15 research outputs found
Stacked Weak Lensing Mass Calibration: Estimators, Systematics, and Impact on Cosmological Parameter Constraints
When extracting the weak lensing shear signal, one may employ either locally
normalized or globally normalized shear estimators. The former is the standard
approach when estimating cluster masses, while the latter is the more common
method among peak finding efforts. While both approaches have identical
signal-to-noise in the weak lensing limit, it is possible that higher order
corrections or systematics considerations make one estimator preferable over
the other. In this paper, we consider the efficacy of both estimators within
the context of stacked weak lensing mass estimation in the Dark Energy Survey
(DES). We find the two estimators have nearly identical statistical precision,
even after including higher order corrections, but that these corrections must
be incorporated into the analysis to avoid observationally relevant biases in
the recovered masses. We also demonstrate that finite bin-width effects may be
significant if not properly accounted for, and that the two estimators exhibit
different systematics, particularly with respect to contamination of the source
catalog by foreground galaxies. Thus, the two estimators may be employed as a
systematics cross-check of each other. Stacked weak lensing in the DES should
allow for the mean mass of galaxy clusters to be calibrated to about 2%
precision (statistical only), which can improve the figure of merit of the DES
cluster abundance experiment by a factor of ~3 relative to the self-calibration
expectation. A companion paper (Schmidt & Rozo, 2010) investigates how the two
types of estimators considered here impact weak lensing peak finding efforts.Comment: 14 pages, 9 figures; comments welcom
Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CFI Bubble Chamber
Data from the operation of a bubble chamber filled with 3.5 kg of CFI
in a shallow underground site are reported. An analysis of ultrasound signals
accompanying bubble nucleations confirms that alpha decays generate a
significantly louder acoustic emission than single nuclear recoils, leading to
an efficient background discrimination. Three dark matter candidate events were
observed during an effective exposure of 28.1 kg-day, consistent with a neutron
background. This observation provides the strongest direct detection constraint
to date on WIMP-proton spin-dependent scattering for WIMP masses
GeV/c.Comment: 4 pages, 4 figures V2 submitted to match journal versio
Primordial non-Gaussianity and Dark Energy constraints from Cluster Surveys
Galaxy cluster surveys will be a powerful probe of dark energy. At the same
time, cluster abundance is sensitive to any non-Gaussianity of the primordial
density field. It is therefore possible that non-Gaussian initial conditions
might be misinterpreted as a sign of dark energy or at least degrade the
expected constraints on dark energy parameters. To address this issue, we
perform a likelihood analysis of an ideal cluster survey similar in size and
depth to the upcoming South Pole Telescope/Dark Energy Survey (SPT-DES). We
analyze a model in which the strength of the non-Gaussianity is parameterized
by the constant fNL; this model has been used extensively to derive Cosmic
Microwave Background (CMB) anisotropy constraints on non-Gaussianity, allowing
us to make contact with those works. We find that the constraining power of the
cluster survey on dark energy observables is not significantly diminished by
non-Gaussianity provided that cluster redshift information is included in the
analysis. We also find that even an ideal cluster survey is unlikely to improve
significantly current and future CMB constraints on non-Gaussianity. However,
when all systematics are under control, it could constitute a valuable cross
check to CMB observations.Comment: 10 pages, 4 figures. Corrected a minor discrepancy between our
earlier definition of fNL and CMB constraints. References adde
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Optically-Selected Cluster Catalogs As a Precision Cosmology Tool
We introduce a framework for describing the halo selection function of optical cluster finders. We treat the problem as being separable into a term that describes the intrinsic galaxy content of a halo (the Halo Occupation Distribution, or HOD) and a term that captures the effects of projection and selection by the particular cluster finding algorithm. Using mock galaxy catalogs tuned to reproduce the luminosity dependent correlation function and the empirical color-density relation measured in the SDSS, we characterize the maxBCG algorithm applied by Koester et al. to the SDSS galaxy catalog. We define and calibrate measures of completeness and purity for this algorithm, and demonstrate successful recovery of the underlying cosmology and HOD when applied to the mock catalogs. We identify principal components--combinations of cosmology and HOD parameters--that are recovered by survey counts as a function of richness, and demonstrate that percent-level accuracies are possible in the first two components, if the selection function can be understood to {approx} 15% accuracy
Isolating Triggered Star Formation
Galaxy pairs provide a potentially powerful means of studying triggered star
formation from galaxy interactions. We use a large cosmological N-body
simulation coupled with a well-tested semi-analytic substructure model to
demonstrate that the majority of galaxies in close pairs reside within cluster
or group-size halos and therefore represent a biased population, poorly suited
