1,088 research outputs found
HI Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud
We present a 93' by 93' map of the area near the Local Group dwarf galaxy LGS
3, centered on an HI cloud 30' away from the galaxy. Previous authors
associated this cloud with LGS 3 but relied on observations made with a 36'
beam. Our high-resolution (3.4'), wide-field Arecibo observations of the region
reveal that the HI cloud is distinct from the galaxy and suggest an interaction
between the two. We point out faint emission features in the map that may be
gas that has been tidally removed from the HI cloud by LGS 3. We also derive
the rotation curve of the cloud and find that it is in solid-body rotation out
to a radius of 10', beyond which the rotation velocity begins to decline.
Assuming a spherical geometry for the cloud, the implied mass is 2.8 x 10^7
(d/Mpc) M_{Sun}, where d is the distance in Mpc. The observed HI mass is 5.5 x
10^6 (d/Mpc)^2 M_{Sun}, implying that the cloud is dark-matter dominated unless
its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity
cloud that is undergoing a tidal interaction with LGS 3 and therefore is
located roughly 700 kpc away from the Milky Way. The cloud then contains a
total mass of ~2.0 x 10^7 M_{Sun}, 82% of which consists of dark matter.Comment: 5 pages, 2 color figures. Accepted for publication in ApJ Letter
OH yields from the CH3CO+O-2 reaction using an internal standard
Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time
OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield
close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone
photolysis in the presence of O2
Tightly Correlated HI and FUV Emission in the Outskirts of M83
We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and
deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps
show a stunning spatial correlation out to almost 4 optical radii (r25),
roughly the extent of our maps. This underscores that HI traces the gas
reservoir for outer disk star formation and it implies that massive (at least
low level) star formation proceeds almost everywhere HI is observed. Whereas
the average FUV intensity decreases steadily with increasing radius before
leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low
HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk,
which is not the case within r25. The time for star formation to consume the
available HI, inferred from the ratio of HI to FUV intensity, rises with
increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times,
near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the
inner parts of galaxies hold in outer disks, the conversion of HI into bound,
molecular clouds seems to limit star formation in outer galaxy disks. The long
consumption times suggest that most of the extended HI observed in M83 will not
be consumed by in situ star formation. However, even these low star formation
rates are enough to expect moderate chemical enrichment in a closed outer disk.Comment: Accepted for Publication in ApJ
The Distances of SNR W41 and overlapping HII regions
New HI images from the VLA Galactic Plane Survey show prominent absorption
features associated with the supernovae remnant G23.3-0.3 (SNR W41). We
highlight the HI absorption spectra and the CO emission spectra of eight
small regions on the face of W41, including four HII regions, three non-thermal
emission regions and one unclassified region. The maximum velocity of
absorption for W41 is 782 km/s and the CO cloud at radial velocity
955 km/s is behind W41. Because an extended TeV source, a diffuse X-ray
enhancement and a large molecular cloud at radial velocity 775 km/s are
also projected at the center of W41, these yield the kinematic distance of 3.9
to 4.5 kpc for W41. For HII regions, our analyses reveal that both G23.42-0.21
and G23.07+0.25 are at the far kinematic distances (9.9 kpc and
10.6 kpc respectively) of their recombination-line velocities (1030.5 km/s
and 89.62.1 km/s respectively), G23.07-0.37 is at the near kinematic
distance (4.40.3 kpc) of its recombination-line velocity (82.72.0
km/s), and G23.27-0.27 is probably at the near kinematic distance (4.10.3
kpc) of its recombination-line velocity (76.10.6 km/s).Comment: 11 pages, 3 figs., 2 tables, accepted by A
Gas Rich Dwarf Spheroidals
We present evidence that nearly half of the dwarf spheroidal galaxies (dSph
and dSph/dIrr) in the Local Group are associated with large reservoirs of
atomic gas, in some cases larger than the stellar mass. The gas is sometimes
found at large distance (~10 kpc) from the center of a galaxy and is not
necessarily centered on it. Similarly large quantities of ionized gas could be
hidden in these systems as well. The properties of some of the gas reservoirs
are similar to the median properties of the High-Velocity Clouds (HVCs); two of
the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf
spheroidals might thus provide a link between the HVCs and stars. We show that
the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if
the galaxy is within 250 kpc of either the Milky Way or M31. This can be
explained if both galaxies have a sufficiently massive x-ray emitting halo that
produces ram-pressure stripping if a dwarf ventures too close to either giant
spiral. We also investigate tidal stripping of the dwarf galaxies and find that
although it may play a role, it cannot explain the apparent total absence of
neutral gas in most dSph galaxies at distances less than 250 kpc. For the
derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping
is found to be more than an order of magnitude more effective in removing the
gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar
masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs
or true dwarf galaxies similar to those we observe now.Comment: AASTex preprint style, 27 pages including 12 figures. Submitted to
ApJ. See also http://astro.berkeley.edu/~robisha
Is There a Fundamental Line for Disk Galaxies?
We show that there are strong local correlations between metallicity, surface
brightness, and dynamical mass-to-light ratio within M33, analogous to the
fundamental line of dwarf galaxies identified by Prada & Burkert (2002). Using
near-infrared imaging from 2MASS, the published rotation curve of M33, and
literature measurements of the metallicities of HII regions and supergiant
stars, we demonstrate that these correlations hold for points at radial
distances between 140 pc and 6.2 kpc from the center of the galaxy. At a given
metallicity or surface brightness, M33 has a mass-to-light ratio approximately
four times as large as the Local Group dwarf galaxies; other than this constant
offset, we see broad agreement between the M33 and dwarf galaxy data. We use
analytical arguments to show that at least two of the three fundamental line
correlations are basic properties of disk galaxies that can be derived from
very general assumptions. We investigate the effect of supernova feedback on
the fundamental line with numerical models and conclude that while feedback
clearly controls the scatter in the fundamental line, it is not needed to
create the fundamental line itself, in agreement with our analytical
calculations. We also compare the M33 data with measurements of a simulated
disk galaxy, finding that the simulation reproduces the trends in the data
correctly and matches the fundamental line, although the metallicity of the
simulated galaxy is too high, and the surface brightness is lower than that of
M33.Comment: 14 pages, 14 figures (5 in color). Accepted for publication in Ap
A Large-scale CO Imaging of the Galactic Center. II. Dynamical Properties of Molecular Clouds
The data from the Nobeyama Radio Observatory 45 m telescope Galactic Center
CO survey have been analyzed to generate a compilation of molecular clouds with
intense CO emission in this region. Clouds are identified in an automated
manner throughout the main part of the survey data for all CO emission peaks
exceeding 10 K (). Correlations between the size, velocity dispersion,
virial mass, and the CO luminosity, for the molecular clouds in the Galactic
center were shown. We diagnosed gravitational stabilities of identified clouds
assuming that the disk clouds are nearly at the onset of gravitational
instability. Most of the clouds and cloud complexes in the Galactic center are
gravitationally stable, while some clouds with intense CO emission are
gravitationally unstable.Comment: 4 pages, 4 figures, to appear in the Proceedings of the 32nd COSPAR
Scientific Assembl
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