5,879 research outputs found
Photometric Response Functions of the SDSS Imager
The monochromatic illumination system is constructed to carry out in situ
measurements of the response function of the mosaicked CCD imager used in the
Sloan Digital Sky Survey (SDSS). The system is outlined and the results of the
measurements, mostly during the first 6 years of the SDSS, are described. We
present the reference response functions for the five colour passbands derived
from these measurements, and discuss column to column variations and variations
in time, and also their effects on photometry. We also discuss the effect
arising from various, slightly different response functions of the associated
detector systems that were used to give SDSS photometry. We show that the
calibration procedures of SDSS remove these variations reasonably well with the
resulting final errors from variant response functions being unlikely to be
larger than 0.01 mag for g, r, i, and z bands over the entire duration of the
survey. The considerable aging effect is uncovered in the u band, the response
function showing a 30% decrease in the throughput in the short wavelength side
during the survey years, which potentially causes a systematic error in
photometry. The aging effect is consistent with variation of the instrumental
sensitivity in u-band, which is calibrated out. The expected colour variation
is consistent with measured colour variation in the catalog of repeated
photometry. The colour variation is delta (u-g) ~ 0.01 for most stars, and at
most delta (u-g) ~ 0.02 mag for those with extreme colours. We verified in the
final catalogue that no systematic variations in excess of 0.01 mag are
detected in the photometry which can be ascribed to aging and/or seasonal
effects except for the secular u-g colour variation for stars with extreme
colours.Comment: 54 pages, 18 figures, 7 tables, accepted for publication in A
Testing gravity with motion of satellites around galaxies: Newtonian gravity against Modified Newtonian Dynamics
The motion of satellite galaxies around normal galaxies at distances 50-500
kpc provides a sensitive test for the theories. We study the surface density
and the velocities of satellites around isolated galaxies in the Sloan Digital
Sky Survey. We find that the surface number-density of satellites declines with
the projected distance as a power law with the slope -1.5-2. The rms velocities
gradually decline: observations exclude constant velocities at a 10 sigma
level. We show that observational data strongly favor the standard model: all
three major statistics of satellites - the number-density profile, the
line-of-sight velocity dispersion, and the distribution function of the
velocities -- agree remarkably well with the predictions of the standard
cosmological model. Thus, that the success of the standard model extends to
scales (50-500) kpc, much lower than what was previously considered. MOND fails
on these scales for models which assume any single power-law number-density
profile of satellites and any constant velocity anisotropy by predicting nearly
constant rms velocities of satellites. Satellite data can be fit by fine-tuned
models, which require (1) specific non-power-law density profile, (2) very
radial orbits at large distances (velocity anisotropy beta =0.6-0.7 at 200-300
kpc), and (3) 2-2.5 times more stellar mass than what is found in the galaxies.
The external gravity force - a necessary component for MOND -- makes the
situation even worse. We argue that a combination of satellite data and
observational constraints on stellar masses make these models very problematic.Comment: 14 pages, 7 figures. Accepted for publication in Ap
Properties of Disks and Bulges of Spiral and Lenticular Galaxies in the Sloan Digital Sky Survey
A bulge-disk decomposition is made for 737 spiral and lenticular galaxies
drawn from a SDSS galaxy sample for which morphological types are estimated. We
carry out the bulge-disk decomposition using the growth curve fitting method.
It is found that bulge properties, effective radius, effective surface
brightness, and also absolute magnitude, change systematically with the
morphological sequence; from early to late types, the size becomes somewhat
larger, and surface brightness and luminosity fainter. In contrast disks are
nearly universal, their properties remaining similar among disk galaxies
irrespective of detailed morphologies from S0 to Sc. While these tendencies
were often discussed in previous studies, the present study confirms them based
on a large homogeneous magnitude-limited field galaxy sample with morphological
types estimated. The systematic change of bulge-to-total luminosity ratio,
, along the morphological sequence is therefore not caused by disks but
mostly by bulges. It is also shown that elliptical galaxies and bulges of
spiral galaxies are unlikely to be in a single sequence. We infer the stellar
mass density (in units of the critical mass density) to be 0.0021 for
spheroids, i.e., elliptical galaxies plus bulges of spiral galaxies, and
0.00081 for disks.Comment: 30 pages, 9 figure
Spectral-based k-corrections and implications for the colour-magnitude relation of E/S0s and its evolution
We select a sample of 70378 E/S0 (early-type) galaxies at 0<z<0.36 from the
Sloan Digital Sky Survey, excluding disk and star-forming galaxies. We estimate
g and r magnitudes in the observer- and rest-frames directly from the SDSS DR6
spectra; this provides an object-by-object estimate of the k-correction. We use
the k-corrections from the spectra to study the evolution of the rest-frame
colour-magnitude relation (CMR) and colour-sigma (velocity dispersion) relation
(CsigmaR). The evolution is very sensitive to the k-correction. Both the CMR
and CsigmaR evolve blueward with increasing redshift, approximately in
agreement with passive evolution models with age approx. 12 Gyr. The slope and
zero-point of the CMR depend on whether colours were defined in fixed physical
or angular apertures, a consequence of the fact that the centers of these
objects tend to be redder: the relation is steeper for fixed angular apertures.
