554 research outputs found
KK246, a dwarf galaxy with extended H I disk in the Local Void
We have found that KK 246, the only confirmed galaxy located within the
nearby Tully Void, is a dwarf galaxy with an extremely extended H I disk and
signs of an H I cloud with anomalous velocity. It also exhibits clear
misalignment between the kinematical major and minor axes, indicative of an
oval distortion, and a general misalignment between the H I and optical major
axes. We measure a H I mass of 1.05 +- 0.08 x 10^8 M_sun, and a H I extent 5
times that of the stellar disk, one of the most extended H I disks known. We
estimate a dynamical mass of 4.1 x 10^9 M_sun, making this also one of the
darkest galaxies known, with a mass-to-light ratio of 89. The relative
isolation and extreme underdense environment make this an interesting case for
examining the role of gas accretion in galaxy evolution.Comment: 15 pages, 8 figures, 2 tables, accepted for publication in A
Cosmological Origin of the Stellar Velocity Dispersions in Massive Early-Type Galaxies
We show that the observed upper bound on the line-of-sight velocity
dispersion of the stars in an early-type galaxy, sigma<400km/s, may have a
simple dynamical origin within the LCDM cosmological model, under two main
hypotheses. The first is that most of the stars now in the luminous parts of a
giant elliptical formed at redshift z>6. Subsequently, the stars behaved
dynamically just as an additional component of the dark matter. The second
hypothesis is that the mass distribution characteristic of a newly formed dark
matter halo forgets such details of the initial conditions as the stellar
"collisionless matter" that was added to the dense parts of earlier generations
of halos. We also assume that the stellar velocity dispersion does not evolve
much at z<6, because a massive host halo grows mainly by the addition of
material at large radii well away from the stellar core of the galaxy. These
assumptions lead to a predicted number density of ellipticals as a function of
stellar velocity dispersion that is in promising agreement with the Sloan
Digital Sky Survey data.Comment: ApJ, in press (2003); matches published versio
The Kinematics of the Outer Halo of M87
Radial velocities are presented for a new sample of globular clusters in the
outer halo of M87 at a distance of 300 to 540 arcsec (24 to 43 kpc) from the
center of this galaxy. These are used to augment our previously published data
and an analysis of the rotation and velocity dispersion of the M87 globular
cluster system is carried out. The rotation is \kms at R = 32 kpc, at
which point the velocity dispersion is also still quite high, \kms.
The high rotation is interesting. The outer halo of M87 is, as was found in our
previous kinematic analysis, very massive.Comment: Accepted for publication in the AJ. 13 pages with 3 figure
Enhanced Peculiar Velocities in Brane-Induced Gravity
The mounting evidence for anomalously large peculiar velocities in our
Universe presents a challenge for the LCDM paradigm. The recent estimates of
the large scale bulk flow by Watkins et al. are inconsistent at the nearly 3
sigma level with LCDM predictions. Meanwhile, Lee and Komatsu have recently
estimated that the occurrence of high-velocity merging systems such as the
Bullet Cluster (1E0657-57) is unlikely at a 6.5-5.8 sigma level, with an
estimated probability between 3.3x10^{-11} and 3.6x10^{-9} in LCDM cosmology.
We show that these anomalies are alleviated in a broad class of
infrared-modifed gravity theories, called brane-induced gravity, in which
gravity becomes higher-dimensional at ultra large distances. These theories
include additional scalar forces that enhance gravitational attraction and
therefore speed up structure formation at late times and on sufficiently large
scales. The peculiar velocities are enhanced by 24-34% compared to standard
gravity, with the maximal enhancement nearly consistent at the 2 sigma level
with bulk flow observations. The occurrence of the Bullet Cluster in these
theories is 10^4 times more probable than in LCDM cosmology.Comment: 15 pages, 6 figures. v2: added reference
Three-Point Correlations in Weak Lensing Surveys: Model Predictions and Applications
We use the halo model of clustering to compute two- and three-point
correlation functions for weak lensing, and apply them in a new statistical
technique to measure properties of massive halos. We present analytical results
on the eight shear three-point correlation functions constructed using
combination of the two shear components at each vertex of a triangle. We
compare the amplitude and configuration dependence of the functions with
ray-tracing simulations and find excellent agreement for different scales and
models. These results are promising, since shear statistics are easier to
measure than the convergence. In addition, the symmetry properties of the shear
three-point functions provide a new and precise way of disentangling the
lensing E-mode from the B-mode due to possible systematic errors.
We develop an approach based on correlation functions to measure the
properties of galaxy-group and cluster halos from lensing surveys. Shear
correlations on small scales arise from the lensing matter within halos of mass
M > 10^13 solar masses. Thus the measurement of two- and three-point
correlations can be used to extract information on halo density profiles,
primarily the inner slope and halo concentration. We demonstrate the
feasibility of such an analysis for forthcoming surveys. We include covariances
in the correlation functions due to sample variance and intrinsic ellipticity
noise to show that 10% accuracy on profile parameters is achievable with
surveys like the CFHT Legacy survey, and significantly better with future
surveys. Our statistical approach is complementary to the standard approach of
identifying individual objects in survey data and measuring their properties.Comment: 30 pages, 21 figures. Corrected typos in equations (23) and (28).
