230 research outputs found
High-resolution mass models of dwarf galaxies from LITTLE THINGS
We present high-resolution rotation curves and mass models of 26 dwarf galaxies from LITTLE THINGS. LITTLE THINGS is a high-resolution Very Large Array HI survey for nearby dwarf galaxies in the local volume within 11 Mpc. The rotation curves of the sample galaxies derived in a homogeneous and consistent manner are combined with Spitzer archival 3.6 micron and ancillary optical U, B, and V images to construct mass models of the galaxies. We decompose the rotation curves in terms of the dynamical contributions by baryons and dark matter halos, and compare the latter with those of dwarf galaxies from THINGS as well as Lambda CDM SPH simulations in which the effect of baryonic feedback processes is included. Being generally consistent with THINGS and simulated dwarf galaxies, most of the LITTLE THINGS sample galaxies show a linear increase of the rotation curve in their inner regions, which gives shallower logarithmic inner slopes alpha of their dark matter density profiles. The mean value of the slopes of the 26 LITTLE THINGS dwarf galaxies is alpha =-0.32 +/- 0.24 which is in accordance with the previous results found for low surface brightness galaxies (alpha = -0.2 +/- 0.2) as well as the seven THINGS dwarf galaxies (alpha =-0.29 +/- 0.07). However, this significantly deviates from the cusp-like dark matter distribution predicted by dark-matter-only Lambda CDM simulations. Instead our results are more in line with the shallower slopes found in the Lambda CDM SPH simulations of dwarf galaxies in which the effect of baryonic feedback processes is included. In addition, we discuss the central dark matter distribution of DDO 210 whose stellar mass is relatively low in our sample to examine the scenario of inefficient supernova feedback in low mass dwarf galaxies predicted from recent Lambda SPH simulations of dwarf galaxies where central cusps still remain.Peer reviewe
Halo mass - concentration relation from weak lensing
We perform a statistical weak lensing analysis of dark matter profiles around
tracers of halo mass from galactic- to cluster-size halos. In this analysis we
use 170,640 isolated ~L* galaxies split into ellipticals and spirals, 38,236
groups traced by isolated spectroscopic Luminous Red Galaxies (LRGs) and 13,823
MaxBCG clusters from the Sloan Digital Sky Survey (SDSS) covering a wide range
of richness. Together these three samples allow a determination of the density
profiles of dark matter halos over three orders of magnitude in mass, from
10^{12} M_{sun} to 10^{15} M_{sun}. The resulting lensing signal is consistent
with an NFW or Einasto profile on scales outside the central region. We find
that the NFW concentration parameter c_{200b} decreases with halo mass, from
around 10 for galactic halos to 4 for cluster halos. Assuming its dependence on
halo mass in the form of c_{200b} = c_0 [M/(10^{14}M_{sun}/h)]^{\beta}, we find
c_0=4.6 +/- 0.7 (at z=0.22) and \beta=0.13 +/- 0.07, with very similar results
for the Einasto profile. The slope (\beta) is in agreement with theoretical
predictions, while the amplitude is about two standard deviations below the
predictions for this mass and redshift, but we note that the published values
in the literature differ at a level of 10-20% and that for a proper comparison
our analysis should be repeated in simulations. We discuss the implications of
our results for the baryonic effects on the shear power spectrum: since these
are expected to increase the halo concentration, the fact that we see no
evidence of high concentrations on scales above 20% of the virial radius
suggests that baryonic effects are limited to small scales, and are not a
significant source of uncertainty for the current weak lensing measurements of
the dark matter power spectrum. [ABRIDGED]Comment: 17 pages, 5 figures, accepted to JCAP pending minor revisions that
are included in v2 here on arXi
The sky brightness and transparency in i-band at Dome A, Antarctica
The i-band observing conditions at Dome A on the Antarctic plateau have been
investigated using data acquired during 2008 with the Chinese Small Telescope
ARray. The sky brightness, variations in atmospheric transparency, cloud cover,
and the presence of aurorae are obtained from these images. The median sky
brightness of moonless clear nights is 20.5 mag arcsec^{-2} in the SDSS
band at the South Celestial Pole (which includes a contribution of about 0.06
mag from diffuse Galactic light). The median over all Moon phases in the
Antarctic winter is about 19.8 mag arcsec^{-2}. There were no thick clouds in
2008. We model contributions of the Sun and the Moon to the sky background to
obtain the relationship between the sky brightness and transparency. Aurorae
are identified by comparing the observed sky brightness to the sky brightness
expected from this model. About 2% of the images are affected by relatively
strong aurorae.Comment: There are 1 Latex file and 14 figures accepted by A
VERITAS Search for VHE Gamma-ray Emission from Dwarf Spheroidal Galaxies
Indirect dark matter searches with ground-based gamma-ray observatories
provide an alternative for identifying the particle nature of dark matter that
is complementary to that of direct search or accelerator production
experiments. We present the results of observations of the dwarf spheroidal
galaxies Draco, Ursa Minor, Bootes 1, and Willman 1 conducted by VERITAS. These
galaxies are nearby dark matter dominated objects located at a typical distance
of several tens of kiloparsecs for which there are good measurements of the
dark matter density profile from stellar velocity measurements. Since the
conventional astrophysical background of very high energy gamma rays from these
objects appears to be negligible, they are good targets to search for the
secondary gamma-ray photons produced by interacting or decaying dark matter
particles. No significant gamma-ray flux above 200 GeV was detected from these
four dwarf galaxies for a typical exposure of ~20 hours. The 95% confidence
upper limits on the integral gamma-ray flux are in the range 0.4-2.2x10^-12
photons cm^-2s^-1. We interpret this limiting flux in the context of pair
annihilation of weakly interacting massive particles and derive constraints on
the thermally averaged product of the total self-annihilation cross section and
the relative velocity of the WIMPs. The limits are obtained under conservative
assumptions regarding the dark matter distribution in dwarf galaxies and are
approximately three orders of magnitude above the generic theoretical
prediction for WIMPs in the minimal supersymmetric standard model framework.
