409 research outputs found
Are disappearing dwarfs just lying low ?
Recent redshift surveys have shown that the excess galaxies seen in faint
galaxy number counts (above those expected given the local galaxy luminosity
function) are not evolved giants at high redshifts, but low to moderate
luminosity objects at more modest redshifts. This has led to the suggestion
that there was once an additional population of dwarf galaxies which has since
disappeared, ie. there is non-conservation of galaxy number. Here we
investigate the possibility that these disappearing dwarfs have actually
evolved to become the population of very low surface brightness galaxies which
is now being detected in nearby clusters.Comment: 12 pages, 7 figures. Figures available from
http://www.phys.unsw.edu.au/~spd/bib.htm
Luminosity Distributions within Rich Clusters - III: A comparative study of seven Abell/ACO clusters
We recover the luminosity distributions over a wide range of absolute
magnitude (-24.5 < M_{R} < -16.5) for a sample of seven rich southern galaxy
clusters. We find a large variation in the ratio of dwarf to giant galaxies,
DGR: 0.8\le \le 3.1. This variation is shown to be inconsistent with a
ubiquitous cluster luminosity function. The DGR shows a smaller variation from
cluster to cluster in the inner regions (r \ls 0.56 Mpc). Outside these regions
we find the DGR to be strongly anti-correlated with the mean local projected
galaxy density with the DGR increasing towards lower densities. In addition the
DGR in the outer regions shows some correlation with Bautz-Morgan type. Radial
analysis of the clusters indicate that the dwarf galaxies are less centrally
clustered than the giants and form a significant halo around clusters. We
conclude that measurements of the total cluster luminosity distribution based
on the inner core alone are likely to be severe underestimates of the dwarf
component, the integrated cluster luminosity and the contribution of galaxy
masses to the cluster's total mass. Further work is required to quantify this.
The observational evidence that the unrelaxed, lower density outer regions of
clusters are dwarf-rich, adds credence to the recent evidence and conjecture
that the field is a predominantly dwarf rich environment and that the dwarf
galaxies are under-represented in measures of the local field luminosity
function.Comment: 31 pages including 11 figures. Also available from
http://star-www.st-and.ac.uk/~spd3/bib.htm
Luminosity Distributions within Rich Clusters - II: Demonstration and Verification via Simulation
We present detailed simulations of long exposure CCD images. The simulations
are used to explore the validity of the statistical method for reconstructing
the luminosity distribution of galaxies within a rich cluster i.e. by the
subtraction of field number-counts from those of a sight-line through the
cluster. In particular we use the simulations to establish the reliability of
our observational data presented in Paper 3. Based on our intended CCD
field-of-view (6.5 by 6.5 arcmins) and a 1-sigma detection limit of 26 mags per
sq arcsecond, we conclude that the luminosity distribution can be robustly
determined over a wide range of absolute magnitude (-23 < M_{R} < -16)
provided:
(a) the cluster has an Abell richness 1.5 or greater,
(b) the cluster's redshift lies in the range 0.1 < z < 0.3,
(c) the seeing is better than FWHM 1.25'' and
(d) the photometric zero points are accurate to within Delta m = \pm 0.12.
