7,565 research outputs found
Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies
We combine state-of-the-art models for the production of stellar radiation
and its transfer through the interstellar medium (ISM) to investigate
ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in
star-forming galaxies. We start by assessing the reliability of our stellar
population synthesis modelling by fitting absorption-line indices in the
ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing
so, we find that neglecting stochastic sampling of the stellar initial mass
function in these young (-100 Myr), low-mass clusters affects
negligibly ultraviolet-based age and metallicity estimates but can lead to
significant overestimates of stellar mass. Then, we proceed and develop a
simple approach, based on an idealized description of the main features of the
ISM, to compute in a physically consistent way the combined influence of
nebular emission and interstellar absorption on ultraviolet spectra of
star-forming galaxies. Our model accounts for the transfer of radiation through
the ionized interiors and outer neutral envelopes of short-lived stellar birth
clouds, as well as for radiative transfer through a diffuse intercloud medium.
We use this approach to explore the entangled signatures of stars, the ionized
and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find
that, aside from a few notable exceptions, most standard ultraviolet indices
defined in the spectra of ISM-free stellar populations are prone to significant
contamination by the ISM, which increases with metallicity. We also identify
several nebular-emission and interstellar-absorption features, which stand out
as particularly clean tracers of the different phases of the ISM.Comment: 27 pages, 21 figures. Accepted for publication in MNRA
The Celestial Reference Frame at 24 and 43 GHz. II. Imaging
We have measured the sub-milli-arcsecond structure of 274 extragalactic
sources at 24 and 43 GHz in order to assess their astrometric suitability for
use in a high frequency celestial reference frame (CRF). Ten sessions of
observations with the Very Long Baseline Array have been conducted over the
course of 5 years, with a total of 1339 images produced for the 274
sources. There are several quantities that can be used to characterize the
impact of intrinsic source structure on astrometric observations including the
source flux density, the flux density variability, the source structure index,
the source compactness, and the compactness variability. A detailed analysis of
these imaging quantities shows that (1) our selection of compact sources from
8.4 GHz catalogs yielded sources with flux densities, averaged over the
sessions in which each source was observed, of about 1 Jy at both 24 and 43
GHz, (2) on average the source flux densities at 24 GHz varied by 20%-25%
relative to their mean values, with variations in the session-to-session flux
density scale being less than 10%, (3) sources were found to be more compact
with less intrinsic structure at higher frequencies, and (4) variations of the
core radio emission relative to the total flux density of the source are less
than 8% on average at 24 GHz. We conclude that the reduction in the effects due
to source structure gained by observing at higher frequencies will result in an
improved CRF and a pool of high-quality fiducial reference points for use in
spacecraft navigation over the next decade.Comment: 63 pages, 18 figures, 6 tables, accepted by the Astronomical Journa
The ages and metallicities of galaxies in the local universe
We derive stellar metallicities, light-weighted ages and stellar masses for a
magnitude-limited sample of 175,128 galaxies drawn from the Sloan Digital Sky
Survey Data Release Two (SDSS DR2). We compute median-likelihood estimates of
these parameters using a large library of model spectra at medium-high
resolution, covering a comprehensive range of star formation histories. The
constraints we derive are set by the simultaneous fit of five spectral
absorption features, which are well reproduced by our population synthesis
models. By design, these constraints depend only weakly on the alpha/Fe element
abundance ratio. Our sample includes galaxies of all types spanning the full
range in star formation activity, from dormant early-type to actively
star-forming galaxies. We show that, in the mean, galaxies follow a sequence of
increasing stellar metallicity, age and stellar mass at increasing 4000AA-break
strength (D4000). For galaxies of intermediate mass, stronger Balmer absorption
at fixed D4000 is associated with higher metallicity and younger age. We
investigate how stellar metallicity and age depend on total galaxy stellar
mass. Low-mass galaxies are typically young and metal-poor, massive galaxies
old and metal-rich, with a rapid transition between these regimes over the
stellar mass range 3x10^9<M/Msun<3x10^10. Both high- and low-concentration
galaxies follow these relations, but there is a large dispersion in stellar
metallicity at fixed stellar mass, especially for low-concentration galaxies of
intermediate mass. Despite the large scatter, the relation between stellar
metallicity and stellar mass is similar to the correlation between gas-phase
oxygen abundance and stellar mass for star-forming galaxies. [abriged]Comment: 22 pages, 14 figures, accepted for publication on MNRAS, data