However significant uncertainty exists in the dark matter distribution as well
as the neutralino cross sections which under favorable assumptions could
further lower the limits.Comment: 21 pages, 2 figures, updated to reflect version published in ApJ.
NOTE: M.D. Wood added as autho
Metabolic Rift or Metabolic Shift? Dialectics, Nature, and the World-Historical Method
Abstract In the flowering of Red-Green Thought over the past two decades, metabolic rift thinking is surely one of its most colorful varieties. The metabolic rift has captured the imagination of critical environmental scholars, becoming a shorthand for capitalismâs troubled relations in the web of life. This article pursues an entwined critique and reconstruction: of metabolic rift thinking and the possibilities for a post-Cartesian perspective on historical change, the world-ecology conversation. Far from dismissing metabolic rift thinking, my intention is to affirm its dialectical core. At stake is not merely the mode of explanation within environmental sociology. The impasse of metabolic rift thinking is suggestive of wider problems across the environmental social sciences, now confronted by a double challenge. One of course is the widespreadâand reasonableâsense of urgency to evolve modes of thought appropriate to an era of deepening biospheric instability. The second is the widely recognizedâbut inadequately internalizedâunderstanding that humans are part of nature
The PLATO Dome A Site-Testing Observatory : instrumentation and first results
The PLATeau Observatory (PLATO) is an automated self-powered astrophysical observatory that was deployed to Dome A, the highest point on the Antarctic plateau, in 2008 January. PLATO consists of a suite of site-testing instruments designed to quantify the benefits of the Dome A site for astronomy, and science instruments designed to take advantage of the unique observing conditions. Instruments include CSTAR, an array of optical telescopes for transient astronomy; Gattini, an instrument to measure the optical sky brightness and cloud cover statistics; DASLE, an experiment to measure the statistics of the meteorological conditions within the near-surface layer; Pre-HEAT, a submillimeter tipping radiometer measuring the atmospheric transmission and water vapor content and performing spectral line imaging of the Galactic plane; and Snodar, an acoustic radar designed to measure turbulence within the near-surface layer. PLATO has run completely unattended and collected data throughout the winter 2008 season. Here we present a detailed description of the PLATO instrument suite and preliminary results obtained from the first season of operation
Transport Properties of the Quark-Gluon Plasma -- A Lattice QCD Perspective
Transport properties of a thermal medium determine how its conserved charge
densities (for instance the electric charge, energy or momentum) evolve as a
function of time and eventually relax back to their equilibrium values. Here
the transport properties of the quark-gluon plasma are reviewed from a
theoretical perspective. The latter play a key role in the description of
heavy-ion collisions, and are an important ingredient in constraining particle
production processes in the early universe. We place particular emphasis on
lattice QCD calculations of conserved current correlators. These Euclidean
correlators are related by an integral transform to spectral functions, whose
small-frequency form determines the transport properties via Kubo formulae. The
universal hydrodynamic predictions for the small-frequency pole structure of
spectral functions are summarized. The viability of a quasiparticle description
implies the presence of additional characteristic features in the spectral
functions. These features are in stark contrast with the functional form that
is found in strongly coupled plasmas via the gauge/gravity duality. A central
goal is therefore to determine which of these dynamical regimes the quark-gluon
plasma is qualitatively closer to as a function of temperature. We review the
analysis of lattice correlators in relation to transport properties, and
tentatively estimate what computational effort is required to make decisive
progress in this field.Comment: 54 pages, 37 figures, review written for EPJA and APPN; one parag.
added end of section 3.4, and one at the end of section 3.2.2; some Refs.
added, and some other minor change
Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas
Shear viscosity is a measure of the amount of dissipation in a simple fluid.
