55 research outputs found
Massive Milky Way Satellites in Cold and Warm Dark Matter: Dependence on Cosmology
We investigate the claim that the largest subhaloes in high resolution
dissipationless cold dark matter (CDM) simulations of the Milky Way are
dynamically inconsistent with observations of its most luminous satellites. We
find that the inconsistency is largely attributable to the large values of
\sigma_8 and n_s adopted in the discrepant simulations producing satellites
that form too early and therefore are too dense. We find the tension between
observations and simulations adopting parameters consistent with WMAP9 is
greatly diminished making the satellites a sensitive test of CDM. We find the
Via Lactea II halo to be atypical for haloes in a WMAP3 cosmology, a
discrepancy that we attribute to its earlier formation epoch than the mean for
its mass. We also explore warm dark matter (WDM) cosmologies for 1--4 keV
thermal relics. In 1 keV cosmologies subhaloes have circular velocities at kpc
scales ~ 60% lower than their CDM counterparts, but are reduced by only 10% in
4 keV cosmologies. Since relic masses < 2-3 keV are ruled out by constraints
from the number of Milky Way satellites and Lyman-\alpha\ forest, WDM has a
minor effect in reducing the densities of massive satellites. Given the
uncertainties on the mass and formation epoch of the Milky Way, the need for
reducing the satellite densities with baryonic effects or WDM is alleviated.Comment: 11 pages, 7 figures, submitted to MNRA
High-J CO SLEDs in nearby infrared bright galaxies observed by Herschel-PACS
We report the detection of far-infrared (FIR) CO rotational emission from
nearby active galactic nuclei (AGN) and starburst galaxies, as well as several
merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using
Herschel-PACS, we have detected transitions in the J = 14 - 20 range
( 130 - 185 m, 1612 - 2300 GHz) with upper limits
on (and in two cases, detections of) CO line fluxes up to J = 30. The
PACS CO data obtained here provide the first well-sampled FIR extragalactic CO
SLEDs for this range, and will be an essential reference for future high
redshift studies. We find a large range in the overall SLED shape, even amongst
galaxies of similar type, demonstrating the uncertainties in relying solely on
high-J CO diagnostics to characterize the excitation source of a galaxy.
Combining our data with low-J line intensities taken from the literature, we
present a CO ratio-ratio diagram and discuss its potential diagnostic value in
distinguishing excitation sources and physical properties of the molecular gas.
The position of a galaxy on such a diagram is less a signature of its
excitation mechanism, than an indicator of the presence (or absence) of warm,
dense molecular gas. We then quantitatively analyze the CO emission from a
subset of the detected sources with Large Velocity Gradient (LVG) radiative
transfer models to fit the CO SLEDs. Using both single-component and
two-component LVG models to fit the kinetic temperature, velocity gradient,
number density and column density of the gas, we derive the molecular gas mass
and the corresponding CO-to-H conversion factor, , for each
respective source. For the ULIRGs we find values in the canonical
range 0.4 - 5 M/(K kmspc), while for the other objects,
varies between 0.2 and 14.} Finally, we compare our best-fit LVG model
..Comment: 39 pages, 3 figures; Accepted to Ap
Radio Spectra of Luminous, Heavily Obscured WISE-NVSS Selected Quasars
We present radio spectra spanning GHz for the sample of heavily
obscured luminous quasars with extremely red mid-infrared-optical colors and
compact radio emission. The spectra are constructed from targeted 10 GHz
observations and archival radio survey data, which together yield flux
density measurements for each object. Our suite of Python tools for modeling
the radio spectra is publicly available on Github. Our primary result is that
most (61%) of the sample have peaked or curved radio spectra and many (36%)
could be classified as Gigahertz Peaked Spectrum (GPS) sources. This indicates
compact emission regions likely arising from recently triggered radio jets.
Assuming synchrotron self-absorption (SSA) generates the peaks, we infer
compact source sizes ( pc) with strong magnetic fields ( mG)
and young ages ( years). Conversely, free-free absorption (FFA)
could also create peaks due to the high column densities associated with the
deeply embedded nature of the sample. However, we find no correlations between
the existence or frequency of the peaks and any parameters of the MIR emission.