One the other hand, the CsigmaR slope does not show this dependence on the
aperture in which the colour was defined, suggesting that colour gradients are
correlated with residuals from the sigma-M_r relation. As these residuals are
age indicators, our findings suggest that colour gradients correlate with the
age of the stellar population.Comment: 16 pages, 24 figures, accepted for publication in MNRAS (minor
revisions and 3 new figures added following referees comments
The Role of Mergers in Early-type Galaxy Evolution and Black Hole Growth
Models of galaxy formation invoke the major merger of gas-rich progenitor
galaxies as the trigger for significant phases of black hole growth and the
associated feedback that suppresses star formation to create red spheroidal
remnants. However, the observational evidence for the connection between
mergers and active galactic nucleus (AGN) phases is not clear. We analyze a
sample of low-mass early-type galaxies known to be in the process of migrating
from the blue cloud to the red sequence via an AGN phase in the green valley.
Using deeper imaging from SDSS Stripe 82, we show that the fraction of objects
with major morphological disturbances is high during the early starburst phase,
but declines rapidly to the background level seen in quiescent early-type
galaxies by the time of substantial AGN radiation several hundred Myr after the
starburst. This observation empirically links the AGN activity in low-redshift
early-type galaxies to a significant merger event in the recent past. The large
time delay between the merger-driven starburst and the peak of AGN activity
allows for the merger features to decay to the background and hence may explain
the weak link between merger features and AGN activity in the literature.Comment: 5 pages, 3 figures. ApJ Letters, in press
Dependence of the optical/UV variability on the emission line properties and Eddington ratio in active galactic nuclei
The dependence of the long-term optical/UV variability on the spectral and
the fundamental physical parameters for radio-quiet active galactic nuclei
(AGNs) is investigated. The multi-epoch repeated photometric scanning data in
the Stripe-82 region of the Sloan Digital Sky Survey (SDSS) are exploited for
two comparative AGN samples (mostly quasars) selected therein, a broad-line
Seyfert\,1 (BLS1) type sample and a narrow-line Seyfert\,1 (NLS1) type AGN
sample within redshifts 0.3--0.8. Their spectral parameters are derived from
the SDSS spectroscopic data. It is found that on rest-frame timescales of
several years the NLS1-type AGNs show systematically smaller variability
compared to the BLS1-type. In fact, the variability amplitude is found to
correlate, though only moderately, with the Eigenvector\,1 parameters, i.e.,
the smaller the \hb\ linewidth, the weaker the [O\,III] and the stronger the
\feii\ emission, the smaller the variability amplitude is. Moreover, an
interesting inverse correlation is found between the variability and the
Eddington ratio, which is perhaps more fundamental. The previously known
dependence of the variability on luminosity is not significant, and that on
black hole mass---as claimed in recent papers and also present in our
data---fades out when controlling for the Eddington ratio in the correlation
analysis, though these may be partly due to the limited ranges of luminosity
and black hole mass of our samples. Our result strongly supports that an
accretion disk is likely to play a major role in producing the opitcal/UV
variability.Comment: 6 pages, 4 figures, 1 tables, accepted for publication in ApJ Lette
Combined effects of salicylic acid and furosemide and noise on hearing
<p>Abstract</p> <p>Background</p> <p>A major cause of the hearing loss following exposure to intense noise involves release of free radicals resulting from the elevated metabolism. The free radicals induce damage to several of the components of the cochlear amplifier including the outer hair cells and indirectly to the transduction currents. Salicylic acid induces a reversible hearing loss since it binds to the motor protein prestin in the outer hair cells, reducing electromotility. Furosemide also induces a reversible hearing loss since it reduces the endocochlear potential which is a major component of the cochlear transduction currents. On the other hand, each of these drugs also provides protection from a noise induced hearing loss if they are injected just before a noise exposure, probably as a result of the decreased metabolism induced in their presence, with release of lower levels of free radicals. In this study, both drugs were administered in order to assess whether their protective effects would be additive.</p> <p>Methods</p> <p>The study was conducted on normal hearing albino mice of the Sabra strain. They were injected with either salicylic acid alone (N = 11), or furosemide alone (N = 14), or both together (N = 14), or with saline control (N = 11) and exposed to broad band noise for 3.5 hours. An additional group of 9 mice was injected with both salicylic acid and furosemide, but not exposed to noise. The degree of the resulting hearing loss was assessed by recording thresholds of the auditory nerve brainstem evoked responses to broad band clicks before the injections and noise, and 7, 14 and 21 days after.