585 research outputs found
Probing the Cosmic Star Formation History by Brightness Distribution of Gamma-Ray Bursts
Brightness distribution of Gamma-Ray Bursts (GRBs) is studied in detail under
the assumption that GRB rate is related to cosmic star formation rate. The two
populations of the long- and short-duration bursts in the 4B BATSE catalog are
analyzed separately. Taking account of current uncertainties in the
observational estimate of star formation rate (SFR), we have tried various
models of the cosmic star formation history and we find that the SFR evolution
in = 0--1 is strongly constrained by the GRB distribution if the standard
candle approximation is valid. The strong SFR evolution by a factor of
15 from = 0 to 1 inferred from UV observations is too steep to be
consistent with the GRB distribution for any distance scale of GRBs. Some
possibilities to reconcile this discrepancy are discussed, including the
intrinsic luminosity dispersion of GRBs and/or modification of star formation
history estimated by UV observations. We argue that SFR increase factor from
= 0 to 1 may be as low as about 4 if we choose different sets of
cosmological parameters and/or take account of the evolution of metallicity and
dust extinction in the UV data, and this would significantly remedy the
discrepancy.Comment: Accepted version by ApJ. Discussion on luminosity dispersion of GRBs
is newly added. 30 pages including 16 figure
Finding Galaxy Groups In Photometric Redshift Space: the Probability Friends-of-Friends (pFoF) Algorithm
We present a structure finding algorithm designed to identify galaxy groups
in photometric redshift data sets: the probability friends-of-friends (pFoF)
algorithm. This algorithm is derived by combining the friends-of-friends
algorithm in the transverse direction and the photometric redshift probability
densities in the radial dimension. The innovative characteristic of our
group-finding algorithm is the improvement of redshift estimation via the
constraints given by the transversely connected galaxies in a group, based on
the assumption that all galaxies in a group have the same redshift. Tests using
the Virgo Consortium Millennium Simulation mock catalogs allow us to show that
the recovery rate of the pFoF algorithm is larger than 80% for mock groups of
at least 2\times10^{13}M_{\sun}, while the false detection rate is about 10%
for pFoF groups containing at least net members. Applying the algorithm
to the CNOC2 group catalogs gives results which are consistent with the mock
catalog tests. From all these results, we conclude that our group-finding
algorithm offers an effective yet simple way to identify galaxy groups in
photometric redshift catalogs.Comment: AJ accepte
Galaxy Evolution, Deep Galaxy Counts and the Near-IR Cosmic Infrared Background
Accurate synthetic models of stellar populations are constructed and used in
evolutionary models of stellar populations in forming galaxies. Following their
formation, the late type galaxies are assumed to follow the Schmidt law for
star formation, while early type galaxies are normalized to the present-day
fundamental plane relations assumed to mimic the metallicity variations along
their luminosity sequence. We then compute predictions of these models for the
observational data at early epochs for various cosmological parameters and . We find good match to the metallicity data from the
damped systems and the evolution of the luminosity density out to
. Likewise, our models provide good fits for low values of
to the deep number counts of galaxies in all bands where data is available;
this is done without assuming existence of extra populations of galaxies at
high . Our models also match the data on the redshift distribution of galaxy
counts in and bands. We compute the predicted mean levels and angular
distribution of the cosmic infrared background produced from the early
evolution of galaxies. The predicted fluxes and fluctuations are still below
the current observational limits, but not by a large factor. Finally, we find
that the recent detection of the diffuse extragalactic light in the visible
bands requires for our models high redshift of galaxy formation, (3-4); otherwise the produced flux of the extragalactic light at optical
bands exceeds the current observational limits.Comment: Accepted to Ap
Mass Models for Spiral Galaxies from 2-D Velocity Maps
We model the mass distributions of 40 high surface brightness spiral galaxies
inside their optical radii, deriving parameters of mass models by matching the
predicted velocities to observed velocity maps. We use constant mass-to-light
disk and bulge models, and we have tried fits with no halo and with three
different halo density profiles. The data require a halo in most, but not all,
cases, while in others the best fit occurs with negligible mass in the luminous
component, which we regard as unphysical. All three adopted halo profiles lead
to fits of about the same quality, and our data therefore do not constrain the
functional form of the halo profile. The halo parameters display large
degeneracies for two of the three adopted halo functions, but the separate
luminous and dark masses are better constrained. However, the fitted disk and
halo masses vary substantially between the adopted halo models, indicating that
even high quality 2-D optical velocity maps do not provide significant
constraints on the dark matter content of a galaxy. We demonstrate that data
from longslit observations are likely to provide still weaker constraints. We
conclude that additional information is needed in order to constrain the
separate disk and halo masses in a galaxy.Comment: 41 pages, 13 figures, accepted for publication in A
Simulating the Hot X-ray Emitting Gas in Elliptical Galaxies
We study the chemo-dynamical evolution of elliptical galaxies and their hot
X-ray emitting gas using high-resolution cosmological simulations. Our Tree
N-body/SPH code includes a self-consistent treatment of radiative cooling, star
formation, supernovae feedback, and chemical enrichment. We present a series of
LCDM cosmological simulations which trace the spatial and temporal evolution of
heavy element abundance patterns in both the stellar and gas components of
galaxies. X-ray spectra of the hot gas are constructed via the use of the
vmekal plasma model, and analysed using XSPEC with the XMM EPN response
function. Simulation end-products are quantitatively compared with the
observational data in both the X-ray and optical regime. We find that radiative
cooling is important to interpret the observed X-ray luminosity, temperature,
and metallicity of the interstellar medium of elliptical galaxies. However,
this cooled gas also leads to excessive star formation at low redshift, and
therefore results in underlying galactic stellar populations which are too blue
with respect to observations.Comment: 6 pages, 3 figures, to appear in the proceedings of "The IGM/Galaxy
Connection - The Distribution of Baryons at z=0", ed. M. Putman & J.
