681 research outputs found
The Star Formation History of IZw18
The star formation history in IZw18 has been inferred from HST/WFPC2 archival
data. This is done by comparing the derived V, B-V and V, V-I color-magnitude
diagrams and luminosity functions with synthetic ones, based on various sets of
stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the
field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main
body is not experiencing its first episode of star formation. Instead, it has
been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol
per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year
per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body,
the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to
the shallower diagrams and the small number of resolved stars. The derived
range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The
IMF providing the best fit to the observed stellar populations in the main body
has a slope 1.5, much flatter than in any similar galaxy analyzed with the same
method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope
(2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical
Journa
The Star Formation Epoch of the Most Massive Early-Type Galaxies
We present new Keck spectroscopy of early-type galaxies in three galaxy
clusters at z~0.5. We focus on the fundamental plane (FP) relation, and combine
the kinematics with structural parameters determined from HST images. The
galaxies obey clear FP relations, which are offset from the FP of the nearby
Coma cluster due to passive evolution of the stellar populations. The z~0.5
data are combined with published data for 11 additional clusters at
0.18<z<1.28, to determine the evolution of the mean M/L(B) ratio of cluster
galaxies with masses M>10^11 M_sun, as implied by the FP. We find
dlog(M/L(B))/dz = -0.555+-0.042, stronger evolution than was previously
inferred from smaller samples. The observed evolution depends on the
luminosity-weighted mean age of the stars in the galaxies, the initial mass
function (IMF), selection effects due to progenitor bias, and other parameters.
Assuming a normal IMF but allowing for various other sources of uncertainty we
find z* = 2.01+-0.20 for the luminosity-weighted mean star formation epoch. The
main uncertainty is the slope of the IMF in the range 1-2 Solar masses: we find
z* = 4.0 for a top-heavy IMF with slope x=0. The M/L(B) ratios of the cluster
galaxies are compared to those of recently published samples of field
early-type galaxies at 0.32<z<1.14. Assuming that progenitor bias and the IMF
do not depend on environment we find that the present-day age of stars in
massive field galaxies is 4.1 +- 2.0 % (~0.4 Gyr) less than that of stars in
massive cluster galaxies, consistent with most, but not all, previous studies
of local and distant early-type galaxies. This relatively small age difference
is surprising in the context of expectations from ``standard'' hierarchical
galaxy formation models. [ABRIDGED]Comment: Accepted for publication in ApJ. Minor corrections to match published
versio
Halos of Spiral Galaxies. III. Metallicity Distributions
(Abriged) We report results of a campaign to image the stellar populations in
the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc
(projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the
Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended
halo populations are detected in all galaxies. The color-magnitude diagrams
appear to be completely dominated by giant-branch stars, with no evidence for
the presence of young stellar populations in any of the fields. We find that
the metallicity distribution functions are dominated by metal-rich populations,
with a tail extending toward the metal poor end. To first order, the overall
shapes of the metallicity distribution functions are similar to what is
predicted by simple, single-component model of chemical evolution with the
effective yields increasing with galaxy luminosity. However, metallicity
distributions significantly narrower than the simple model are observed for a
few of the most luminous galaxies in the sample. It appears clear that more
luminous spiral galaxies also have more metal-rich stellar halos. The
increasingly significant departures from the closed-box model for the more
luminous galaxies indicate that a parameter in addition to a single yield is
required to describe chemical evolution. This parameter, which could be related
to gas infall or outflow either in situ or in progenitor dwarf galaxies that
later merge to form the stellar halo, tends to act to make the metallicity
distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press
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
Morphological Evolution and the Ages of Early-Type Galaxies in Clusters
Morphological and spectroscopic studies of high redshift clusters indicate
that a significant fraction of present-day early-type galaxies was transformed
from star forming galaxies at z<1. On the other hand, the slow luminosity
evolution of early-type galaxies and the low scatter in their color-magnitude
relation indicate a high formation redshift of their stars. In this paper we
construct models which reconcile these apparently contradictory lines of
evidence, and we quantify the effects of morphological evolution on the
observed photometric properties of early-type galaxies in distant clusters. We
show that in the case of strong morphological evolution the apparent luminosity
and color evolution of early-type galaxies are similar to that of a single age
stellar population formed at z=infinity, irrespective of the true star
formation history of the galaxies. Furthermore, the scatter in age, and hence
the scatter in color and luminosity, is approximately constant with redshift.
These results are consequences of the ``progenitor bias'': the progenitors of
the youngest low redshift early-type galaxies drop out of the sample at high
redshift. We construct models which reproduce the observed evolution of the
number fraction of early-type galaxies in rich clusters and their color and
luminosity evolution simultaneously. Our modelling indicates that approx. 50%
of early-type galaxies were transformed from other galaxy types at z<1, and
their progenitor galaxies may have had roughly constant star formation rates
prior to morphological transformation. After correcting the observed evolution
of the mean M/L_B ratio for the maximum progenitor bias we find that the mean
luminosity weighted formation redshift of stars in early-type galaxies
z_*=2.0^{+0.3}_{-0.2} for Omega_m=0.3 and Omega_Lambda=0.7. [ABRIDGED]Comment: Accepted for publication in The Astrophysical Journal. 13 pages, 6
figure
Search for Blue Compact Dwarf Galaxies During Quiescence
Blue Compact Dwarf (BCD) galaxies are metal poor systems going through a
major starburst that cannot last for long. We have identified galaxies which
may be BCDs during quiescence (QBCD), i.e., before the characteristic starburst
sets in or when it has faded away. These QBCD galaxies are assumed to be like
the BCD host galaxies. The SDSS/DR6 database provides ~21500 QBCD candidates.
