1,356 research outputs found
Cosmological Gamma-Ray Bursts and Evolution of Galaxies
Evolution of the rate density of cosmological gamma-ray bursts (GRBs) is
calculated and compared to the BATSE brightness distribution in the context of
binary neutron-star mergers as the source of GRBs, taking account of the
realistic star formation history in the universe and evolution of compact
binary systems. We tried two models of the evolution of cosmic star formation
rate (SFR): one is based on recent observations of SFRs at high redshifts,
while the other is based on a galaxy evolution model of stellar population
synthesis that reproduces the present-day colors of galaxies. It is shown that
the binary merger scenario of GRBs naturally results in the comoving
rate-density evolution of \propto (1+z)^{2-2.5} up to z ~ 1, that has been
suggested independently from the compatibility between the number-brightness
distribution and duration-brightness correlation. If the cosmic SFR has its
peak at z ~ 1--2 as suggested by recent observations, the effective power-index
of GRB photon spectrum, \alpha >~ 1.5$ is favored, that is softer than the
recent observational determination of \alpha = 1.1 \pm 0.3. However, high
redshift starbursts (z >~ 5) in elliptical galaxies, that have not yet been
detected, can alleviate this discrepancy. The redshift of GRB970508 is likely
about 2, just below the upper limit that is recently determined, and the
absorption system at z = 0.835 seems not to be the site of the GRB.Comment: ApJ Lett. in press, very minor change just making clear that the
predicted rate-density evolution is in a comoving sense. (Received 1997 May
15; Accepted 1997 July 2
A Robust Age Indicator for Old Stellar Populations
We derive new spectral H_gamma index definitions which are robust age
indicators for old and relatively old stellar populations and thus have great
potential for solving the age-metallicity degeneracy of galaxy spectra. To
study H_gamma as a function of age, metallicity and resolution, we used a new
spectral synthesis model which predicts SEDs of single-age, single-metallicity
stellar populations at resolution FWHM=1.8A (which can be smoothed to different
resolutions), allowing direct measurements of the equivalent widths of
particular absorption features. We find that the H_gamma strong age
disentangling power strongly depends strongly on the adopted resolution and
galaxy velocity dispersion. We propose a system of indices which are completely
insensitive to metallicity and stable against resolution, allowing the study of
galaxies up to ~300 km/s. Observational spectra of very high S/N and relatively
high dispersion, are required to gain this unprecedented age discriminating
power. Once such spectra are obtained, accurate and reliable estimates for the
luminosity-weighted average stellar ages of these galaxies will become possible
for the first time, without assessing their metallicities. We measured this
index for two globular clusters, a number of low-luminosity elliptical galaxies
and a standard S0 galaxy. We find a large spread in the average stellar ages of
a sample of low-luminosity ellipticals. In particular these indices yield 4 Gyr
for M32, in agreement with the age provided by an extraordinary fit to the full
spectrum of this galaxy that we achieve here.Comment: 22 pages, 4 figures. ApJ, in press. Models and details can be found
at http://www.ioa.s.u-tokyo.ac.jp/~vazdekis
The Ages of Dwarf Ellipticals
We present narrow band photometry of 91 dwarf ellipticals in the Coma and
Fornax clusters taken through the Stromgren (uvby) filter system. Dividing the
sample by dwarf morphology into nucleated (dEN) and non-nucleated (dE) dwarfs
reveals two distinct populations of early-type systems based on integrated
colors. The class of dEN galaxies are redder in their continuum colors as
compared to bright cluster ellipticals and dE type dwarfs, and their position
in multi-color diagrams can only be explained by an older mean age for their
underlying stellar populations. By comparison with the narrow band photometry
of the M87 globular cluster system (Jordan et al. 2002), we find that dENs are
a higher metallicity continuation of the old, metal-poor color sequence of
galactic globulars and the blue population of M87 globulars. Bright ellipticals
and dE dwarfs, on the other hand, follow the color sequence of the metal-rich,
red population of M87 globulars. A comparison to SED models, convolved to a
simple metallicity model, finds that dENs and blue globulars are 3 to 4 Gyrs
older than cluster ellipticals and 5 Gyrs older than dE type galaxies. The
implication is that globulars and dEN galaxies are primordial and have
metallicities set by external constraints such as the enrichment of their
formation clouds. Bright ellipticals and dE galaxies have metallicities and
