2,332 research outputs found
On the Halo Occupation of Dark Baryons
We introduce a new technique that adopts the halo occupation framework for
understanding the origin of QSO absorption-line systems. Our initial study
focuses specifically on MgII absorbers. We construct a model of the gaseous
content in which the absorption equivalent width W_r is determined by the the
amount of cold gas, in the form of discrete clouds, along a sightline through a
halo. The two quantities that we specify per halo in the model are (1) the mean
absorption strength per unit surface mass density A_W(M), and (2) the mean
covering factor kappa_g(M) of the gaseous clouds. These parameters determine
the conditional probability distribution of W_r as a function of halo mass,
P(W_r|M). Two empirical measurements are applied to constrain the model: (i)
the absorber frequency distribution function and (ii) the W_r-dependent
clustering amplitude. We find that the data demand a rapid transition in the
gas content of halos at ~10^11.5 Msol/h, below which halos contain
predominantly cold gas and beyond which gas becomes predominantly hot. In order
to reproduce the observed overall strong clustering of the absorbers and the
anti-correlation between W_r and halo mass M, roughly 5% of gas in halos up to
10^14 Msol/h is required to be cold. The gas covering factor is near unity over
a wide range of halo mass, supporting that Mg II systems probe an unbiased
sample of typical galaxies. We discuss the implications of our study in the
contexts of mass assembly of distant galaxies and the origin of QSO absorption
line systems.Comment: 15 emulateapj pages, 7 figures, replaced with revised version
incorporating referee's comment
Micro/Nanosatellite Mars Network for Global Lower Atmosphere Characterization
To address multiple key challenge areas for robotic exploration of Mars, to achieve scientific goals and reduce risk for future human missions, a micro/nanosatellite constellation for lower atmosphere characterization is proposed. A microsatellite design is discussed that can operate (1) in tandem with another microsat or (2) as a "mother-ship" to deploy a network of nanosatellites (CubeSats). Either configuration of the network would perform radio occultation-based atmospheric measurements. Advantages of the proposed network are low development cost based on an existing microsatellite bus, and proven performance of the bus to date. Continued efforts in miniaturization of instruments are needed to fully enable the mother-ship/nanosat version of the proposed network
Mock galaxy catalogs using the quick particle mesh method
Sophisticated analysis of modern large-scale structure surveys requires mock
catalogs. Mock catalogs are used to optimize survey design, test reduction and
analysis pipelines, make theoretical predictions for basic observables and
propagate errors through complex analysis chains. We present a new method,
which we call "quick particle mesh", for generating many large-volume,
approximate mock catalogs at low computational cost. The method is based on
using rapid, low-resolution particle mesh simulations that accurately reproduce
the large-scale dark matter density field. Particles are sampled from the
density field based on their local density such that they have N-point
statistics nearly equivalent to the halos resolved in high-resolution
simulations, creating a set of mock halos that can be populated using halo
occupation methods to create galaxy mocks for a variety of possible target
classes.Comment: 13 pages, 16 figures. Matches version accepted by MNRAS. Code
available at http://github.com/mockFactor
Star Formation Quenching Timescale of Central Galaxies in a Hierarchical Universe
Central galaxies make up the majority of the galaxy population, including the
majority of the quiescent population at . Thus, the mechanism(s) responsible for quenching
central galaxies plays a crucial role in galaxy evolution as whole. We combine
a high resolution cosmological -body simulation with observed evolutionary
trends of the "star formation main sequence," quiescent fraction, and stellar
mass function at to construct a model that statistically tracks the
star formation histories and quenching of central galaxies. Comparing this
model to the distribution of central galaxy star formation rates in a group
catalog of the SDSS Data Release 7, we constrain the timescales over which
physical processes cease star formation in central galaxies. Over the stellar
mass range to we infer quenching
e-folding times that span to with more massive
central galaxies quenching faster. For , this implies a total migration time of from the star formation main sequence to quiescence. Compared
to satellites, central galaxies take longer to quench
their star formation, suggesting that different mechanisms are responsible for
quenching centrals versus satellites. Finally, the central galaxy quenching
timescale we infer provides key constraints for proposed star formation
quenching mechanisms. Our timescale is generally consistent with gas depletion
timescales predicted by quenching through strangulation. However, the exact
physical mechanism(s) responsible for this still remain unclear.Comment: 16 pages, 11 figure
On the Mass-to-Light Ratio of Large Scale Structure
We examine the dependence of the mass-to-light (M/L) ratio of large-scale
structure on cosmological parameters, in models that are constrained to match
observations of the projected galaxy correlation function w(rp). For a sequence
of cosmological models with a fixed P(k) shape and increasing normalization
\sig8, we find parameters of the galaxy halo occupation distribution (HOD) that
reproduce SDSS measurements of w(rp) as a function of luminosity. Using these
HOD models we calculate mean M/L ratios as a function of halo mass and populate
halos of N-body simulations to compute M/L in larger scale environments,
including cluster infall regions. For all cosmological models, the M/L ratio in
high mass halos or high density regions is approximately independent of halo
mass or smoothing scale. However, the "plateau" value of M/L depends on \sig8
as well as \Omega_m, and it represents the universal mass-to-light ratio
only for models in which the galaxy correlation function is approximately
unbiased, i.e., with \sig8 ~ \sig8_gal. Our results for cluster mass halos
follow the trend M/L = 577(\Omega_m/0.3)(\sig8/0.9)^{1.7} h Msun/Lsun. Combined
with Carlberg et al.'s (1996) mean M/L ratio of CNOC galaxy clusters, this
relation implies (\sig8/0.9)(\Omega_m/0.3)^{0.6} = 0.75 +/- 0.06. M/L ratios of
clusters from the SDSS and CAIRNS surveys yield similar results. This
constraint is inconsistent with parameter values \Omega_m ~ 0.3, \sig8 ~ 0.9
favored by recent joint analyses of CMB measurements and other large-scale
structure data. We discuss possible resolutions, none of which seems entirely
satisfactory. Appendices present an improved formula for halo bias factors and
an improved analytic technique for calculating the galaxy correlation function
from a given cosmological model and HOD. (Abridged)Comment: Accepted to ApJ (v 630, no 2). Replaced with accepted versio
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