741 research outputs found
Fermi Gamma-ray Haze via Dark Matter and Millisecond Pulsars
We study possible astrophysical and dark matter (DM) explanations for the
Fermi gamma-ray haze in the Milky Way halo. As representatives of various DM
models, we consider DM particles annihilating into W+W-, b-bbar, and e+e-. In
the first two cases, the prompt gamma-ray emission from DM annihilations is
significant or even dominant at E > 10 GeV, while inverse Compton scattering
(ICS) from annihilating DM products is insignificant. For the e+e- annihilation
mode, we require a boost factor of order 100 to get significant contribution to
the gamma-ray haze from ICS photons. Possible astrophysical sources of high
energy particles at high latitudes include type Ia supernovae (SNe) and
millisecond pulsars (MSPs). Based on our current understanding of Ia SNe rates,
they do not contribute significantly to gamma-ray flux in the halo of the Milky
Way. As the MSP population in the stellar halo of the Milky Way is not well
constrained, MSPs may be a viable source of gamma-rays at high latitudes
provided that there are ~ 20 000 - 60 000 of MSPs in the Milky Way stellar
halo. In this case, pulsed gamma-ray emission from MSPs can contribute to
gamma-rays around few GeV's while the ICS photons from MSP electrons and
positrons may be significant at all energies in the gamma-ray haze. The
plausibility of such a population of MSPs is discussed. Consistency with the
Wilkinson Microwave Anisotropy Probe (WMAP) microwave haze requires that either
a significant fraction of MSP spin-down energy is converted into e+e- flux or
the DM annihilates predominantly into leptons with a boost factor of order 100.Comment: 18 pages, 1 table, 5 figures; v2: references and a few discussions
added, v3: minor change
DETERMINATION OF THE THIXOCASTING TEMPERATURES OF AZ91D AND OTHER Mg ALLOYS USING A QUENCHING METHOD
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2
We directly measure the evolution of the intergalactic Lya effective optical
depth, tau_eff, over the redshift range 2<z<4.2 from a sample of 86
high-resolution, high-signal-to-noise quasar spectra obtained with the ESI and
HIRES spectrographs on Keck, and with the MIKE spectrograph on Magellan. This
represents an improvement over previous analyses of the Lya forest from
high-resolution spectra in this redshift interval of a factor of two in the
size of the data set alone. We pay particular attention to robust error
estimation and extensively test for systematic effects. We find that our
estimates of the quasar continuum levels in the Lya forest obtained by spline
fitting are systematically biased low, with the magnitude of the bias
increasing with redshift, but that this bias can be accounted for using mock
spectra. The mean fractional error is <1% at z=2, 4% at z=3, and 12% at z=4.
Previous measurements of tau_eff at z>~3 based on directly fitting the quasar
continua in the Lya forest, which have generally neglected this effect, are
therefore likely biased low. We provide estimates of the level of absorption
arising from metals in the Lya forest based on both direct and statistical
metal removal results in the literature, finding that this contribution is
~6-9% at z=3 and decreases monotonically with redshift. The high precision of
our measurement, attaining 3% in redshift bins of width Delta z=0.2 around z=3,
indicates significant departures from the best-fit power-law redshift evolution
(tau_eff=0.0018(1+z)^3.92, when metals are left in), particularly near z=3.2.
The observed downward departure is statistically consistent with a similar
feature detected in a precision statistical measurement using Sloan Digital Sky
Survey spectra by Bernardi and coworkers using an independent approach.Comment: 27 pages, including 18 figures, published in Ap
New limits on the population of normal and millisecond pulsars in the Large and Small Magellanic Clouds
We model the potentially observable populations of normal and millisecond
radio pulsars in the Large and Small Magellanic Clouds (LMC and SMC) where the
known population currently stands at 19 normal radio pulsars. Taking into
account the detection thresholds of previous surveys, and assuming optimal
period and luminosity distributions based on studies of Galactic pulsars, we
estimate there are (1.79 +/- 0.20) x 10^4 and (1.09 +/- 0.16) x 10^4 normal
pulsars in the LMC and SMC respectively. When we attempt to correct for beaming
effects, and the fraction of high-velocity pulsars which escape the clouds, we
estimate birth rates in both the LMC and SMC to be comparable and in the range
0.5--1 pulsar per century. Although higher than estimates for the rate of
core-collapse supernovae in the clouds, these pulsar birth rates are consistent
with historical supernova observations in the past 300 yr. A substantial
population of active radio pulsars (of order a few hundred thousand) have
escaped the LMC and SMC and populate the local intergalactic medium. For the
millisecond pulsar (MSP) population, the lack of any detections from current
surveys leads to respective upper limits (at the 95% confidence level) of
15,000 for the LMC and 23,000 for the SMC. Several MSPs could be detected by a
currently ongoing survey of the SMC with improved time and frequency resolution
using the Parkes multibeam system. Giant-pulse emitting neutron stars could
also be seen by this survey.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
SIDM on FIRE: Hydrodynamical Self-Interacting Dark Matter simulations of low-mass dwarf galaxies
We compare a suite of four simulated dwarf galaxies formed in 10 haloes of collisionless Cold Dark Matter (CDM) with galaxies
simulated in the same haloes with an identical galaxy formation model but a
non-zero cross-section for dark matter self-interactions. These cosmological
zoom-in simulations are part of the Feedback In Realistic Environments (FIRE)
project and utilize the FIRE-2 model for hydrodynamics and galaxy formation
physics. We find the stellar masses of the galaxies formed in Self-Interacting
Dark Matter (SIDM) with are very similar to those in CDM
(spanning ) and all runs lie on a
similar stellar mass -- size relation. The logarithmic dark matter density
slope () in the central pc remains
steeper than for the CDM-Hydro simulations with stellar mass
and core-like in the most massive galaxy.
