309 research outputs found
Cross-correlation Weak Lensing of SDSS Galaxy Clusters III: Mass-to-light Ratios
We present measurements of the excess mass-to-light ratio measured
aroundMaxBCG galaxy clusters observed in the SDSS. This red sequence cluster
sample includes objects from small groups with masses ranging from ~5x10^{12}
to ~10^{15} M_{sun}/h. Using cross-correlation weak lensing, we measure the
excess mass density profile above the universal mean \Delta \rho(r) = \rho(r) -
\bar{\rho} for clusters in bins of richness and optical luminosity. We also
measure the excess luminosity density \Delta l(r) = l(r) - \bar{l} measured in
the z=0.25 i-band. For both mass and light, we de-project the profiles to
produce 3D mass and light profiles over scales from 25 kpc/ to 22 Mpc/h. From
these profiles we calculate the cumulative excess mass M(r) and excess light
L(r) as a function of separation from the BCG. On small scales, where \rho(r)
>> \bar{\rho}, the integrated mass-to-light profile may be interpreted as the
cluster mass-to-light ratio. We find the M/L_{200}, the mass-to-light ratio
within r_{200}, scales with cluster mass as a power law with index 0.33+/-0.02.
On large scales, where \rho(r) ~ \bar{\rho}, the M/L approaches an asymptotic
value independent of cluster richness. For small groups, the mean M/L_{200} is
much smaller than the asymptotic value, while for large clusters it is
consistent with the asymptotic value. This asymptotic value should be
proportional to the mean mass-to-light ratio of the universe . We find
/b^2_{ml} = 362+/-54 h (statistical). There is additional uncertainty in
the overall calibration at the ~10% level. The parameter b_{ml} is primarily a
function of the bias of the L <~ L_* galaxies used as light tracers, and should
be of order unity. Multiplying by the luminosity density in the same bandpass
we find \Omega_m/b^2_{ml} = 0.02+/-0.03, independent of the Hubble parameter.Comment: Third paper in a series; v2.0 incorporates ApJ referee's suggestion
Cross-correlation Weak Lensing of SDSS Galaxy Clusters I: Measurements
This is the first in a series of papers on the weak lensing effect caused by
clusters of galaxies in Sloan Digital Sky Survey. The photometrically selected
cluster sample, known as MaxBCG, includes ~130,000 objects between redshift 0.1
and 0.3, ranging in size from small groups to massive clusters. We split the
clusters into bins of richness and luminosity and stack the surface density
contrast to produce mean radial profiles. The mean profiles are detected over a
range of scales, from the inner halo (25 kpc/h) well into the surrounding large
scale structure (30 Mpc/h), with a significance of 15 to 20 in each bin. The
signal over this large range of scales is best interpreted in terms of the
cluster-mass cross-correlation function. We pay careful attention to sources of
systematic error, correcting for them where possible. The resulting signals are
calibrated to the ~10% level, with the dominant remaining uncertainty being the
redshift distribution of the background sources. We find that the profiles
scale strongly with richness and luminosity. We find the signal within a given
richness bin depends upon luminosity, suggesting that luminosity is more
closely correlated with mass than galaxy counts. We split the samples by
redshift but detect no significant evolution. The profiles are not well
described by power laws. In a subsequent series of papers we invert the
profiles to three-dimensional mass profiles, show that they are well fit by a
halo model description, measure mass-to-light ratios and provide a cosmological
interpretation.Comment: Paper I in a series; v2.0 includes ApJ referee's suggestion
SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar
We report the discovery of SDSSJ115517.35+634622.0, a previously unknown
gravitationally lensed quasar. The lens system exhibits two images of a quasar, with an image separation of 1{\farcs}832 \pm 0.007 . Near-IR
imaging of the system reveals the presence of the lensing galaxy between the
two quasar images. Based on absorption features seen in the Sloan Digital Sky
Survey (SDSS) spectrum, we determine a lens galaxy redshift of .
The lens is rather unusual in that one of the quasar images is only
0{\farcs}22\pm0{\farcs}07 () from the center of the
lens galaxy and photometric modeling indicates that this image is significantly
brighter than predicted by a SIS model. This system was discovered in the
course of an ongoing search for strongly lensed quasars in the dataset from the
SDSS.Comment: 18 pages, 6 figures. Accepted for publication in A
Ultraefficient Thermophotovoltaic Power Conversion By Band-Edge Spectral Filtering
Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical \u3e50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C
First Measurement of the Clustering Evolution of Photometrically-Classified Quasars
We present new measurements of the quasar autocorrelation from a sample of
\~80,000 photometrically-classified quasars taken from SDSS DR1. We find a
best-fit model of for the angular
autocorrelation, consistent with estimates from spectroscopic quasar surveys.
