lensing Survey) is a dense redshift survey
covering a 4 square degree region to a limiting R = 20.6. In the construction
of the galaxy catalog and in the acquisition of spectroscopic targets, we paid
careful attention to the survey completeness for lower surface brightness dwarf
galaxies. Thus, although the survey covers a small area, it is a robust basis
for computation of the slope of the faint end of the galaxy luminosity function
to a limiting M_R = -13.3 + 5logh. We calculate the faint end slope in the
R-band for the subset of SHELS galaxies with redshif ts in the range 0.02 <= z
< 0.1, SHELS_{0.1}. This sample contains 532 galaxies with R< 20.6 and with a
median surface brightness within the half light radius of SB_{50,R} = 21.82 mag
arcsec^{-2}. We used this sample to make one of the few direct measurements of
the dependence of the faint end of the galaxy luminosity function on surface
brightness. For the sample as a whole the faint end slope, alpha = -1.31 +/-
0.04, is consistent with both the Blanton et al. (2005b) analysis of the SDSS
and the Liu et al. (2008) analysis of the COSMOS field. This consistency is
impressive given the very different approaches of th ese three surveys. A
magnitude limited sample of 135 galaxies with optical spectroscopic reds hifts
with mean half-light surface brightness, SB_{50,R} >= 22.5 mag arcsec^{-2} is
unique to SHELS_{0.1}. The faint end slope is alpha_{22.5} = -1.52+/- 0.16.
SHELS_{0.1} shows that lower surface brightness objects dominate the faint end
slope of the l uminosity function in the field, underscoring the importance of
surface brightness limits in evaluating measurements of the faint end slope and
its evolution.Comment: 34 pages, 13 figures, 3 tables, Astronomical Journal, in press
(updated based on review
The spectral-type/luminosity and the spectral type/satellite-density relations in the 2dFGRS
We examine the relative fractions of passive (Type 1), quiet-SF (Type 2) and
active-SF (Type 3+4) galaxies as a function of luminosity and number of
neighbours in several volume limited samples selected from the 2dFGRS.
Neighbours are counted within 1 Mpc projected distance, and
1000 km s depth. We apply a maximum magnitude difference criterion and
require neighbours to be fainter than the galaxy itself. We show that, whatever
the environment, passive galaxies dominate in bright samples and active-SF
galaxies in faint samples, whereas quiet-SF galaxies never dominate. We further
show that in bright samples (M -- 5 ) the fraction of passive galaxies grows steadily with fainter neighbour
density, whereas in faint samples a threshold like dependence is observed. This
suggests that the spectral-type / density ( morphology / density)
relation extends to intermediate dense environment, but only in the surrounding
of luminous galaxies and that it reflects an enhancement of the number of
satellites rather than stronger clustering among galaxies themselves. Our
analysis indicates that, in general, luminosity is a good tracer of a galaxy
halo mass and that it dominates over environment (satellite density) in setting
the spectral type mix of a population. However, minority populations exist,
such as luminous SF galaxies and faint passive galaxies, whose luminosity is an
unfair tracer of halo mass.Comment: 8 pages, 5 figur
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