Rosenberg; High resolution version is available at
http://astronomy.swin.edu.au/staff/dkawata/research/papers.htm
VLT Spectroscopy of Globular Clusters in Low Surface Brightness Dwarf Galaxies
We present VLT/FORS2 spectroscopic observations of globular clusters (GCs) in
five low surface brightness (LSB) dwarf galaxies: KK211 and KK221, which are
both dwarf spheroidal satellites (dSph) of NGC 5128, dSph KK84 located close to
the isolated S0 galaxy NGC 3115, and two isolated dwarf irregular (dIrr)
galaxies UGC 3755 and ESO 490-17. Our sample is selected from the Sharina et
al. (2005) database of Hubble Space Telescope WFPC2 photometry of GC candidates
in dwarf galaxies. For objects with accurate radial velocity measurements we
confirm 26 as genuine GCs out of the 27 selected candidates from our WFPC2
survey. Lick absorption line indices in the spectra of confirmed GCs and the
subsequent comparison with SSP model predictions show that all confirmed GCs in
dSphs are old, except GC KK211-3-149 (6 +/- 2 Gyr), which we consider to be the
nucleus of KK211. GCs in UGC 3755 and ESO 490-17 show a large spread in ages
ranging from old objects (t > 10 Gyr) to clusters with ages around 1 Gyr. Most
of our sample GCs have low metallicities [Z/H] <= -1. Two relatively metal-rich
clusters with [Z/H] ~ -0.3 are likely to be associated with NGC 3115. Our
sample GCs show in general a complex distribution of alpha-element enhancement
with a mean [alpha/Fe]=0.19 +/-0.04 derived with the chi2 minimization
technique and 0.18+/-0.12 dex computed with the iterative approach. These
values are slightly lower than the mean [alpha/Fe]=0.29+/-0.01 for typical
Milky Way GCs. We compare other abundance ratios with those of Local Group GCs
and find indications for systematic differences in N and Ca abundance. The
specific frequencies, S_N, of our sample galaxies are in line with the
predictions of a simple mass-loss model for dwarf galaxies and compare well
with S_N values found for dwarf galaxies in nearby galaxy clusters.Comment: accepted for publication in Ap
Molecular Gas in the Lensed Lyman Break Galaxy cB58
We have used the IRAM Plateau de Bure Interferometer to map CO(3-2) emission
from the gravitationally lensed Lyman break galaxy MS1512-cB58. This is the
first detection of a molecular emission line in any Lyman break system; its
integrated intensity implies a total molecular gas mass of 6.6e9 Msun, while
its width implies a dynamical mass of 1.0e10 csc^2i Msun (for a flat Lambda=0.7
cosmology). These estimates are in excellent concordance with nearly all
parameters of the system measured at other wavelengths, and yield a consistent
picture of past and future star formation with no obvious discrepancies
requiring explanation by differential lensing. In particular, we find that the
age and remaining lifetime of the current episode of star formation are likely
to be similar; the surface densities of star formation and molecular gas mass
are related by a Schmidt law; and the fraction of baryonic mass already
converted into stars is sufficient to account for the observed enrichment of
the interstellar medium to 0.4 Zsun. Barring substantial gas inflow or a major
merger, the stars forming in the current episode will have mass and coevality
at z=0 similar to those of a spiral bulge. Assuming cB58 is a typical Lyman
break galaxy apart from its magnification, its global parameters suggest that
the prescriptions for star formation used in some semi-analytic models of
galaxy evolution require moderate revision, although the general prediction
that gas mass fraction should increase with redshift is validated. [abridged]Comment: 41 pages, 6 figures, accepted by Ap
Virgo cluster early-type dwarf galaxies with the Sloan Digital Sky Survey. IV. The color-magnitude relation
We present an analysis of the optical colors of 413 Virgo cluster early-type
dwarf galaxies (dEs), based on Sloan Digital Sky Survey imaging data. Our study
comprises (1) a comparison of the color-magnitude relation (CMR) of the
different dE subclasses that we identified in Paper III of this series, (2) a
comparison of the shape of the CMR in low and high-density regions, (3) an
analysis of the scatter of the CMR, and (4) an interpretation of the observed
colors with ages and metallicities from population synthesis models. We find
that the CMRs of nucleated (dE(N)) and non-nucleated dEs (dE(nN)) are
significantly different from each other, with similar colors at fainter
magnitudes (r > 17 mag), but increasingly redder colors of the dE(N)s at
brighter magnitudes. We interpret this with older ages and/or higher