If these conditions are not met then the recovered luminosity distribution is
unreliable and potentially grossly miss-leading. Finally although the method
clearly has limitations, within these limitations the technique represents an
extremely promising probe of galaxy evolution and environmental dependencies.Comment: 24 pages, 8 figures accepted for publication in MNRAS also available
from http://star-www.st-and.ac.uk/~spd3/bib.htm
Optical/near-infrared colours of early-type galaxies and constraints on their star formation histories
(abridged) We introduce and discuss the properties of a theoretical
(B-K)-(J-K) integrated colour diagram for single-age, single-metallicity
stellar populations. This combination of integrated colours is able to largely
disentangle the well known age-metallicity degeneracy when the age of the
population is greater than ~300 Myr. We discuss in detail the effect on this
colour-colour diagram of alpha-enhanced metal abundance ratios, the presence of
blue horizontal branch stars unaccounted for in the theoretical calibration,
and of statistical colour fluctuations in low mass stellar systems. In the case
of populations with multiple stellar generations, the luminosity-weighted mean
age obtained from this diagram is shown to be heavily biased towards the
youngest stellar components. We apply this method to several datasets for which
optical and near-IR photometry are available in the literature. For the two
Local Group dwarf galaxies NGC185 and NGC6822, the mean ages derived from the
integrated colours are consistent with the star formation histories inferred
independently from photometric observations of their resolved stellar
populations. A sample of bright field and Virgo cluster elliptical galaxies is
found to exhibit a range of luminosity-weighted mean ages from 3 to 14 Gyr,
with a mean of 8 Gyr, independent of environment, and mean metallicities at or
just above the solar value. Colour gradients are found in all of the galaxies
studied, in the sense that central regions are redder. Aperture data for five
Virgo early-type dwarf galaxies show that these galaxies appear to be shifted
to lower mean metallicities and lower mean ages (range 1 to 6 Gyr) than their
higher luminosity counterparts.Comment: (1) Liverpool John Moores University, UK; (2) University of Cardiff,
UK; (3) University of Bristol, UK; (4) INAF-Osservatorio Astronomico di
Collurania, I; 12 pages, 9 figures, MNRAS in pres
Is the luminosity distribution of field galaxies really flat ?
Recent observations of the galaxy population within rich clusters have found
a characteristic luminosity distribution described by a flat (alpha = -1.0)
Schechter function which exhibits an upturn at faint absolute magnitudes (B Mag
= -18). Here we discuss whether such a form for the field luminosity
distribution is ruled out by local and/or faint magnitude limited redshift
surveys (MLRS).
Our conclusions are that existing redshift surveys provide little constraints
on the volume-density distribution of field galaxies faintwards of B Mag = -18.
The local MLRS suffer from poor statistics over inhomogeneous volumes, while
the faint MLRS are ambiguous because of the unknown nature of the ``faint blue
excess'' and the ``normalization'' problem.
Adopting a functional form similar to that seen in rich clusters we find that
the maximum allowable faint end slope, based on the Mt Stromlo-APM redshift
survey, is faintwards of B Mag = -18.0 (Ho = 50
km/s/Mpc^{3}).Comment: To appear in The Astrophysical Journal (Sept 20th, 1996). 27 pages
including 4 figures (gzipped uuencoded
Morphological number-count and redshift distributions to I < 26 from the Hubble Deep Field: Implications for the evolution of Ellipticals, Spirals and Irregulars
We combine the photometric redshift data of Fernandez-Soto et al. (1997) with
the morphological data of Odewahn et al. (1996) for all galaxies with I < 26.0
detected in the Hubble Deep Field. From this combined catalog we generate the
morphological galaxy number-counts and corresponding redshift distributions and
compare these to the predictions of high normalization zero- and passive-
evolution models. From this comparison we conclude the following: (1) E/S0s are
seen in numbers and over a redshift range consistent with zero- or minimal
passive- evolution to I = 24. Beyond this limit fewer E/S0s are observed than
predicted implying a net negative evolutionary process --- luminosity dimming,
disassembly or masking by dust --- at I > 24. (2) Spiral galaxies are present
in numbers consistent with zero- evolution predictions to I = 22. Beyond this
magnitude some net- positive evolution is required. Although the number-counts
are consistent with the passive-evolution predictions to I=26.0 the redshift
distributions favor number AND luminosity evolution. (3) There is no obvious
explanation for the late-type/irregular class and this category requires
further subdivision. While a small fraction of the population lies at low
redshift (i.e. true irregulars), the majority lie at redshifts, 1 < z < 3. At z
> 1.5 mergers are frequent and, taken in conjunction with the absence of normal
spirals at z > 2, the logical inference is that they represent the progenitors
of normal spirals forming via hierarchical merging.Comment: Accepted for publication in ApJ Letters, colour plates available from
http://www.phys.unsw.edu.au/~spd/bib.htm
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