ages that suggest an extended phase of initial star formation to produce a
younger mean age, even if their formation epoch is similar to that of dENs and
blue globulars, and an internally driven chemical evolutionary history.Comment: 13 pages AAS LaTeX, 6 figures, accepted for publication in A
Extended dust emission and atomic hydrogen, a reservoir of diffuse H_2 in NGC 1068
We report on sensitive sub-mm imaging observations of the prototype
Seyfert~2/starburst galaxy NGC 1068 at 850 m and 450 m using the
Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell
Telescope (JCMT). We find clear evidence of dust emission associated with the
extended HI component which together with the very faint CO J=1--0
emission give a gas-to-dust ratio of . This contrasts with the larger ratio estimated within a galactocentric radius of kpc, where the
gas is mostly molecular and starburst activity occurs. The large gas-to-dust
ratio found for the starburst region is attributed to a systematic overestimate
of the molecular gas mass in starburst environments when the luminosity of the
CO J=1--0 line and a standard galactic conversion factor is used. On
the other hand sub-mm imaging proves to be a more powerful tool than
conventional CO imaging for revealing the properties of the diffuse
that coexists with HI. This molecular gas phase is characterized by low
densities ( cm), very faint emission from
sub-thermally excited CO, and contains more mass than HI, namely .Comment: Accepted for publication in the Astrophysical Journal Letter
Does the Number Density of Elliptical Galaxies Change at z<1?
We have performed a detailed V/Vmax test for a sample of the Canada-France
Redshift Survey (CFRS) for the purpose of examining whether the comoving number
density of field galaxies changes significantly at redshifts of z<1. Taking
into account the luminosity evolution of galaxies which depends on their
morphological type through different history of star formation, we obtain
\sim 0.5 in the range of 0.3<z<0.8, where reliable redshifts were
secured by spectroscopy of either absorption or emission lines for the CFRS
sample. This indicates that a picture of mild evolution of field galaxies
without significant mergers is consistent with the CFRS data. Early-type
galaxies, selected by their (V-I)_{AB} color, become unnaturally deficient in
number at z>0.8 due to the selection bias, thereby causing a fictitious
decrease of . We therefore conclude that a reasonable choice of upper
bound of redshift z \sim 0.8 in the V/Vmax test saves the picture of passive
evolution for field ellipticals in the CFRS sample, which was rejected by
Kauffman, Charlot, & White (1996) without confining the redshift range.
However, about 10% of the CFRS sample consists of galaxies having colors much
bluer than predicted for irregular galaxies, and their \avmax is significantly
larger than 0.5. We discuss this population of extremely blue galaxies in terms
of starburst that has just turned on at their observed redshifts.Comment: 11 pages including 3 figures, to appear in ApJ Letter
Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant
We show that evolution of the luminosity density of galaxies in the universe
provides a powerful test for the geometry of the universe. Using reasonable
galaxy evolution models of population synthesis which reproduce the colors of
local galaxies of various morphological types, we have calculated the
luminosity density of galaxies as a function of redshift . Comparison of the
result with recent measurements by the Canada-France Redshift Survey in three
wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the
\Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the
lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if
\Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0,
\lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however
ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels
quoted apply unless the standard assumptions on galaxy evolution are
drastically violated. We have also calculated a global star formation rate in
the universe to be compared with the observed rate beyond z \sim 2. We find
from this comparison that spiral galaxies are formed from material accretion
over an extended period of a few Gyrs, while elliptical galaxies are formed
from initial star burst at z >~ 5 supplying enough amount of metals and
ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ
Letter
A correlation between light profile and [Mg/Fe] abundance ratio in early-type galaxies
We explore possible correlations between light profile shapes, as
parameterized by the Sersic index or the concentration index C_re(1/3), and
relevant stellar population parameters in early-type galaxies. Mean luminosity
weighted ages, metallicities and abundance ratios were obtained from spectra of