The high-frequency spectral indices are steep () and
correlate, weakly, with the ratio of MIR photon energy density to magnetic
energy density, suggesting that the spectral steepening could arise from
inverse Compton scattering off the intense MIR photon field. This study
provides a foundation for combining multi-frequency and mixed-resolution radio
survey data for understanding the impact of young radio jets on the ISM and
star formation rates of their host galaxies.Comment: 48 pages, 17 figures, published in Astrophysical Journa
Surveying the Dynamic Radio Sky with the Long Wavelength Demonstrator Array
This paper presents a search for radio transients at a frequency of 73.8 MHz
(4 m wavelength) using the all-sky imaging capabilities of the Long Wavelength
Demonstrator Array (LWDA). The LWDA was a 16-dipole phased array telescope,
located on the site of the Very Large Array in New Mexico. The field of view of
the individual dipoles was essentially the entire sky, and the number of
dipoles was sufficiently small that a simple software correlator could be used
to make all-sky images. From 2006 October to 2007 February, we conducted an
all-sky transient search program, acquiring a total of 106 hr of data; the time
sampling varied, being 5 minutes at the start of the program and improving to 2
minutes by the end of the program. We were able to detect solar flares, and in
a special-purpose mode, radio reflections from ionized meteor trails during the
2006 Leonid meteor shower. We detected no transients originating outside of the
solar system above a flux density limit of 500 Jy, equivalent to a limit of no
more than about 10^{-2} events/yr/deg^2, having a pulse energy density >~ 1.5 x
10^{-20} J/m^2/Hz at 73.8 MHz for pulse widths of about 300 s. This event rate
is comparable to that determined from previous all-sky transient searches, but
at a lower frequency than most previous all-sky searches. We believe that the
LWDA illustrates how an all-sky imaging mode could be a useful operational
model for low-frequency instruments such as the Low Frequency Array, the Long
Wavelength Array station, the low-frequency component of the Square Kilometre
Array, and potentially the Lunar Radio Array.Comment: 20 pages; accepted for publication in A
Constraining warm dark matter with cosmic shear power spectra
We investigate potential constraints from cosmic shear on the dark matter
particle mass, assuming all dark matter is made up of light thermal relic
particles. Given the theoretical uncertainties involved in making cosmological
predictions in such warm dark matter scenarios we use analytical fits to linear
warm dark matter power spectra and compare (i) the halo model using a mass
function evaluated from these linear power spectra and (ii) an analytical fit
to the non-linear evolution of the linear power spectra. We optimistically
ignore the competing effect of baryons for this work. We find approach (ii) to
be conservative compared to approach (i). We evaluate cosmological constraints
using these methods, marginalising over four other cosmological parameters.
Using the more conservative method we find that a Euclid-like weak lensing
survey together with constraints from the Planck cosmic microwave background
mission primary anisotropies could achieve a lower limit on the particle mass
of 2.5 keV.Comment: 26 pages, 9 figures, minor changes to match the version accepted for
publication in JCA
Nonlinear Evolution of Cosmological Structures in Warm Dark Matter Models
The dark energy dominated warm dark matter (WDM) model is a promising
alternative cosmological scenario. We explore large-scale structure formation
in this paradigm. We do this in two different ways: with the halo model
approach and with the help of an ensemble of high resolution N-body
simulations. Combining these quasi-independent approaches, leads to a physical
understanding of the important processes which shape the formation of
structures. We take a detailed look at the halo mass function, the
concentrations and the linear halo bias of WDM. In all cases we find
interesting deviations with respect to CDM. In particular, the
concentration-mass relation displays a turnover for group scale dark matter
haloes, for the case of WDM particles with masses of the order ~0.25 keV. This
may be interpreted as a hint for top-down structure formation on small scales.
We implement our results into the halo model and find much better agreement
with simulations. On small scales the WDM halo model now performs as well as
its CDM counterpart.Comment: accepted for publication in MNRA
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