261 research outputs found
First measurement of gravitational lensing by cosmic voids in SDSS
We report the first measurement of the diminutive lensing signal arising from
matter underdensities associated with cosmic voids. While undetectable
individually, by stacking the weak gravitational shear estimates around 901
voids detected in SDSS DR7 by Sutter et al. (2012a), we find substantial
evidence for a depression of the lensing signal compared to the cosmic mean.
This depression is most pronounced at the void radius, in agreement with
analytical models of void matter profiles. Even with the largest void sample
and imaging survey available today, we cannot put useful constraints on the
radial dark-matter void profile. We invite independent investigations of our
findings by releasing data and analysis code to the public at
https://github.com/pmelchior/void-lensingComment: 6 pages, 5 figures, as accepted by MNRA
A Galaxy Photometric Redshift Catalog for the Sloan Digital Sky Survey Data Release 6
We present and describe a catalog of galaxy photometric redshifts (photo-z's)
for the Sloan Digital Sky Survey (SDSS) Data Release 6 (DR6). We use the
Artificial Neural Network (ANN) technique to calculate photo-z's and the
Nearest Neighbor Error (NNE) method to estimate photo-z errors for ~ 77 million
objects classified as galaxies in DR6 with r < 22. The photo-z and photo-z
error estimators are trained and validated on a sample of ~ 640,000 galaxies
that have SDSS photometry and spectroscopic redshifts measured by SDSS, 2SLAQ,
CFRS, CNOC2, TKRS, DEEP, and DEEP2. For the two best ANN methods we have tried,
we find that 68% of the galaxies in the validation set have a photo-z error
smaller than sigma_{68} =0.021 or $0.024. After presenting our results and
quality tests, we provide a short guide for users accessing the public data.Comment: 16 pages, 12 figure
Cross-correlation Weak Lensing of SDSS galaxy Clusters II: Cluster Density Profiles and the Mass--Richness Relation
We interpret and model the statistical weak lensing measurements around
130,000 groups and clusters of galaxies in the Sloan Digital Sky Survey
presented by Sheldon et al. 2007 (Paper I). We present non-parametric
inversions of the 2D shear profiles to the mean 3D cluster density and mass
profiles in bins of both optical richness and cluster i-band luminosity. We
correct the inferred 3D profiles for systematic effects, including non-linear
shear and the fact that cluster halos are not all precisely centered on their
brightest galaxies. We also model the measured cluster shear profile as a sum
of contributions from the brightest central galaxy, the cluster dark matter
halo, and neighboring halos. We infer the relations between mean cluster virial
mass and optical richness and luminosity over two orders of magnitude in
cluster mass; the virial mass at fixed richness or luminosity is determined
with a precision of 13% including both statistical and systematic errors. We
also constrain the halo concentration parameter and halo bias as a function of
cluster mass; both are in good agreement with predictions of LCDM models. The
methods employed here will be applicable to deeper, wide-area optical surveys
that aim to constrain the nature of the dark energy, such as the Dark Energy
Survey, the Large Synoptic Survey Telescope and space-based surveys
Photometric Redshift Probability Distributions for Galaxies in the SDSS DR8
We present redshift probability distributions for galaxies in the SDSS DR8
imaging data. We used the nearest-neighbor weighting algorithm presented in
Lima et al. 2008 and Cunha et al. 2009 to derive the ensemble redshift
distribution N(z), and individual redshift probability distributions P(z) for
galaxies with r < 21.8. As part of this technique, we calculated weights for a
set of training galaxies with known redshifts such that their density
distribution in five dimensional color-magnitude space was proportional to that
of the photometry-only sample, producing a nearly fair sample in that space. We
then estimated the ensemble N(z) of the photometric sample by constructing a
weighted histogram of the training set redshifts. We derived P(z) s for
individual objects using the same technique, but limiting to training set
objects from the local color-magnitude space around each photometric object.
Using the P(z) for each galaxy, rather than an ensemble N(z), can reduce the
statistical error in measurements that depend on the redshifts of individual
galaxies. The spectroscopic training sample is substantially larger than that
used for the DR7 release, and the newly added PRIMUS catalog is now the most
important training set used in this analysis by a wide margin. We expect the
primary source of error in the N(z) reconstruction is sample variance: the
training sets are drawn from relatively small volumes of space. Using
simulations we estimated the uncertainty in N(z) at a given redshift is 10-15%.
The uncertainty on calculations incorporating N(z) or P(z) depends on how they
are used; we discuss the case of weak lensing measurements. The P(z) catalog is
publicly available from the SDSS website.Comment: 29 pages, 9 figures, single colum
Studying Inter-Cluster Galaxy Filaments Through Stacking GMBCG Galaxy Cluster Pairs
We present a method to study the photometric properties of galaxies in
filaments by stacking the galaxy populations between pairs of galaxy clusters.
Using Sloan Digital Sky Survey data, this method can detect the inter-cluster
filament galaxy overdensity with a significance of out to
. Using this approach, we study the color and luminosity
distribution of filament galaxies as a function of redshift. Consistent with
expectation, filament galaxies are bimodal in their color distribution and
contain a larger blue galaxy population than clusters. Filament galaxies are
also generally fainter than cluster galaxies. More interestingly, the observed
filament population seems to show redshift evolution at : the blue
galaxy fraction has a trend to increase at higher redshift: a filament "Butcher
Oemler Effect". We test the dependence of the observed filament density on the
richness of the cluster pair: richer clusters are connected by higher density
filaments. We also test the spatial dependence of filament galaxy overdensity:
this quantity decreases when moving away from the inter-cluster axis between a
cluster pair. This method provides an economical way to probe the photometric
properties of filament galaxies and should prove useful for upcoming projects
like the Dark Energy Survey.Comment: Submitted to Ap
Dynamical Confirmation of SDSS Weak Lensing Scaling Laws
Galaxy masses can be estimated by a variety of methods; each applicable in
different circumstances, and each suffering from different systematic
uncertainties. Confirmation of results obtained by one technique with analysis
by another is particularly important. Recent SDSS weak lensing measurements of
the projected-mass correlation function reveal a linear relation between galaxy
luminosities and the depth of their dark matter halos (measured on 260 \hinv
kpc scales). In this work we use an entirely independent dynamical method to
confirm these results. We begin by assembling a sample of 618 relatively
isolated host galaxies, surrounded by a total of 1225 substantially fainter
satellites. We observe the mean dynamical effect of these hosts on the motions
of their satellites by assembling velocity difference histograms. Dividing the
sample by host properties, we find significant variations in satellite velocity
dispersion with host luminosity. We quantify these variations using a simple
dynamical model, measuring \mtsd a dynamical mass within 260 \hinv kpc. The
appropriateness of this mass reconstruction is checked by conducting a similar
analysis within an N-body simulation. Comparison between the dynamical and
lensing mass-to-light scalings shows reasonable agreement, providing some
quantitative confirmation for the lensing results.Comment: 7 pages, 3 figures, accepted for publication in ApJ Letter
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
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