483 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
Astrometric calibration and performance of the Dark Energy Camera
We characterize the ability of the Dark Energy Camera (DECam) to perform
relative astrometry across its 500~Mpix, 3 deg^2 science field of view, and
across 4 years of operation. This is done using internal comparisons of ~4x10^7
measurements of high-S/N stellar images obtained in repeat visits to fields of
moderate stellar density, with the telescope dithered to move the sources
around the array. An empirical astrometric model includes terms for: optical
distortions; stray electric fields in the CCD detectors; chromatic terms in the
instrumental and atmospheric optics; shifts in CCD relative positions of up to
~10 um when the DECam temperature cycles; and low-order distortions to each
exposure from changes in atmospheric refraction and telescope alignment. Errors
in this astrometric model are dominated by stochastic variations with typical
amplitudes of 10-30 mas (in a 30 s exposure) and 5-10 arcmin coherence length,
plausibly attributed to Kolmogorov-spectrum atmospheric turbulence. The size of
these atmospheric distortions is not closely related to the seeing. Given an
astrometric reference catalog at density ~0.7 arcmin^{-2}, e.g. from Gaia, the
typical atmospheric distortions can be interpolated to 7 mas RMS accuracy (for
30 s exposures) with 1 arcmin coherence length for residual errors. Remaining
detectable error contributors are 2-4 mas RMS from unmodelled stray electric
fields in the devices, and another 2-4 mas RMS from focal plane shifts between
camera thermal cycles. Thus the astrometric solution for a single DECam
exposure is accurate to 3-6 mas (0.02 pixels, or 300 nm) on the focal plane,
plus the stochastic atmospheric distortion.Comment: Submitted to PAS
Semliki Forest virus induced, immune mediated demyelination: the effect of irradiation
International audienceThe Dark Energy Camera has captured a large set of images as part of Science Verification (SV) for the Dark Energy Survey (DES). The SV footprint covers a large portion of the outer Large Magellanic Cloud (LMC), providing photometry 1.5 mag fainter than the main sequence turn-off of the oldest LMC stellar population. We derive geometrical and structural parameters for various stellar populations in the LMC disc. For the distribution of all LMC stars, we find an inclination of i = -38.14° ± 0.08° (near side in the north) and a position angle for the line of nodes of θ0 = 129.51° ± 0.17°. We find that stars younger than ∼4 Gyr are more centrally concentrated than older stars. Fitting a projected exponential disc shows that the scale radius of the old populations is R>4 Gyr = 1.41 ± 0.01 kpc, while the younger population has R = 0.72 ± 0.01 kpc. However, the spatial distribution of the younger population deviates significantly from the projected exponential disc model. The distribution of old stars suggests a large truncation radius of Rt = 13.5 ± 0.8 kpc. If this truncation is dominated by the tidal field of the Galaxy, we find that the LMC is {∼eq } 24^{+9}_{-6} times less massive than the encircled Galactic mass. By measuring the Red Clump peak magnitude and comparing with the best-fitting LMC disc model, we find that the LMC disc is warped and thicker in the outer regions north of the LMC centre. Our findings may either be interpreted as a warped and flared disc in the LMC outskirts, or as evidence of a spheroidal halo component
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
Evolution of Galaxy Luminosity Function Using Photometric Redshifts
We examine the impact of using photometric redshifts for studying the
evolution of both the global galaxy luminosity function (LF) and that for
different galaxy types. To this end we compare LFs obtained using photometric
redshifts from the CFHT Legacy Survey (CFHTLS) D1 field with those from the
spectroscopic survey VIMOS VLT Deep Survey (VVDS) comprising ~4800 galaxies. We
find that for z<2, in the interval of magnitudes considered by this survey, the
LFs obtained using photometric and spectroscopic redshifts show a remarkable
agreement. This good agreement led us to use all four Deep fields of CFHTLS
comprising ~386000 galaxies to compute the LF of the combined fields and
estimate directly the error in the parameters based on field-to-field
variation. We find that the characteristic absolute magnitude M* of Schechter
fits fades by ~0.7mag from z~1.8 to z~0.3, while the characteristic density
phi* increases by a factor of ~4 in the same redshift bin. We use the galaxy
classification provided by the template fitting program used to compute
photometric redshifts and split the sample into galaxy types. We find that
these Schechter parameters evolve differently for each galaxy type, an
indication that their evolution is a combination of several effects: galaxy
merging, star formation quenching and mass assembly. All these results are
compatible with those obtained by different spectroscopic surveys such as VVDS,
DEEP2 and zCosmos, which reinforces the fact that photometric redshifts can be
used to study galaxy evolution, at least for the redshift bins adopted so far.
This is of great interest since future very large imaging surveys containing
hundreds of millions of galaxies will allow to obtain important precise
measurements to constrain the evolution of the LF and to explore the dependence
of this evolution on morphology and/or color helping constrain the mechanisms
of galaxy evolution.Comment: 29 pages, 10 figures. Approved for publication in The Astronomical
Journa
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