available at http://www.mpa-garching.mpg.de/SDSS
Properties of the galaxy population in hydrodynamical simulations of clusters
We present a study of the galaxy population predicted by hydrodynamical
simulations for a set of 19 galaxy clusters based on the GADGET-2 Tree+SPH
code. These simulations include gas cooling, star formation, a detailed
treatment of stellar evolution and chemical enrichment, as well as SN energy
feedback in the form of galactic winds. We compute the spectro-photometric
properties of the simulated galaxies. All simulations have been performed for
two choices of the stellar initial mass function: a standard Salpeter IMF, and
a top-heavier IMF. Several of the observational properties of the galaxy
population in nearby clusters are reproduced fairly well by simulations. A
Salpeter IMF is successful in accounting for the slope and the normalization of
the color-magnitude relation for the bulk of the galaxy population. Simulated
clusters have a relation between mass and optical luminosity which generally
agrees with observations, both in normalization and slope. We find that
galaxies are generally bluer, younger and more star forming in the cluster
outskirts, thus reproducing the observational trends. However, simulated
clusters have a total number of galaxies which is significantly smaller than
the observed one, falling short by about a factor 2-3. Finally, the brightest
cluster galaxies are always predicted to be too massive and too blue, when
compared to observations, due to gas overcooling in the core cluster regions,
even in the presence of a rather efficient SN feedback.Comment: 15 pages, 17 figures, to appear in MNRA
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
Results from the Blazar Monitoring Campaign at the Whipple 10m Gamma-ray Telescope
In September 2005, the observing program of the Whipple 10 m gamma-ray
telescope was redefined to be dedicated almost exclusively to AGN monitoring.
Since then the five Northern Hemisphere blazars that had already been detected
at Whipple are monitored routinely each night that they are visible. Thanks to
the efforts of a large number of multiwavelength collaborators, the first year
of this program has been very successful. We report here on the analysis of
Markarian 421 observations taken from November, 2005 to May, 2006 in the
gamma-ray, X-ray, optical and radio bands.Comment: 4 pages; contribution to the 30th International Cosmic Ray
Conference, Merida, Mexico, July 200
Integrated Stellar Populations: Confronting Photometry with Spectroscopy
We investigate the ability of spectroscopic techniques to yield realistic
star formation histories (SFHs) for the bulges of spiral galaxies based on a
comparison with their observed broadband colors. Full spectrum fitting to
optical spectra indicates that recent (within ~1 Gyr) star formation activity
can contribute significantly to the V-band flux, whilst accounting for only a
minor fraction of the stellar mass budget which is made up primarily of old
stars. Furthermore, recent implementations of stellar population (SP) models
reveal that the inclusion of a more complete treatment of the thermally
pulsating asymptotic giant branch (TP-AGB) phase to SP models greatly increases
the NIR flux for SPs of ages 0.2-2 Gyr. Comparing the optical--NIR colors
predicted from population synthesis fitting, using models which do not include
all stages of the TP-AGB phase, to the observed colors reveals that observed
optical--NIR colors are too red compared to the model predictions. However,
when a 1 Gyr SP from models including a full treatment the TP-AGB phase is
used, the observed and predicted colors are in good agreement. This has strong
implications for the interpretation of stellar populations, dust content, and
SFHs derived from colors alone.Comment: 6 pages, 6 figures, accepted for publication in the Astrophysical
Journa
A fundamental plane for field star-forming galaxies
Star formation rate (SFR), metallicity and stellar mass are within the
important parameters of star--forming galaxies that characterize their
formation and evolution. They are known to be related to each other at low and
high redshift in the mass--metallicity, mass--SFR, and metallicity--SFR
relations. In this work we demonstrate the existence of a plane in the 3D space
defined by the axes SFR [log(SFR)(M_sun yr^-1)], gas metallicity [12+log(O/H)],
and stellar mass [log(M_star/M_sun)] of star-forming galaxies. We used
star--forming galaxies from the "main galaxy sample" of the Sloan Digital Sky
Survey--Data Release 7 (SDSS-DR7) in the redshift range 0.04 < z < 0.1 and
r-magnitudes between 14.5 and 17.77. Metallicities, SFRs, and stellar masses
were taken from the Max-Planck-Institute for Astrophysics-John Hopkins
University (MPA-JHU) emission line analysis database. From a final sample of
44214 galaxies, we find for the first time a fundamental plane for field
galaxies relating the SFR, gas metallicity, and stellar mass for star--forming
galaxies in the local universe. One of the applications of this plane would be
estimating stellar masses from SFR and metallicity. High redshift data from the
literature at redshift ~2.2 and 3.5, do not show evidence for evolution in this
fundamental plane.Comment: Accepted for publication in A&A. 4 pages, 4 Figures, and 2 online
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