We also select from SDSS/DR6 a complete sample of BCD galaxies to serve as
reference. The properties of these two galaxy sets have been computed and
compared. The QBCD candidates are thirty times more abundant than the BCDs,
with their luminosity functions being very similar except for the scaling
factor, and the expected luminosity dimming associated with the end of the
starburst. QBCDs are redder than BCDs, and they have larger HII region based
oxygen abundance. QBCDs also have lower surface brightness. The BCD candidates
turn out to be the QBCD candidates with the largest specific star formation
rate (actually, with the largest H_alpha equivalent width). One out of each
three dwarf galaxies in the local universe may be a QBCD. The properties of the
selected BCDs and QBCDs are consistent with a single sequence in galactic
evolution, with the quiescent phase lasting thirty times longer than the
starburst phase. The resulting time-averaged star formation rate is low enough
to allow this cadence of BCD -- QBCD phases during the Hubble time.Comment: Accepted for publication in ApJ. 17 pages. 13 Fig
Probing the Neutron-Capture Nucleosynthesis History of Galactic Matter
The heavy elements formed by neutron capture processes have an interesting
history from which we can extract useful clues to and constraints upon both the
characteristics of the processes themselves and the star formation and
nucleosynthesis history of Galactic matter. Of particular interest in this
regard are the heavy element compositions of extremely metal-deficient stars.
At metallicities [Fe/H] <= -2.5, the elements in the mass region past barium (A
>= 130-140 have been found (in non carbon-rich stars) to be pure r-process
products. The identification of an environment provided by massive stars and
associated Type II supernovae as an r-process site seems compelling. Increasing
levels of heavy s-process (e.g., barium) enrichment with increasing
metallicity, evident in the abundances of more metal-rich halo stars and disk
stars, reflect the delayed contributions from the low- and intermediate-mass (M
\~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesis
component during the AGB phase of their evolution. New abundance data in the
mass region 60 <~ A <~ 130 is providing insight into the identity of possible
alternative r-process sites. We review recent observational studies of heavy
element abundances both in low metallicity halo stars and in disk stars,
discuss the observed trends in light of nucleosynthesis theory, and explore
some implications of these results for Galactic chemical evolution,
nucleosynthesis, and nucleocosmochronology.Comment: 47 pages, 2 tables, 11 figures; To appear in PAS
Chemical Evolution of the Galaxy Based on the Oscillatory Star Formation History
We model the star formation history (SFH) and the chemical evolution of the
Galactic disk by combining an infall model and a limit-cycle model of the
interstellar medium (ISM). Recent observations have shown that the SFH of the
Galactic disk violently variates or oscillates. We model the oscillatory SFH
based on the limit-cycle behavior of the fractional masses of three components
of the ISM. The observed period of the oscillation ( Gyr) is reproduced
within the natural parameter range. This means that we can interpret the
oscillatory SFH as the limit-cycle behavior of the ISM. We then test the
chemical evolution of stars and gas in the framework of the limit-cycle model,
since the oscillatory behavior of the SFH may cause an oscillatory evolution of
the metallicity. We find however that the oscillatory behavior of metallicity
is not prominent because the metallicity reflects the past integrated SFH. This
indicates that the metallicity cannot be used to distinguish an oscillatory SFH
from one without oscillations.Comment: 21 pages LaTeX, to appear in Ap
The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (III): Cosmological Implications
(Abridged) We have used HST WFPC2 and ground-based spectroscopy to measure
the integrated extragalactic background light (EBL) at optical wavelengths. We
have also computed the integrated light from individual galaxy counts in the
images used to measure the EBL and in the Hubble Deep Field. We find that the
flux in galaxies as measured by standard galaxy photometry methods has
generally been underestimated by about 50%. Further, we find that the total
flux in individually detected galaxies is a factor of 2 to 3 less than the EBL
at 3000--8000A. We show that a significant fraction of the EBL may come from
normal galaxies at z<4, which are simply undetectable as a result of
K-corrections and cosmological surface brightness dimming. This is consistent
with recent redshift surveys at z<4. In the context of some simple models, we
discuss the constraints placed by the EBL on the evolution of the luminosity
density at z>1. Based on our optical EBL and published UV and IR EBL
measurements, we estimate that the total EBL from 0.1--1000 microns is 100+/-20
nW/m^2/sr. If the total EBL were produced entirely by stellar nucleosynthesis,
then we estimate that the total baryonic mass processed through stars is
Omega_* = 0.0062 (+/- 0.0022) h^{-2}, which corresponds to 0.33+/-0.12 Omega_B
for currently favored values of the baryon density. This estimate is smaller by
roughly 7% if 7 h_{0.7} nW/m^2/sr of the total EBL comes from accretion onto
central black holes. This estimate of Omega_* suggests that the universe has
been enriched to a total metal mass of 0.21(+/-0.13) Z_sun Omega_B. Our
estimate is consistent with other measurements of the cumulative metal mass
fraction of stars, stellar remnants, and the intracluster medium of galaxy
clusters in the local universe.Comment: Accepted for publication in ApJ, 20 pages using emulateapj.sty,
version with higher resolution figures available at
http://www.astro.lsa.umich.edu/~rab/publications.html or at
http://nedwww.ipac.caltech.edu/level5/Sept01/Bernstein3/frames.htm
Corotation: its influence on the chemical abundance pattern of the Galaxy
A simple theory for the chemical enrichment of the Galaxy which takes into
account the effects of spiral arms on heavy elements output was developed. In
the framework of the model with the corotation close to the position of the Sun
in the Galaxy the observed abundance features are explained.Comment: LaTeX, 6 pages, 5 jpg figures, uses aastex.sty, submitted to ApJ Let
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