162 research outputs found
Ages and Abundances of Red Sequence Galaxies as a Function of LINER Emission Line Strength
Although the spectrum of a prototypical early-type galaxy is assumed to lack
emission lines, a substantial fraction (likely as high as 30%) of nearby red
sequence galaxy spectra contain emission lines with line ratios characteristic
of low ionization nuclear emission-line regions (LINERs). We use spectra of
~6000 galaxies from the Sloan Digital Sky Survey (SDSS) in a narrow redshift
slice (0.06 < z < 0.08) to compare the stellar populations of red sequence
galaxies with and without LINER-like emission. The spectra are binned by
internal velocity dispersion and by emission properties to produce high S/N
stacked spectra. The recent stellar population models of R. Schiavon (2007)
make it possible to measure ages, [Fe/H], and individual elemental abundance
ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We
find that red sequence galaxies with strong LINER-like emission are
systematically 2-3.5 Gyr (10-40%) younger than their emission-free counterparts
at the same velocity dispersion. This suggests a connection between the
mechanism powering the emission (whether AGN, post-AGB stars, shocks, or
cooling flows) and more recent star formation in the galaxy. We find that mean
stellar age and [Fe/H] increase with velocity dispersion for all galaxies.
Elemental abundance [Mg/Fe] increases modestly with velocity dispersion in
agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly
with velocity dispersion than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar
for all galaxies. At fixed velocity dispersion, galaxies with fainter r-band
luminosities have lower [Fe/H] and older ages but similar abundance ratios
compared to brighter galaxies.Comment: 25 pages, 17 figures, Accepted for publication in ApJ as of 16 July
2007; acceptance status updated, paper unchange
The Dark Energy Survey Data Management System
The Dark Energy Survey collaboration will study cosmic acceleration with a
5000 deg2 griZY survey in the southern sky over 525 nights from 2011-2016. The
DES data management (DESDM) system will be used to process and archive these
data and the resulting science ready data products. The DESDM system consists
of an integrated archive, a processing framework, an ensemble of astronomy
codes and a data access framework. We are developing the DESDM system for
operation in the high performance computing (HPC) environments at NCSA and
Fermilab. Operating the DESDM system in an HPC environment offers both speed
and flexibility. We will employ it for our regular nightly processing needs,
and for more compute-intensive tasks such as large scale image coaddition
campaigns, extraction of weak lensing shear from the full survey dataset, and
massive seasonal reprocessing of the DES data. Data products will be available
to the Collaboration and later to the public through a virtual-observatory
compatible web portal. Our approach leverages investments in publicly available
HPC systems, greatly reducing hardware and maintenance costs to the project,
which must deploy and maintain only the storage, database platforms and
orchestration and web portal nodes that are specific to DESDM. In Fall 2007, we
tested the current DESDM system on both simulated and real survey data. We used
Teragrid to process 10 simulated DES nights (3TB of raw data), ingesting and
calibrating approximately 250 million objects into the DES Archive database. We
also used DESDM to process and calibrate over 50 nights of survey data acquired
with the Mosaic2 camera. Comparison to truth tables in the case of the
simulated data and internal crosschecks in the case of the real data indicate
that astrometric and photometric data quality is excellent.Comment: To be published in the proceedings of the SPIE conference on
Astronomical Instrumentation (held in Marseille in June 2008). This preprint
is made available with the permission of SPIE. Further information together
with preprint containing full quality images is available at
http://desweb.cosmology.uiuc.edu/wik
Accurate Atmospheric Parameters at Moderate Resolution Using Spectral Indices: Preliminary Application to the MARVELS Survey
Studies of Galactic chemical and dynamical evolution in the solar
neighborhood depend on the availability of precise atmospheric parameters
(Teff, [Fe/H] and log g) for solar-type stars. Many large-scale spectroscopic
surveys operate at low to moderate spectral resolution for efficiency in
observing large samples, which makes the stellar characterization difficult due
to the high degree of blending of spectral features. While most surveys use
spectral synthesis, in this work we employ an alternative method based on
spectral indices to determine the atmospheric parameters of a sample of nearby
FGK dwarfs and subgiants observed by the MARVELS survey at moderate resolving
power (R~12,000). We have developed three codes to automatically normalize the
observed spectra, measure the equivalent widths of the indices and, through the
comparison of those with values calculated with pre-determined calibrations,
derive the atmospheric parameters of the stars. The calibrations were built
using a sample of 309 stars with precise stellar parameters obtained from the
analysis of high-resolution FEROS spectra. A validation test of the method was
conducted with a sample of 30 MARVELS targets that also have reliable
atmospheric parameters from high-resolution spectroscopic analysis. Our
approach was able to recover the parameters within 80 K for Teff, 0.05 dex for
[Fe/H] and 0.15 dex for log g, values that are lower or equal to the typical
external uncertainties found between different high-resolution analyzes. An
additional test was performed with a subsample of 138 stars from the ELODIE
stellar library and the literature atmospheric parameters were recovered within
125 K for Teff, 0.10 dex for [Fe/H] and 0.29 dex for log g. These results show
that the spectral indices are a competitive tool to characterize stars with the
intermediate resolution spectra.Comment: Accepted for publication in AJ. Abstract edited to comply with arXiv
standards regarding the number of character
Do observed metallicity gradients of early-type galaxies support a hybrid formation scenario?
