436 research outputs found
Simulating intergalactic quasar scintillation
Intergalactic scintillation of distant quasars is sensitive to free electrons
and therefore complements Ly absorption line experiments probing the
neutral intergalactic medium (IGM). We present a new scheme to compute IGM
refractive scintillation effects on distant sources in combination with
Adaptive Mesh Refinement cosmological simulations. First we validate our model
by reproducing the well-known interstellar scintillation (ISS) of Galactic
sources. The simulated cosmic density field is then used to infer the
statistical properties of intergalactic scintillation. Contrary to previous
claims, we find that the scattering measure of the simulated IGM at is
\langle \mbox{SM}_{\equ}\rangle=3.879, i.e. almost 40 times larger than for
the usually assumed smooth IGM. This yield an average modulation index ranging
from 0.01 ( GHz) up to 0.2 ( GHz); above \nu_{s}\gsim30
GHz the IGM contribution dominates over ISS modulation. We compare our model
with data from a quasar sample observed at \nu_{\obs}=8.4
GHz. For this high frequency (), high galactic
latitude sample ISS is negligible, and IGM scintillation can reproduce the
observed modulation with a 4% accuracy, without invoking intrinsic source
variability. We conclude by discussing the possibility of using IGM
scintillation as a tool to pinpoint the presence of intervening high-
groups/clusters along the line of sight, thus making it a probe suitably
complementing Sunyaev-Zeldovich data recently obtained by \textit{Planck}.Comment: 14 pages, 13 figures, accepted for publication in MNRA
Two phase galaxy formation: The Evolutionary Properties of Galaxies
We use our model for the formation and evolution of galaxies within a
two-phase galaxy formation scenario, showing that the high-redshift domain
typically supports the growth of spheroidal systems, whereas at low redshifts
the predominant baryonic growth mechanism is quiescent and may therefore
support the growth of a disc structure. Under this framework we investigate the
evolving galaxy population by comparing key observations at both low and
high-redshifts, finding generally good agreement. By analysing the evolutionary
properties of this model, we are able to recreate several features of the
evolving galaxy population with redshift, naturally reproducing number counts
of massive star-forming galaxies at high redshifts, along with the galaxy
scaling relations, star formation rate density and evolution of the stellar
mass function. Building upon these encouraging agreements, we make model
predictions that can be tested by future observations. In particular, we
present the expected evolution to z=2 of the super-massive black hole mass
function, and we show that the gas fraction in galaxies should decrease with
increasing redshift in a mass, with more and more evolution going to higher and
higher masses. Also, the characteristic transition mass from disc to bulge
dominated system should decrease with increasing redshift.Comment: 15 pages, 11 figures. Version polished for publication in MNRA
Particle energy cascade in the intergalactic medium
We study the development of high-energy (Ein <= 1 TeV) cascades produced by a primary electron of energy Ein injected into the intergalactic medium (IGM). To this aim we have developed the new code MEDEA (Monte Carlo Energy Deposition Analysis) which includes Bremsstrahlung and inverse Compton (IC) processes, along with H/He collisional ionizations and excitations, and electron-electron collisions. The cascade energy partition into heating, excitations and ionizations depends primarily not only on the IGM ionized fraction, xe, but also on redshift, z, due to IC on cosmic microwave background (CMB) photons. While Bremsstrahlung is unimportant under most conditions, IC becomes largely dominant at energies Ein >= 1 MeV. The main effect of IC at injection energies Ein <= 100 MeV is a significant boost of the fraction of energy converted into low-energy photons (h\u3bd < 10.2 eV) which do not further interact with the IGM. For energies Ein >= 1 GeV CMB photons are preferentially upscattered within the X-ray spectrum (h\u3bd > 104 eV) and can free stream to the observer. Complete tables of the fractional energy depositions as a function of redshift, Ein and ionized fraction are given. Our results can be used in many astrophysical contexts, with an obvious application related to the study of decaying/annihilating dark matter (DM) candidates in the high-z Universe
Discovery of the supernova remnant G351.0-5.4
Context. While searching the NRAO VLA Sky Survey (NVSS) for diffuse radio
emission, we have serendipitously discovered extended radio emission close to
the Galactic plane. The radio morphology suggests the presence of a previously
unknown Galactic supernova remnant. An unclassified {\gamma}-ray source
detected by EGRET (3EG J1744-3934) is present in the same location and may stem
from the interaction between high-speed particles escaping the remnant and the
surrounding interstellar medium.