very high signal-to-noise and stellar population models that synthesize galaxy
spectra at the resolution given by their velocity dispersions, in combination
with an age indicator that is virtually free of the effects of metallicity. We
do not find any significant correlation between the Sersic index (or C_re(1/3))
and mean age or metallicity, but a strong positive correlation of the shape
parameters with [Mg/Fe] abundance ratio. This dependence is as strong as the
[Mg/Fe] vs. velocity dispersion and C_re(1/3) vs. velocity dispersion
relations. We speculate that early-type galaxies settle up their structure on
time-scales in agreement with those imposed by their [Mg/Fe] ratios. This
suggest that the global structure of larger galaxies, with larger [Mg/Fe]
ratios and shorter time-scales, was already at place at high z, without
experiencing a significant time evolution.Comment: 5 pages, 3 figures (to appear in The Astrophysical Journal Letters
Galaxy number counts in the Hubble Deep Field as a strong constraint on a hierarchical galaxy formation model
Number counts of galaxies are re-analyzed using a semi-analytic model (SAM)
of galaxy formation based on the hierarchical clustering scenario. We have
determined the astrophysical parameters in the SAM that reproduce observations
of nearby galaxies, and used them to predict the number counts and redshifts of
faint galaxies for three cosmological models for (1) the standard cold dark
matter (CDM) universe, (2) a low-density flat universe with nonzero
cosmological constant, and (3) a low-density open universe with zero
cosmological constant. The novelty of our SAM analysis is the inclusion of
selection effects arising from the cosmological dimming of surface brightness
of high-redshift galaxies, and also from the absorption of visible light by
internal dust and intergalactic \ion{H}{1} clouds. Contrary to previous SAM
analyses which do not take into account such selection effects, we find, from
comparison with observed counts and redshifts of faint galaxies in the Hubble
Deep Field (HDF), that the standard CDM universe is {\it not} preferred, and a
low-density universe either with or without cosmological constant is favorable,
as suggested by other recent studies. Moreover, we find that a simple
prescription for the time scale of star formation (SF), being proportional to
the dynamical time scale of the formation of the galactic disk, is unable to
reproduce the observed number- redshift relation for HDF galaxies, and that the
SF time scale should be nearly independent of redshift, as suggested by other
SAM analyses for the formation of quasars and the evolution of damped
Ly- systems.Comment: 16 pages, 13 figures, LaTeX, using emulateapj5.st
Unavoidable Selection Effects in the Analysis of Faint Galaxies in the Hubble Deep Field: Probing the Cosmology and Merger History of Galaxies
(Abridged) We present a detailed analysis of the number count and photometric
redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying
a special attention to the selection effects including the cosmological dimming
of surface brightness of galaxies. We find a considerably different result from
previous studies ignoring the selection effects, and these effects should
therefore be taken into account in the analysis. We find that the model of pure
luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe
predicts much smaller counts than those observed at faint magnitude limits by a
factor of more than 10, so that a very strong number evolution of galaxies with
\eta > 3-4 must be invoked to reproduce the I_{814} counts, when parametrized
as \phi^* \propto (1+z)^\eta. However we show that such a strong number
evolution under realistic merging processes of galaxies can not explain the
steep slope of the B_{450} and V_{606} counts, and it is seriously inconsistent
with their photometric redshift distribution. We find that these difficulties
still persist in an open universe with \Omega_0 > 0.2, but are resolved only
when we invoke a -dominated flat universe, after examining various
systematic uncertainties in modeling the formation and evolution of galaxies.
The present analysis revitalizes the practice of using faint number counts as
an important cosmological test, giving one of the arguments against the EdS
universe and suggests acceleration of the cosmic expansion by vacuum energy
density. While a modest number evolution of galaxies with \eta ~ 1 is still
necessary even in a Lambda-dominated universe, a stronger number evolution with
\eta > 1 is rejected from the HDF data, giving a strong constraint on the
merger history of galaxies.Comment: 24 pages, 15 figures, final version matching publication in ApJ. Some
references added. The complete ps file of Table 3 is available at
http://th.nao.ac.jp/~totani/images/paper/ty2000-table3.p
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