for direct comparison to ``field'' galaxies. Thus, the frequent observation
that some types of galaxies in pairs have redder colors than ``field'' galaxies
is primarily a selection effect. We select galaxy pairs that are isolated in
their dark matter halos with respect to other massive subhalos (N=2 halos) and
a control sample of isolated galaxies (N=1 halos) for comparison. We then apply
these selection criteria to a volume-limited subset of the 2dF Galaxy Redshift
Survey with M_Bj <= -19 and obtain the first clean measure of the typical
fraction of galaxies affected by triggered star formation and the average
elevation in the star formation rate. We find that 24% (30.5%) of these L^\star
and sub-L^{\star} galaxies in isolated 50 (30) kpc/h pairs exhibit star
formation that is boosted by a factor of >~ 5 above their average past value,
while only 10% of isolated galaxies in the control sample show this level of
enhancement. Thus, 14% (20 %) of the galaxies in these close pairs show clear
triggered star formation. The isolation criteria we develop provide a means to
constrain star formation and feedback prescriptions in hydrodynamic simulations
and a very general method of understanding the importance of triggered star
formation in a cosmological context. (Abridged.)Comment: 12 pages, 10 figures, emulateapj format, accepted by Ap
The fate of high redshift massive compact galaxies in dense environments
Massive compact galaxies seem to be more common at high redshift than in the
local universe, especially in denser environments. To investigate the fate of
such massive galaxies identified at z~2 we analyse the evolution of their
properties in three cosmological hydrodynamical simulations that form
virialised galaxy groups of mass ~10^13 Msun hosting a central massive
elliptical/S0 galaxy by redshift zero. We find that at redshift ~2 the
population of galaxies with M_*> 2 10^10 Msun is diverse in terms of mass,
velocity dispersion, star formation and effective radius, containing both very
compact and relatively extended objects. In each simulation all the compact
satellite galaxies have merged into the central galaxy by redshift 0 (with the
exception of one simulation where one of such satellite galaxy survives).
Satellites of similar mass at z = 0 are all less compact than their high
redshift counterparts. They form later than the galaxies in the z = 2 sample
and enter the group potential at z < 1, when dynamical friction times are
longer than the Hubble time. Also, by z = 0 the central galaxies have increased
substantially their characteristic radius via a combination of in situ star
formation and mergers. Hence in a group environment descendants of compact
galaxies either evolve towards larger sizes or they disappear before the
present time as a result of the environment in which they evolve. Since the
group-sized halos that we consider are representative of dense environments in
the LambdaCDM cosmology, we conclude that the majority of high redshift compact
massive galaxies do not survive until today as a result of the environment.Comment: 10 pages, 4 figures, submitted to MNRA
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Impact of astrophysical processes on the gamma-ray background from dark matter annihilations
We study the impact of astrophysical processes on the gamma-ray background produced by the annihilation of dark matter particles in cosmological halos, with particular attention to the consequences of the formation of supermassive black holes. In scenarios where these objects form adiabatically from the accretion of matter on small seeds, dark matter is first compressed into very dense 'spikes', then its density progressively decreases due to annihilations and scattering of stellar cusps. With respect to previous analyses, based on non-evolving halos, the predicted annihilation signal is higher and significantly distorted at low energies, reflecting the large contribution to the total flux from unevolved spikes at high redshifts. The peculiar spectral feature arising from the specific redshift distribution of the signal, would discriminate the proposed scenario from more conventional astrophysical explanations. We discuss how this affects the prospects for detection and demonstrate that the gamma-ray background from DM annihilations might be detectable even in absence of a signal from the Galactic center
Report of the Dark Energy Task Force
Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible
Discovery of GeV Emission from the Circinus galaxy with the Fermi-LAT
We report the discovery of gamma-ray emission from the Circinus galaxy using
the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope.
Circinus is a nearby (~4 Mpc) starburst with a heavily obscured Seyfert-type
active nucleus, bipolar radio lobes perpendicular to the spiral disk, and
kpc-scale jet-like structures. Our analysis of 0.1-100 GeV events collected
during 4 years of LAT observations reveals a significant (~ 7.3 sigma) excess
above the background. We find no indications of variability or spatial
extension beyond the LAT point-spread function. A power-law model used to
describe the 0.1-100 GeV gamma-ray spectrum yields a flux of
(18.8+/-5.8)x10^{-9} ph cm^{-2} s^{-1} and photon index 2.19+/-0.12,
corresponding to an isotropic gamma-ray luminosity of 3 x 10^{40} erg s^{-1}.
This observed gamma-ray luminosity exceeds the luminosity expected from
cosmic-ray interactions in the interstellar medium and inverse Compton
radiation from the radio lobes. Thus the origin of the GeV excess requires
further investigation.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical
Journa
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