Aims. Our aim is to confirm the presence of a previously unknown supernova
remnant and to determine a possible association with the {\gamma}-ray emission
3EG J1744-3934.
Methods. We have conducted optical and radio follow-ups of the target using
the Dark Energy Camera (DECam) on the Blanco telescope at Cerro Tololo
Inter-American Observatory (CTIO) and the Giant Meterwave Radio Telescope
(GMRT). We then combined these data with archival radio and {\gamma}-ray
observations.
Results. While we detected the extended emission in four different radio
bands (325, 1400, 2417, and 4850 MHz), no optical counterpart has been
identified. Given its morphology and brightness, it is likely that the radio
emission is caused by an old supernova remnant no longer visible in the optical
band. Although an unclassified EGRET source is co-located with the supernova
remnant, Fermi-LAT data do not show a significant {\gamma}-ray excess that is
correlated with the radio emission. However, in the radial distribution of the
{\gamma}-ray events, a spatially extended feature is related with SNR at a
confidence level {\sigma}.
Conclusions. We classify the newly discovered extended emission in the radio
band as the old remnant of a previously unknown Galactic supernova: SNR
G351.0-5.4.Comment: 6 pages, 6 figures, accepted A&
Neutrino emission from dark matter annihilation/decay in light of cosmic and data
A self-consistent global fitting method based on the Markov Chain Monte Carlo
technique to study the dark matter (DM) property associated with the cosmic ray
electron/positron excesses was developed in our previous work. In this work we
further improve the previous study to include the hadronic branching ratio of
DM annihilation/decay. The PAMELA data are employed to constrain
the hadronic branching ratio. We find that the 95% () upper limits of
the quark branching ratio allowed by the PAMELA data is for DM annihilation and for DM decay respectively. This
result shows that the DM coupling to pure leptons is indeed favored by the
current data. Based on the global fitting results, we further study the
neutrino emission from DM in the Galactic center. Our predicted neutrino flux
is some smaller than previous works since the constraint from -rays is
involved. However, it is still capable to be detected by the forth-coming
neutrino detector such as IceCube. The improved points of the present study
compared with previous works include: 1) the DM parameters, both the particle
physical ones and astrophysical ones, are derived in a global fitting way, 2)
constraints from various species of data sets, including -rays and
antiprotons are included, and 3) the expectation of neutrino emission is fully
self-consistent.Comment: 13 pages, 2 figures, 1 table; Published in IJMPA 201
New Constraints from PAMELA anti-proton data on Annihilating and Decaying Dark Matter
Recently the PAMELA experiment has released its updated anti-proton flux and
anti-proton to proton flux ratio data up to energies of ~200GeV. With no clear
excess of cosmic ray anti-protons at high energies, one can extend constraints
on the production of anti-protons from dark matter. In this letter, we consider
both the cases of dark matter annihilating and decaying into standard model
particles that produce significant numbers of anti-protons. We provide two sets
of constraints on the annihilation cross-sections/decay lifetimes. In the one
set of constraints we ignore any source of anti-protons other than dark matter,
which give the highest allowed cross-sections/inverse lifetimes. In the other
set we include also anti-protons produced in collisions of cosmic rays with
interstellar medium nuclei, getting tighter but more realistic constraints on
the annihilation cross-sections/decay lifetimes.Comment: 7 pages, 3 figures, 3 table
The HI Content of Local Late-Type Galaxies
We present a solid relationship between the neutral hydrogen (HI) disk mass
and the stellar disk mass of late-type galaxies in the local universe. This
relationship is derived by comparing the stellar disk mass function from the
Sloan Digital Sky Survey and the HI mass function from the HI Parkes All Sky
Survey (HIPASS). We find that the HI mass in late-type galaxies tightly
correlates with the stellar mass over three orders of magnitude in stellar disk
mass. We cross-check our result with that obtained from a sample of HIPASS
objects for which the stellar mass has been obtained by inner kinematics. In
addition, we derive the HI versus halo mass relationship and the dependence of
all the baryonic components in spirals on the host halo mass. These
relationships bear the imprint of the processes ruling galaxy formation, and
highlight the inefficiency of galaxies both in forming stars and in retaining
their pristine HI gas.Comment: 6 pages, 5 figures. Match to the published version. References
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