In kinetic theory shear viscosity is related to the rate of momentum transport
by quasi-particles, and the uncertainty relation suggests that the ratio of
shear viscosity eta to entropy density s in units of hbar/k_B is bounded by a
constant. Here, hbar is Planck's constant and k_B is Boltzmann's constant. A
specific bound has been proposed on the basis of string theory where, for a
large class of theories, one can show that eta/s is greater or equal to hbar/(4
pi k_B). We will refer to a fluid that saturates the string theory bound as a
perfect fluid. In this review we summarize theoretical and experimental
information on the properties of the three main classes of quantum fluids that
are known to have values of eta/s that are smaller than hbar/k_B. These fluids
are strongly coupled Bose fluids, in particular liquid helium, strongly
correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the
main theoretical approaches to transport properties of these fluids: kinetic
theory, numerical simulations based on linear response theory, and holographic
dualities. We also summarize the experimental situation, in particular with
regard to the observation of hydrodynamic behavior in ultracold Fermi gases and
the quark gluon plasma.Comment: 76 pages, 11 figures, review article, extensive revision
Inflation, cold dark matter, and the central density problem
A problem with high central densities in dark halos has arisen in the context
of LCDM cosmologies with scale-invariant initial power spectra. Although n=1 is
often justified by appealing to the inflation scenario, inflationary models
with mild deviations from scale-invariance are not uncommon and models with
significant running of the spectral index are plausible. Even mild deviations
from scale-invariance can be important because halo collapse times and
densities depend on the relative amount of small-scale power. We choose several
popular models of inflation and work out the ramifications for galaxy central
densities. For each model, we calculate its COBE-normalized power spectrum and
deduce the implied halo densities using a semi-analytic method calibrated
against N-body simulations. We compare our predictions to a sample of dark
matter-dominated galaxies using a non-parametric measure of the density. While
standard n=1, LCDM halos are overdense by a factor of 6, several of our example
inflation+CDM models predict halo densities well within the range preferred by
observations. We also show how the presence of massive (0.5 eV) neutrinos may
help to alleviate the central density problem even with n=1. We conclude that
galaxy central densities may not be as problematic for the CDM paradigm as is
sometimes assumed: rather than telling us something about the nature of the
dark matter, galaxy rotation curves may be telling us something about inflation
and/or neutrinos. An important test of this idea will be an eventual consensus
on the value of sigma_8, the rms overdensity on the scale 8 h^-1 Mpc. Our
successful models have values of sigma_8 approximately 0.75, which is within
the range of recent determinations. Finally, models with n>1 (or sigma_8 > 1)
are highly disfavored.Comment: 13 pages, 6 figures. Minor changes made to reflect referee's
Comments, error in Eq. (18) corrected, references updated and corrected,
conclusions unchanged. Version accepted for publication in Phys. Rev. D,
scheduled for 15 August 200
Cosmological phase transitions in warped space: gravitational waves and collider signatures
We study the electroweak phase transition within a 5D warped model including
a scalar potential with an exponential behavior, and strong back-reaction over the metric,
in the infrared. By means of a novel treatment of the superpotential formalism, we explore
parameter regions that were previously inaccessible. We nd that for large enough values
of the t'Hooft parameter (e.g. N = 25) the holographic phase transition occurs, and it
can force the Higgs to undergo a rst order electroweak phase transition, suitable for
electroweak baryogenesis. The model exhibits gravitational waves and colliders signatures.
It typically predicts a stochastic gravitational wave background observable both at the
Laser Interferometer Space Antenna and at the Einstein Telescope. Moreover the radion
tends to be heavy enough such that it evades current constraints, but may show up in
future LHC runs.The work of EM is supported by the Spanish MINEICO under
Grant FPA2015-64041-C2-1-P and FIS2017-85053-C2-1-P, by the Junta de Andaluc a under
Grant FQM-225, by the Basque Government under Grant IT979-16, and by the Spanish
Consolider Ingenio 2010 Programme CPAN (CSD2007-00042). The research of EM is also
supported by the Ram on y Cajal Program of the Spanish MINEICO, and by the Universidad
del Pa s Vasco UPV/EHU, Bilbao, Spain, as a Visiting Professor. GN is supported
by the Swiss National Science Foundation (SNF) under grant 200020-168988. The work
of MQ is partly supported by Spanish MINEICO under Grant CICYT-FEDER-FPA2014-
55613-P and FPA2017-88915-P, by the Severo Ochoa Excellence Program of MINEICO
under Grant SEV-2016-0588, and by CNPq PVE fellowship project 405559/2013-5
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