70 research outputs found
On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis
Here we address the question whether the ionized shells associated with giant
HII regions can be progenitors of the larger HI shell-like objects found in the
Milky Way and other spiral and dwarf irregular galaxies. We use for our
analysis a sample of 12 HII shells presented recently by Rela\~no et al. (2005,
2007). We calculate the evolutionary tracks that these shells would have if
their expansion is driven by multiple supernovae explosions from the parental
stellar clusters. We find, contrary to Rela\~no et al. (2007), that the
evolutionary tracks of their sample HII shells are inconsistent with the
observed parameters of the largest and most massive neutral hydrogen
supershells. We conclude that HII shells found inside giant HII regions may
represent the progenitors of small or intermediate HI shells, however they
cannot evolve into the largest HI objects unless, aside from the multiple
supernovae explosions, an additional energy source contributes to their
expansion.Comment: Accepted for publication in ApJ, tentatively scheduled for the ApJ
July 1, 2008, v681n1 issue. 19 pages, 4 figure
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
Nebular C IV 1550 Imaging of the Metal-Poor Starburst Mrk 71: Direct Evidence of Catastrophic Cooling
We use the Hubble Space Telescope ACS camera to obtain the first spatially
resolved, nebular imaging in the light of C IV 1548,1551 by using the F150LP
and F165LP filters. These observations of the local starburst Mrk 71 in NGC
2366 show emission apparently originating within the interior cavity around the
dominant super star cluster (SSC), Knot A. Together with imaging in He II 4686
and supporting STIS FUV spectroscopy, the morphology and intensity of the C IV
nebular surface brightness and the C IV / He II ratio map provide direct
evidence that the mechanical feedback is likely dominated by catastrophic
radiative cooling, which strongly disrupts adiabatic superbubble evolution. The
implied extreme mass loading and low kinetic efficiency of the cluster wind are
reasonably consistent with the wind energy budget, which is probably enhanced
by radiation pressure. In contrast, the Knot B SSC lies within a well-defined
superbubble with associated soft X-rays and He II 1640 emission, which are
signatures of adiabatic, energy-driven feedback from a supernova-driven
outflow. This system lacks clear evidence of C IV from the limb-brightened
shell, as expected for this model, but the observations may not be deep enough
to confirm its presence. We also detect a small C IV-emitting object that is
likely an embedded compact H II region. Its C IV emission may indicate the
presence of very massive stars (> 100 M_sun) or strongly pressure-confined
stellar feedback.Comment: 13 pages, 8 figures. Accepted to ApJ Letter
Detailed Kinematic Study of the Ionized and Neutral Gas in the Complex of Star Formation in the Galaxy IC 1613
We carried out detailed kinematic studies of the complex of multiple HI and
HII shells that represent the only region of ongoing star formation in the
dwarf irregular galaxy IC 1613. We investigated the ionized-gas kinematics by
using Fabry--Perot H-alpha observations with the 6-m Special Astrophysical
Observatory telescope and the neutral-gas kinematics by using VLA 21-cm radio
observations. We identified three extended (300-350 pc) neutral shells with
which the brightest HII shells in the complex of star formation are associated.
The neutral-gas kinematics in the complex has been studied for the first time
and the H~I shells were found to expand at a velocity of 15--18 km/s. We
constructed velocity ellipses for all HII shells in the complex and refined
(increased) the expansion velocities of most of them. The nature of the
interacting ionized and neutral shells is discussed.Comment: 14 pages, 9 EPS-figure
The Sensitivity of HAWC to High-Mass Dark Matter Annihilations
The High Altitude Water Cherenkov (HAWC) observatory is a wide field-of-view
detector sensitive to gamma rays of 100 GeV to a few hundred TeV. Located in
central Mexico at 19 degrees North latitude and 4100 m above sea level, HAWC
will observe gamma rays and cosmic rays with an array of water Cherenkov
detectors. The full HAWC array is scheduled to be operational in Spring 2015.
In this paper, we study the HAWC sensitivity to the gamma-ray signatures of
high-mass (multi- TeV) dark matter annihilation. The HAWC observatory will be
sensitive to diverse searches for dark matter annihilation, including
annihilation from extended dark matter sources, the diffuse gamma-ray emission
from dark matter annihilation, and gamma-ray emission from non-luminous dark
matter subhalos. Here we consider the HAWC sensitivity to a subset of these
sources, including dwarf galaxies, the M31 galaxy, the Virgo cluster, and the
Galactic center. We simulate the HAWC response to gamma rays from these sources
in several well-motivated dark matter annihilation channels. If no gamma-ray
excess is observed, we show the limits HAWC can place on the dark matter
cross-section from these sources. In particular, in the case of dark matter
annihilation into gauge bosons, HAWC will be able to detect a narrow range of
dark matter masses to cross-sections below thermal. HAWC should also be
sensitive to non-thermal cross-sections for masses up to nearly 1000 TeV. The
constraints placed by HAWC on the dark matter cross-section from known sources
should be competitive with current limits in the mass range where HAWC has
similar sensitivity. HAWC can additionally explore higher dark matter masses
than are currently constrained.Comment: 15 pages, 4 figures, version to be published in PR
VAMOS: a Pathfinder for the HAWC Gamma-Ray Observatory
VAMOS was a prototype detector built in 2011 at an altitude of 4100m a.s.l.
in the state of Puebla, Mexico. The aim of VAMOS was to finalize the design,
construction techniques and data acquisition system of the HAWC observatory.
HAWC is an air-shower array currently under construction at the same site of
VAMOS with the purpose to study the TeV sky. The VAMOS setup included six water
Cherenkov detectors and two different data acquisition systems. It was in
operation between October 2011 and May 2012 with an average live time of 30%.
Besides the scientific verification purposes, the eight months of data were
used to obtain the results presented in this paper: the detector response to
the Forbush decrease of March 2012, and the analysis of possible emission, at
energies above 30 GeV, for long gamma-ray bursts GRB111016B and GRB120328B.Comment: Accepted for pubblication in Astroparticle Physics Journal (20 pages,
10 figures). Corresponding authors: A.Marinelli and D.Zaboro
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