metallicities of the brighter dE(N)s. The dEs with disk features have similar
colors as the dE(N)s and seem to be only slightly younger and/or less
metal-rich on average. Furthermore, we find a small but significant dependence
of the CMR on local projected galaxy number density, consistently seen in all
of u-r, g-r, and g-i, and weakly i-z. We deduce that a significant intrinsic
color scatter of the CMR is present, even when allowing for a distance spread
of our galaxies. No increase of the CMR scatter at fainter magnitudes is
observed down to r = 17 mag (Mr = -14 mag). The color residuals, i.e., the
offsets of the data points from the linear fit to the CMR, are clearly
correlated with each other in all colors for the dE(N)s and for the full dE
sample. We conclude that there must be at least two different formation
channels for early-type dwarfs in order to explain the heterogeneity of this
class of galaxy. (Abridged)Comment: 17 pages + 12 figures. Accepted for publication in A
Cluster vs. Field Elliptical Galaxies and Clues on their Formation
Using new observations for a sample of 931 early-type galaxies we investigate
whether the \mg2--\so relation shows any dependence on the local environment.
The galaxies have been assigned to three different environments depending on
the local overdensity: clusters, groups, and field, having used our
completeredshift database to guide the assignment of galaxies. It is found that
cluster, group and field early-type galaxies follow almost identical \mg2--\so\
relations, with the largest \mg2 zero-point difference (clusters minus field)
being only mag. No correlation of the residuals is found with
the morphological type or the bulge to disk ratio. Using stellar population
models in a differential fashion, this small zero-point difference implies a
luminosity-weighted age difference of only Gyr between the
corresponding stellar populations, with field galaxies being younger. The
mass-weighted age difference could be significantly smaller, if minor events of
late star formation took place preferentially in field galaxies. We combine
these results with the existing evidence for the bulk of stars in cluster
early-type galaxies having formed at very high redshift, and conclude that the
bulk of stars in galactic spheroids had to form at high redshifts (z\gsim 3),
no matter whether such spheroids now reside in low or high density regions. The
cosmological implications of these findings are briefly discussed.Comment: 16 pages, 2 figures, accepted for publication in the ApJ.
Near-Infrared Imaging of Early-Type Galaxies IV. The Physical Origins of the Fundamental Plane Scaling Relations
The physical origins of the Fundamental Plane (FP) scaling relations are
investigated for early-type galaxies observed at optical and near-infrared
wavelengths. The slope for the FP is shown to increase systematically with
wavelength from the U-band through the K-band. A distance-independent
construction of the observables is described which provides an accurate
measurement of the change in the FP slope between any pair of bandpasses. The
variation of the FP slope with wavelength is strong evidence of systematic
variations in stellar content along the elliptical galaxy sequence. The
intercept of the diagnostic relationship between log(D_K/D_V) and log(sigma_0)
shows no significant dependence on environment within the uncertainties of the
Galactic extinction corrections, demonstrating the universality of the stellar
populations contributions at the level of Delta(V-K)=0.03 mag to the zero-point
of the global scaling relations.
Several other constraints on the properties of early-type galaxies --- the
slope of the Mg_2-sigma_0 relation, the effects of stellar populations
gradients, and deviations of early-type galaxies from a dynamically homologous
family --- are included to construct an empirical, self-consistent model which
provides a complete picture of the underlying physical properties which are
varying along the early-type galaxy sequence. This empirical approach
demonstrates that there are significant systematic variations in both age and
metallicity along the elliptical galaxy sequence, and that a small, but
systematic, breaking of dynamical homology (or a similar, wavelength
independent effect) is required. Predictions for the evolution of the slope of
the FP with redshift are described. [abriged]Comment: to appear in The Astronomical Journal; 40 pages, including 10
Postscript figures and 3 tables; uses AAS LaTeX style file
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