In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with
. Moreover, the central density profiles predicted in SIDM runs
without baryons are similar to the SIDM runs that include FIRE-2 baryonic
physics. Thus, SIDM appears to be much more robust to the inclusion of
(potentially uncertain) baryonic physics than CDM on this mass scale,
suggesting SIDM will be easier to falsify than CDM using low-mass galaxies. Our
FIRE simulations predict that galaxies less massive than provide potentially ideal targets for discriminating models,
with SIDM producing substantial cores in such tiny galaxies and CDM producing
cusps.Comment: 10 Pages, 7 figures, submitted to MNRA
The Great Observatories Origins Deep Survey: Constraints on the Lyman Continuum Escape Fraction Distribution of Lyman--Break Galaxies at 3.4<z<4.5
We use ultra-deep ultraviolet VLT/VIMOS intermediate-band and VLT/FORS1
narrow-band imaging in the GOODS Southern field to derive limits on the
distribution of the escape fraction (f_esc) of ionizing radiation for L >~
L*(z=3) Lyman Break Galaxies (LBGs) at redshift 3.4--4.5. Only one LBG, at
redshift z=3.795, is detected in its Lyman continuum (LyC; S/N~5.5), the
highest redshift galaxy currently known with a direct detection. Its
ultraviolet morphology is quite compact (R_eff=0.8, kpc physical). Three out of
seven AGN are also detected in their LyC, including one at redshift z=3.951 and
z850 = 26.1. From stacked data (LBGs) we set an upper limit to the average
f_esc in the range 5%--20%, depending on the how the data are selected (e.g.,
by magnitude and/or redshift). We undertake extensive Monte Carlo simulations
that take into account intergalactic attenuation, stellar population synthesis
models, dust extinction and photometric noise in order to explore the moments
of the distribution of the escaping radiation. Various distributions
(exponential, log-normal and Gaussian) are explored. We find that the median
f_esc is lower than ~6% with an 84% percentile limit not larger than 20%. If
this result remains valid for fainter LBGs down to current observational
limits, then the LBG population might be not sufficient to account for the
entire photoionization budget at the redshifts considered here, with the exact
details dependent upon the assumed ionizing background and QSO contribution
thereto. It is possible that f_esc depends on the UV luminosity of the
galaxies, with fainter galaxies having higher f_esc, and estimates of f_esc
from a sample of faint LBG from the HUDF (i775<28.5) are in broad quantitative
agreement with such a scenario.Comment: 58 pages, 23 figures; submitted to ApJ, revised version in response
to referee's comment
A High-Frequency Search for Pulsars Within the Central Parsec of SgrA*
We report results from a deep high-frequency search for pulsars within the
central parsec of Sgr A* using the Green Bank Telescope. The observing
frequency of 15 GHz was chosen to maximize the likelihood of detecting normal
pulsars (i.e. with periods of \,ms and spectral indices of ) close to Sgr A*, that might be used as probes of gravity in the
strong-field regime; this is the highest frequency used for such pulsar
searches of the Galactic Center to date. No convincing candidate was detected
in the survey, with a detection threshold of Jy
achieved in two separate observing sessions. This survey represents a
significant improvement over previous searches for pulsars at the Galactic
Center and would have detected a significant fraction ($\gtrsim 5%) of the
pulsars around Sgr A*, if they had properties similar to those of the known
population. Using our best current knowledge of the properties of the Galactic
pulsar population and the scattering material toward Sgr A*, we estimate an
upper limit of 90 normal pulsars in orbit within the central parsec of Sgr A*.Comment: 10 pages, 7 figures, accepted for publication in the ApJ
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