We measure radial gradients of the Mg2 index in 15 E-E/S0 and 14 S0 galaxies.
Our homogeneous data set covers a large range of internal stellar velocity
dispersions (2.0<logsigma<2.5) and Mg2 gradients (dMg2/dlogr/re* up to
-0.14mag/dex). We find for the first time, a noticeable lower boundary in the
relation between Mg2 gradient and sigma along the full range of sigma, which
may be populated by galaxies predominantly formed by monolithic collapse. At
high sigma, galaxies showing flatter gradients could represent objects which
suffered either important merging episodes or later gas accretion. These
processes contribute to the flattening of the metallicity gradients and their
increasing importance could define the distribution of the objects above the
boundary expected by the ``classical'' monolithic process. This result is in
marked contrast with previous works which found a correlation between
dMg2/dlogr/re* and sigma confined to the low mass galaxies, suggesting that
only galaxies below some limiting sigma were formed by collapse whereas the
massive ones by mergers. We show observational evidence that a hybrid scenario
could arise also among massive galaxies. Finally, we estimated d[Z/H] from Mg2
and Hbeta measurements and single stellar population models. The conclusions
remain the same, indicating that the results cannot be ascribed to age effects
on Mg2.Comment: 11 pages, 2 figures, to appear in ApJLetter
Line strengths of early-type galaxies
In this paper we present measurements of velocity dispersions and Lick
indices for 509 galaxies in the local Universe, based on high signal-to-noise,
long slit spectra obtained with the 1.52 m ESO telescope at La Silla. The
conversion of our measurements into the Lick/IDS system was carried out
following the general prescription of Worthey and Ottaviani 1997. Comparisons
of our measurements with those of other authors show, in general, good
agreement. We also examine the dependence between these indices (e.g., Hbeta,
Mg_2, Fe5270 and NaD) and the central velocity dispersion (sigma), and we find
that they are consistent with those previously reported in the literature.
Benefiting from the relatively large size of the sample, we are able to
investigate the dependence of these relations on morphology and environment,
here represented by the local galaxy density. We find that for metallic lines
these relations show no significant dependence on environment or morphology,
except in the case of NaD, which shows distinct behavior for E and S0. On the
other hand, the Hbeta-logsigma shows a significant difference as a function of
the local density of galaxies, which we interpret as being caused by the
truncation of star formation in high density environments. Comparing our
results with those obtained by other authors we find a few discrepancies,
adding to the ongoing debate about the nature of these relations. Finally, we
report that the scatter of the Mg indices versus sigma relations correlate with
Hbeta, suggesting that age may contribute to the scatter. Furthermore, this
scatter shows no significant dependence on morphology or environment. Our
results are consistent with the current downsizing model, where low mass
galaxies have an extended star formation history (abridged).Comment: 88 pages, 24 figures, to be published in AJ, for further information
see http://staff.on.br/ogand
Orientation bias of optically selected galaxy clusters and its impact on stacked weak-lensing analyses
Weak-lensing measurements of the averaged shear profiles of galaxy clusters binned by some proxy for cluster mass are commonly converted to cluster mass estimates under the assumption that these cluster stacks have spherical symmetry. In this paper, we test whether this assumption holds for optically selected clusters binned by estimated optical richness. Using mock catalogues created from N-body simulations populated realistically with galaxies, we ran a suite of optical cluster finders and estimated their optical richness. We binned galaxy clusters by true cluster mass and estimated optical richness and measure the ellipticity of these stacks. We find that the processes of optical cluster selection and richness estimation are biased, leading to stacked structures that are elongated along the line of sight. We show that weak-lensing alone cannot measure the size of this orientation bias. Weak-lensing masses of stacked optically selected clusters are overestimated by up to 3–6 per cent when clusters can be uniquely associated with haloes. This effect is large enough to lead to significant biases in the cosmological parameters derived from large surveys like the Dark Energy Survey, if not calibrated via simulations or fitted simultaneously. This bias probably also contributes to the observed discrepancy between the observed and predicted Sunyaev–Zel’dovich signal of optically selected clusters
- …