</p> <p>Results</p> <p>The noise induced hearing loss in the mice injected with salicylic acid alone or furosemide alone was smaller than in those injected with saline, i.e. these drugs provided protection, as in previous studies in this laboratory. There was no threshold elevation after two weeks in the mice injected with both drugs without noise exposure, i.e. the effects of the two drugs given together was reversible. On the other hand, there was a significant hearing loss (i.e. threshold elevation) in the group which received both drugs and was also exposed to noise, with mean threshold elevations of 38.8 Ā± 19.0 dB and 28.3 Ā± 11.7 dB 7 days after noise exposure.</p> <p>Conclusions</p> <p>This result is very surprising, if not paradoxical. Drugs which provide protection from a noise induced hearing loss when administered alone, not only do not provide protection when given together, but also induce a greater hearing loss when accompanied by noise. This observation may be related to the finding that the depression of the endocochlear potential normally caused by furosemide is reduced in the presence of salicylic acid, so that the protection usually provided by furosemide is not present when it is administered together with salicylic acid. Thus it seems that each drug may interfere with the protective action of the other when coupled with noise.</p
An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect
We measure hot and cold spots on the microwave background associated with
supercluster and supervoid structures identified in the Sloan Digital Sky
Survey Luminous Red Galaxy catalog. The structures give a compelling visual
imprint, with a mean temperature deviation of 9.6 +/- 2.2 microK, i.e. above 4
sigma. We interpret this as a detection of the late-time Integrated Sachs-Wolfe
(ISW) effect, in which cosmic acceleration from dark energy causes
gravitational potentials to decay, heating or cooling photons passing through
density crests or troughs. In a flat universe, the linear ISW effect is a
direct signal of dark energy.Comment: 4 pages, 2 figures, accepted to ApJ Letters. Minor changes to match
accepted versio
The color gradients of spiral disks in the Sloan Digital Sky Survey
We investigate the radial color gradients of galactic disks using a sample of
about 20,000 face-on spiral galaxies selected from the fourth data release of
the Sloan Digital Sky Survey (SDSS-DR4). We combine galaxies with similar
concentration, size and luminosity to construct composite galaxies, and then
measure their color profiles by stacking the azimuthally averaged radial color
profiles of all the member galaxies. Except for the smallest galaxies (R_{50}<3
kpc), almost all galaxies show negative disk color gradients with mean g-r
gradient G_{gr}=-0.006 mag kpc^{-1} and r-z gradient G_{rz}=-0.018 mag
kpc^{-1}. The disk color gradients are independent of the morphological types
of galaxies and strongly dependent on the disk surface brightness \mu_{d}, with
lower surface brightness galactic disks having steeper color gradients. We
quantify the intrinsic correlation between color gradients and surface
brightness as G_{gr}=-0.011\mu_{d}+0.233 and G_{rz}=-0.015\mu_{d}+0.324. These
quantified correlations provide tight observational constraints on the
formation and evolution models of spiral galaxies.Comment: 20 pages, 5 figures, Accepted for publication in RAA (Research in
Astronomy and Astrophysics
Building the Galactic halo from globular clusters: evidence from chemically unusual red giants
We present a spectroscopic search for halo field stars that originally formed
in globular clusters. Using moderate-resolution SDSS-III/SEGUE-2 spectra of 561
red giants with typical halo metallicities (-1.8 < [Fe/H] < -1.0), we identify
16 stars, 3% of the sample, with CN and CH bandstrength behavior indicating
depleted carbon and enhanced nitrogen abundances relative to the rest of the
data set. Since globular clusters are the only environment known in which stars
form with this pattern of atypical light-element abundances, we claim that
these stars are second-generation globular cluster stars that have been lost to
the halo field via normal cluster mass-loss processes. Extrapolating from
theoretical models of two-generation globular cluster formation, this result
suggests that globular clusters contributed significant numbers of stars to the
construction of the Galactic halo: we calculate that a minimum of 17% of the
present-day mass of the stellar halo was originally formed in globular
clusters. The ratio of CN-strong to CN-normal stars drops with Galactocentric
distance, suggesting that the inner-halo population may be the primary
repository of these stars.Comment: 9 pages including 8 figures, A&A accepte
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