Matches version for publication in MNRA
Anisotropic brane gravity with a confining potential
We consider an anisotropic brane world with Bianchi type I and V geometries
where the mechanism of confining the matter on the brane is through the use of
a confining potential. The resulting equations on the anisotropic brane are
modified by an extra term that may be interpreted as the x-matter, providing a
possible phenomenological explanation for the accelerated expansion of the
universe. We obtain the general solution of the field equations in an exact
parametric form for both Bianchi type I and V space-times. In the special case
of a Bianchi type I the solutions of the field equations are obtained in an
exact analytic form. Finally, we study the behavior of the observationally
important parameters.Comment: 14 pages, 3 figures, to appear in PL
Linear Perturbations in Brane Gas Cosmology
We consider the effect of string inhomogeneities on the time dependent
background of Brane Gas Cosmology. We derive the equations governing the linear
perturbations of the dilaton-gravity background in the presence of string
matter sources. We focus on long wavelength fluctuations and find that there
are no instabilities. Thus, the predictions of Brane Gas Cosmology are robust
against the introduction of linear perturbations. In particular, we find that
the stabilization of the extra dimensions (moduli) remains valid in the
presence of dilaton and string perturbations.Comment: 17 pages, 1 figur
Locally Cold Flows from Large-Scale Structure
We show that the "cold" Hubble flow observed for galaxies around the Milky
Way does not represent a problem in cosmology but is due to the particular
geometry and dynamics of our local wall. The behavior of the perturbed Hubble
flow around the Milky Way is the result of two main factors: at small scales (R
< 1 Mpc) the inflow is dominated by the gravitational influence of the Milky
Way. At large scales (R > 1 Mpc) the out flow reflects the expansion of our
local wall which "cools down" the peculiar velocities. This is an intrinsic
property of walls and is independent of cosmology. We find the dispersion of
the local Hubble flow (1 < R < 3 Mpc) around simulated "Milky Way" haloes
located at the centre of low-density cosmological walls to be {\sigma}_H ~ 30
km/s, in excellent agreement with observations. The expansion of our local wall
is also reflected in the value of the measured local Hubble constant. For
"Milky Way" haloes inside walls, we find super-Hubble flows with h_local \simeq
0.77 - 1.13. The radius of equilibrium (R_0) depends not only on the mass of
the central halo and the Hubble expansion but also on the dynamics given by the
local LSS geometry. The super-Hubble flow inside our local wall has the effect
of reducing the radius at which the local expansion balances the gravitational
influence of the Milky Way. By ignoring the dynamical effect of the local wall,
the mass of the Milky Way estimated from R_0 can be underestimated by as much
as ~ 30%.Comment: 5 pages, 3 figures, Submitted to MNRA
Only the Lonely: H I Imaging of Void Galaxies
Void galaxies, residing within the deepest underdensities of the Cosmic Web,
present an ideal population for the study of galaxy formation and evolution in
an environment undisturbed by the complex processes modifying galaxies in
clusters and groups, as well as provide an observational test for theories of
cosmological structure formation. We have completed a pilot survey for the HI
imaging aspects of a new Void Galaxy Survey (VGS), imaging 15 void galaxies in
HI in local (d < 100 Mpc) voids. HI masses range from 3.5 x 10^8 to 3.8 x 10^9
M_sun, with one nondetection with an upper limit of 2.1 x 10^8 M_sun. Our
galaxies were selected using a structural and geometric technique to produce a
sample that is purely environmentally selected and uniformly represents the
void galaxy population. In addition, we use a powerful new backend of the
Westerbork Synthesis Radio Telescope that allows us to probe a large volume
around each targeted galaxy, simultaneously providing an environmentally
constrained sample of fore- and background control sample of galaxies while
still resolving individual galaxy kinematics and detecting faint companions in
HI. This small sample makes up a surprisingly interesting collection of
perturbed and interacting galaxies, all with small stellar disks. Four galaxies
have significantly perturbed HI disks, five have previously unidentified
companions at distances ranging from 50 to 200 kpc, two are in interacting
systems, and one was found to have a polar HI disk. Our initial findings
suggest void galaxies are a gas-rich, dynamic population which present evidence
of ongoing gas accretion, major and minor interactions, and filamentary
alignment despite the surrounding underdense environment.Comment: 53 pages, 18 figures, accepted for publication in AJ. High resolution
available at http://www.astro.columbia.edu/~keejo/kreckel2010.pd
Quasi-Newtonian dust cosmologies
Exact dynamical equations for a generic dust matter source field in a
cosmological context are formulated with respect to a non-comoving
Newtonian-like timelike reference congruence and investigated for internal
consistency. On the basis of a lapse function (the relativistic
acceleration scalar potential) which evolves along the reference congruence
according to (), we find that
consistency of the quasi-Newtonian dynamical equations is not attained at the
first derivative level. We then proceed to show that a self-consistent set can
be obtained by linearising the dynamical equations about a (non-comoving) FLRW
background. In this case, on properly accounting for the first-order momentum
density relating to the non-relativistic peculiar motion of the matter,
additional source terms arise in the evolution and constraint equations
describing small-amplitude energy density fluctuations that do not appear in
similar gravitational instability scenarios in the standard literature.Comment: 25 pages, LaTeX 2.09 (10pt), to appear in Classical and Quantum
Gravity, Vol. 15 (1998
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