2,887 research outputs found
Where do "red and dead" early-type void galaxies come from?
Void regions of the Universe offer a special environment for studying
cosmology and galaxy formation, which may expose weaknesses in our
understanding of these phenomena. Although galaxies in voids are observed to be
predominately gas rich, star forming and blue, a sub-population of bright red
void galaxies can also be found, whose star formation was shut down long ago.
Are the same processes that quench star formation in denser regions of the
Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy
Redshift Survey, to those from a galaxy formation model built on the Millennium
Simulation. We show that a global star formation suppression mechanism in the
form of low luminosity "radio mode" AGN heating is sufficient to reproduce the
observed population of void early-types. Radio mode heating is environment
independent other than its dependence on dark matter halo mass, where, above a
critical mass threshold of approximately M_vir~10^12.5 M_sun, gas cooling onto
the galaxy is suppressed and star formation subsequently fades. In the
Millennium Simulation, the void halo mass function is shifted with respect to
denser environments, but still maintains a high mass tail above this critical
threshold. In such void halos, radio mode heating remains efficient and red
galaxies are found; collectively these galaxies match the observed space
density without any modification to the model. Consequently, galaxies living in
vastly different large-scale environments but hosted by halos of similar mass
are predicted to have similar properties, consistent with observations.Comment: 6 pages, 3 figures, accepted MNRA
Automatic abstracting: a review and an empirical evaluation
The abstract is a fundamental tool in information retrieval. As condensed representations,
they facilitate conservation of the increasingly precious search time and space of scholars, allowing them to manage more effectively an ever-growing deluge of documentation.
Traditionally the product of human intellectual effort, attempts to automate the abstracting
process began in 1958. Two identifiable automatic abstracting techniques emerged which
reflect differing levels of ambition regarding simulation of the human abstracting process,
namely sentence extraction and text summarisation. This research paradigm has recently
diversified further, with a cross-fertilisation of methods. Commercial systems are beginning
to appear, but automatic abstracting is still mainly confined to an experimental arena.
The purpose of this study is firstly to chart the historical development and current state of
both manual and automatic abstracting; and secondly, to devise and implement an empirical
user-based evaluation to assess the adequacy of automatic abstracts derived from sentence
extraction techniques according to a set of utility criteria. [Continues.
Cosmic Voids and Galaxy Bias in the Halo Occupation Framework
(Abridged) We investigate the power of void statistics to constrain galaxy
bias and the amplitude of dark matter fluctuations. We use the halo occupation
distribution (HOD) framework to describe the relation between galaxies and dark
matter. After choosing HOD parameters that reproduce the mean space density
n_gal and projected correlation function w_p measured for galaxy samples with
M_r<-19 and M_r<-21 from the Sloan Digital Sky Survey (SDSS), we predict the
void probability function (VPF) and underdensity probability function (UPF) of
these samples by populating the halos of a large, high-resolution N-body
simulation. If we make the conventional assumption that the HOD is independent
of large scale environment at fixed halo mass, then models constrained to match
n_gal and w_p predict nearly identical void statistics, independent of the
scatter between halo mass and central galaxy luminosity or uncertainties in HOD
parameters. Models with sigma_8=0.7 and sigma_8=0.9 also predict very similar
void statistics. However, the VPF and UPF are sensitive to environmental
variations of the HOD in a regime where these variations have little impact on
w_p. For example, doubling the minimum host halo mass in regions with large
scale (5 Mpc/h) density contrast delta<-0.65 has a readily detectable impact on
void probabilities of M_r<-19 galaxies, and a similar change for delta<-0.2
alters the void probabilities of M_r<-21 galaxies at a detectable level. The
VPF and UPF provide complementary information about the onset and magnitude of
density- dependence in the HOD. By detecting or ruling out HOD changes in low
density regions, void statistics can reduce systematic uncertainties in the
cosmological constraints derived from HOD modeling, and, more importantly,
reveal connections between halo formation history and galaxy properties.Comment: emulateapj, 16 pages, 13 figure
Redshift space 21 cm power spectra from reionization
We construct a simple but self-consistent analytic ionization model for rapid
exploration of 21cm power spectrum observables in redshift space. It is fully
described by the average ionization fraction and HII patch size
and has the flexibility to accommodate various reionization scenarios. The
model associates ionization regions with dark matter halos of the number
density required to recover and treats redshift space distortions
self-consistently with the virial velocity of such halos. Based on this model,
we study the line-of-sight structures in the brightness fluctuations since they
are the most immune to foreground contamination. We explore the degeneracy
between the HII patch size and nonlinear redshift space distortion in the one
dimensional power spectrum. We also discuss the limitations experimental
frequency and angular resolutions place on their distinguishability. Angular
resolution dilutes even the radial signal and will be a serious limitation for
resolving small bubbles before the end of reionization. Nonlinear redshift
space distortions suggest that a resolution of order 1 -- 10\arcsec and a
frequency resolution of 10kHz will ultimately be desirable to extract the full
information in the radial field at . First generation instruments
such as LOFAR and MWA can potentially measure radial HII patches of a few
comoving Mpc and larger at the end of reionization and are unlikely to be
affected by nonlinear redshift space distortions.Comment: 13 pages, 10 figures. Revised version. Includes minor changes. Adds
appendix on accomodating a distribution of radii for the HII regions.
