598 research outputs found
Stellar Metallicity Gradients in SDSS galaxies
We infer stellar metallicity and abundance ratio gradients for a sample of
red galaxies in the Sloan Digital Sky Survey (SDSS) Main galaxy sample. Because
this sample does not have multiple spectra at various radii in a single galaxy,
we measure these gradients statistically. We separate galaxies into stellar
mass bins, stack their spectra in redshift bins, and calculate the measured
absorption line indices in projected annuli by differencing spectra in
neighboring redshift bins. After determining the line indices, we use stellar
population modeling from the EZ\_Ages software to calculate ages,
metallicities, and abundance ratios within each annulus. Our data covers the
central regions of these galaxies, out to slightly higher than . We
find detectable gradients in metallicity and relatively shallow gradients in
abundance ratios, similar to results found for direct measurements of
individual galaxies. The gradients are only weakly dependent on stellar mass,
and this dependence is well-correlated with the change of with mass.
Based on this data, we report mean equivalent widths, metallicities, and
abundance ratios as a function of mass and velocity dispersion for SDSS
early-type galaxies, for fixed apertures of 2.5 kpc and of 0.5 .Comment: 19 pages; 8 tables, 12 figures. Submitted to ApJ for publicatio
Galaxies in SDSS and DEEP2: a quiet life on the blue sequence?
In the six billion years between redshifts z=1 and z=0.1, galaxies change due
to the aging of their stellar populations, the formation of new stars, and
mergers with other galaxies. Here I explore the relative importance of these
various effects, finding that while mergers are likely to be important for the
red galaxy sequence they are unlikely to affect more than 10% of the blue
galaxy sequence. I compare the galaxy population at redshift z=0.1 from the
Sloan Digital Sky Survey to that at z=1 from the Deep Extragalactic
Evolutionary Probe 2. Galaxies are bluer at z=1: the blue sequence by about 0.3
mag and the red sequence by about 0.1 mag, in redshift z=0.1 (u-g) color. I
evaluate the change in color and in the luminosity functions of the two
sequences using some simplistic stellar population synthesis models. These
models indicate that the luminous end of the red sequence fades less than
passive evolution allows by about 0.2 mag. Due to a lack of luminous blue
progenitors, ``dry'' mergers betweeen red galaxies then must create the
luminous red population at z=0.1, if stellar population models are correct. The
blue sequence colors and luminosity function are consistent with a reduction in
the star-formation rate since redshift z=1 by a factor of about three, with no
change in the number density to within 10%. These results restrict the number
of blue galaxies that can fall onto the red sequence by any process, and in
particular suggest that if mergers are catastrophic events they must be rare
for blue galaxies.Comment: submitted to ApJ, summary and viewgraphs available at
http://cosmo.nyu.edu/blanton/deep2sdss
The Nature of LINER-like Emission in Red Galaxies
Passive red galaxies frequently contain warm ionized gas and have spectra
similar to low-ionization nuclear emission-line regions (LINERs). Here we
investigate the nature of the ionizing sources powering this emission, by
comparing nuclear spectroscopy from the Palomar survey with larger aperture
data from the Sloan Digital Sky Survey. We find the line emission in the
majority of passive red galaxies is spatially extended; the Halpha surface
brightness profile depends on radius (r) as r^(-1.28). We detect strong line
ratio gradients with radius in [N II]/Ha, [S II]/Ha, and [O III]/[S II],
requiring the ionization parameter to increase outwards. Combined with a
realistic gas density profile, this outward increasing ionization parameter
convincingly rules out AGN as the dominant ionizing source, and strongly favors
distributed ionizing sources. Sources that follow the stellar density profile
can additionally reproduce the observed luminosity-dependence of the line ratio
gradient. Post-AGB stars provide a natural ionization source candidate, though
they have an ionization parameter deficit. Velocity width differences among
different emission lines disfavor shocks as the dominant ionization mechanism,
and suggest that the interstellar medium in these galaxies contains multiple
components. We conclude that the line emission in most LINER-like galaxies
found in large aperture (>100pc) spectroscopy is not primarily powered by AGN
activity and thus does not trace the AGN bolometric luminosity. However, they
can be used to trace warm gas in these red galaxies.Comment: 27 pages, 25 figures, ApJ in press; v2: added Figure 20 and made
minor revisions matching the accepted versio
K-corrections and filter transformations in the ultraviolet, optical, and near infrared
Template fits to observed galaxy fluxes allow calculation of K-corrections
and conversions among observations of galaxies at various wavelengths. We
present a method for creating model-based template sets given a set of
heterogeneous photometric and spectroscopic galaxy data. Our technique,
non-negative matrix factorization, is akin to principle component analysis
(PCA), except that it is constrained to produce nonnegative templates, it can
use a basis set of models (rather than the delta function basis of PCA), and it
naturally handles uncertainties, missing data, and heterogeneous data
(including broad-band fluxes at various redshifts). The particular
implementation we present here is suitable for ultraviolet, optical, and
near-infrared observations in the redshift range 0 < z < 1.5. Since we base our
templates on stellar population synthesis models, the results are intepretable
in terms of approximate stellar masses and star-formation histories. We present
templates fit with this method to data from GALEX, Sloan Digital Sky Survey
spectroscopy and photometry, the Two-Micron All Sky Survey, the Deep
Extragalactic Evolutionary Probe and the Great Observatories Origins Deep
Survey. In addition, we present software for using such data to estimate
K-corrections and stellar masses.Comment: 43 pages, 20 figures, submitted to AJ, software and full-resolution
figures available at http://cosmo.nyu.edu/blanton/kcorrec
Improved background subtraction for the Sloan Digital Sky Survey images
We describe a procedure for background subtracting Sloan Digital Sky Survey
(SDSS) imaging that improves the resulting detection and photometry of large
galaxies on the sky. Within each SDSS drift scan run, we mask out detected
sources and then fit a smooth function to the variation of the sky background.
This procedure has been applied to all SDSS-III Data Release 8 images, and the
results are available as part of that data set. We have tested the effect of
our background subtraction on the photometry of large galaxies by inserting
fake galaxies into the raw pixels, reanalyzing the data, and measuring them
after background subtraction. Our technique results in no size-dependent bias
in galaxy fluxes up to half-light radii of 100 arcsec; in contrast, for
galaxies of that size the standard SDSS photometric catalog underestimates
fluxes by about 1.5 mag. Our results represent a substantial improvement over
the standard SDSS catalog results and should form the basis of any analysis of
nearby galaxies using the SDSS imaging data.Comment: accepted by the Astronomical Journa
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