We show that only models with little or no evolution in the clustering of
quasars in comoving coordinates since z~1.4 can recover a scale-length
consistent with local galaxies and Active Galactic Nuclei (AGNs). A model with
little evolution of quasar clustering in comoving coordinates is best explained
in the current cosmological paradigm by rapid evolution in quasar bias. We show
that quasar biasing must have changed from b_Q~3 at a (photometric) redshift of
z=2.2 to b_Q~1.2-1.3 by z=0.75. Such a rapid increase with redshift in biasing
implies that quasars at z~2 cannot be the progenitors of modern L* objects,
rather they must now reside in dense environments, such as clusters. Similarly,
the duration of the UVX quasar phase must be short enough to explain why local
UVX quasars reside in essentially unbiased structures. Our estimates of b_Q are
in good agreement with recent spectroscopic results, which demonstrate the
implied evolution in b_Q is consistent with quasars inhabiting halos of similar
mass at every redshift. Treating quasar clustering as a function of both
redshift and luminosity, we find no evidence for luminosity dependence in
quasar clustering, and that redshift evolution thus affects quasar clustering
more than changes in quasars' luminosity. We provide a new method for
quantifying stellar contamination in photometrically-classified quasar catalogs
via the correlation function.Comment: 34 pages, 10 figures, 1 table, Accepted to ApJ after: (i) Minor
textual changes; (ii) extra points added to Fig.
Quasinormal modes of gravitational perturbation around a Schwarzschild black hole surrounded by quintessence
In this paper, the quasinormal modes of gravitational perturbation around a
Schwarzschild black hole surrounded by quintessence were evaluated by using the
third-order WKB approximation. Due to the presence of quintessence, the
gravitational wave damps more slowly
Crossing the cosmological constant line in a dilatonic brane-world model with and without curvature corrections
We construct a new brane-world model composed of a bulk -with a dilatonic
field-, plus a brane -with brane tension coupled to the dilaton-, cold dark
matter and an induced gravity term. It is possible to show that depending on
the nature of the coupling between the brane tension and the dilaton this model
can describe the late-time acceleration of the brane expansion (for the normal
branch) as it moves within the bulk. The acceleration is produced together with
a mimicry of the crossing of the cosmological constant line (w=-1) on the
brane, although this crossing of the phantom divide is obtained without
invoking any phantom matter neither on the brane nor in the bulk. The role of
dark energy is played by the brane tension, which reaches a maximum positive
value along the cosmological expansion of the brane. It is precisely at that
maximum that the crossing of the phantom divide takes place. We also show that
these results remain valid when the induced gravity term on the brane is
switched off.Comment: 12 pages, 2 figures, RevTeX
Robust Optical Richness Estimation with Reduced Scatter
Reducing the scatter between cluster mass and optical richness is a key goal
for cluster cosmology from photometric catalogs. We consider various
modifications to the red-sequence matched filter richness estimator of Rozo et
al. (2009), and evaluate their impact on the scatter in X-ray luminosity at
fixed richness. Most significantly, we find that deeper luminosity cuts can
reduce the recovered scatter, finding that sigma_lnLX|lambda=0.63+/-0.02 for
clusters with M_500c >~ 1.6e14 h_70^-1 M_sun. The corresponding scatter in mass
at fixed richness is sigma_lnM|lambda ~ 0.2-0.3 depending on the richness,
comparable to that for total X-ray luminosity. We find that including blue
galaxies in the richness estimate increases the scatter, as does weighting
galaxies by their optical luminosity. We further demonstrate that our richness
estimator is very robust. Specifically, the filter employed when estimating
richness can be calibrated directly from the data, without requiring a-priori
calibrations of the red-sequence. We also demonstrate that the recovered
richness is robust to up to 50% uncertainties in the galaxy background, as well
as to the choice of photometric filter employed, so long as the filters span
the 4000 A break of red-sequence galaxies. Consequently, our richness estimator
can be used to compare richness estimates of different clusters, even if they
do not share the same photometric data. Appendix 1 includes "easy-bake"
instructions for implementing our optimal richness estimator, and we are
releasing an implementation of the code that works with SDSS data, as well as
an augmented maxBCG catalog with the lambda richness measured for each cluster.Comment: Submitted to ApJ. 20 pages in emulateapj forma
Photometric Redshifts of Quasars
We demonstrate that the design of the Sloan Digital Sky Survey (SDSS) filter
system and the quality of the SDSS imaging data are sufficient for determining
accurate and precise photometric redshifts (``photo-z''s) of quasars. Using a
sample of 2625 quasars, we show that photo-z determination is even possible for
z<=2.2 despite the lack of a strong continuum break that robust photo-z
techniques normally require. We find that, using our empirical method on our
sample of objects known to be quasars, approximately 70% of the photometric
redshifts are correct to within delta z = 0.2; the fraction of correct
photometric redshifts is even better for z>3. The accuracy of quasar
photometric redshifts does not appear to be dependent upon magnitude to nearly
21st magnitude in i'. Careful calibration of the color-redshift relation to
21st magnitude may allow for the discovery of on the order of 10^6 quasars
candidates in addition to the 10^5 quasars that the SDSS will confirm
spectroscopically. We discuss the efficient selection of quasar candidates from
imaging data for use with the photometric redshift technique and the potential
scientific uses of a large sample of quasar candidates with photometric
redshifts.Comment: 29 pages, 8 figures, submitted to A
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