Accepted for publication in Ap
From Galaxy-Galaxy Lensing to Cosmological Parameters
Galaxy-galaxy lensing measures the mean excess surface density DS(r) around a
sample of lensing galaxies. We develop a method for combining DS(r) with the
galaxy correlation function xi_gg(r) to constrain Omega_m and sigma_8, going
beyond the linear bias model to reach the level of accuracy demanded by current
and future measurements. We adopt the halo occupation distribution (HOD)
framework, and we test its applicability to this problem by examining the
effects of replacing satellite galaxies in the halos of an SPH simulation with
randomly selected dark matter particles from the same halos. The difference
between dark matter and satellite galaxy radial profiles has a ~10% effect on
DS(r) at r<1 Mpc/h. However, if radial profiles are matched, the remaining
impact of individual subhalos around satellite galaxies and environmental
dependence of the HOD at fixed halo mass is <5% in DS(r) for 0.1<r<15 Mpc/h. We
develop an analytic approximation for DS(r) that incorporates halo exclusion
and scale-dependent halo bias, and we demonstrate its accuracy with tests
against a suite of populated N-body simulations. We use the analytic model to
investigate the dependence of DS(r) and the galaxy-matter correlation function
xi_gm(r) on Omega_m and sigma_8, once HOD parameters for a given cosmological
model are pinned down by matching xi_gg(r). The linear bias prediction is
accurate for r>2 Mpc/h, but it fails at the 30-50% level on smaller scales. The
scaling of DS(r) ~ Omega_m^a(r) sigma_8^b(r) approaches the linear bias
expectation a=b=1 at r>10 Mpc/h, but a(r) and b(r) vary from 0.8 to 1.6 at
smaller r. We calculate a fiducial DS(r) and scaling indices a(r) and b(r) for
two SDSS galaxy samples; galaxy-galaxy lensing measurements for these samples
can be combined with our predictions to constrain Omega_m and sigma_8.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Angular Momentum Evolution of Stars in the Orion Nebula Cluster
(Abridged) We present theoretical models of stellar angular momentum
evolution from the Orion Nebula Cluster (ONC) to the Pleiades and the Hyades.
We demonstrate that observations of the Pleiades and Hyades place tight
constraints on the angular momentum loss rate from stellar winds. The observed
periods, masses and ages of ONC stars in the range 0.2--0.5 M, and the
loss properties inferred from the Pleiades and Hyades stars, are then used to
test the initial conditions for stellar evolution models. We use these models
to estimate the distribution of rotational velocities for the ONC stars at the
age of the Pleiades (120 Myr). The modeled ONC and observed Pleiades
distributions of rotation rates are not consistent if only stellar winds are
included. In order to reconcile the observed loss of angu lar momentum between
these two clusters, an extrinsic loss mechanism such as protostar-accretion
disk interaction is required. Our model, which evolves the ONC stars with a
mass dependent saturation threshold normalized such that at 0.5 \m, and which includes a distribution of disk lifetimes
that is uniform over the range 0--6 Myr, is consistent with the Pleiades. This
model for disk-locking lifetimes is also consistent with inferred disk
lifetimes from the percentage of stars with infrared excesses observed in young
clusters. Different models, using a variety of initial period distributions and
different maximum disk lifetimes, are also compared to the Pleiades. For
disk-locking models that use a uniform distribution of disk lifetimes over the
range 0 to , the acceptable range of the maximum lifetime is Myr.Comment: 21 pages, 7 figures, submitted to Ap
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