52 research outputs found
Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy
Using the Westerbork Synthesis Radio Telescope, we have detected HI 21
cm-line absorption in the Warm Neutral Medium of the Galaxy toward the
extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s
with a full width at half maximum of 53+/-6 km/s, is associated with the
Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3).
The estimated spin temperature of the gas is 3600+/-360 K. The volume density
is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273
and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the
3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600
K. In addition, we have also detected HI 21 cm-line absorption from high
velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of
-117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature
components in the high velocity clouds with spin temperatures of greater than
1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table
Small-Scale Variations of HI Spectra from Interstellar Scintillatio
I suggest that radio-wave scattering by the interstellar plasma, in
combination with subsonic gradients in the Doppler velocity of interstellar HI,
is responsible for the observed small-scale variation in HI absorption spectra
of pulsars. Velocity gradients on the order of 0.05 to 0.3 km/s across 1 AU can
produce the observed variations. I suggest observational tests to distinguish
between this model and the traditional picture of small-scale opacity
variations from cloudlets.Comment: 24 pages, 2 figures, Latex, uses AASTe
1420 MHz Continuum Absorption Towards Extragalactic Sources in the Galactic Plane
We present a 21-cm emission-absorption study towards extragalactic sources in
the Canadian Galactic Plane Survey (CGPS). We have analyzed HI spectra towards
437 sources with S > 150 mJy, giving us a source density of 0.6 sources per
square degree at arcminute resolution. We present the results of a first
analysis of the HI temperatures, densities, and feature statistics. Particular
emphasis is placed on 5 features with observed spin temperatures below 40 K. We
find most spin temperatures in the range from 40 K to 300 K. A simple HI
two-component model constrains the bulk of the cold component to temperatures
(T_c) between 40 K and 100 K. T_c peaks in the Perseus arm region and clearly
drops off with Galactocentric radius, R, beyond that. The HI density follows
this trend, ranging from a local value of 0.4 cm^{-3} to less than 0.1 cm^{-3}
at R = 20 kpc. We find that HI emission alone on average traces about 75% of
the total HI column density, as compared to the total inferred by the emission
and absorption. Comparing the neutral hydrogen absorption to CO emission no
correlation is found in general, but all strong CO emission is accompanied by a
visible HI spectral feature. Finally, the number of spectral HI absorption
features per kpc drop off exponentially with increasing R.Comment: 13 pages, 13 figures, Accepted for March 2004 Ap
The Structure of the Cold Neutral ISM on 10-100 Astronomical Unit Scales
We have used the Very Long Baseline Array (VLBA) and the Very Large Array
(VLA) to image Galactic neutral hydrogen in absorption towards four compact
extragalactic radio sources with 10 milliarcsecond resolution. Previous VLBA
data by Faison et al (1998) have shown the existence of prominent structures in
the direction of the extragalactic source 3C~138 with scale sizes of 10-20 AU
with changes in HI optical depth in excess of 0.8 0.1. In this paper we
confirm the small scale \hi optical depth variations toward 3C~147 suggested
earlier at a level up to 20 % 5% . The sources 3C~119, 2352+495 and
0831+557 show no significant change in \hi optical depth across the sources
with one sigma limits of 30%, 50%, and 100%. Of the seven sources recently
investigated with the VLBA and VLA, only 3C~138 and 3C~147 show statistically
significant variations in HI opacities.
Deshpande (2000) have attempted to explain the observed small-scale structure
as an extension of the observed power spectrum of structure on parsec size
scales. The predictions of Deshpande (2000) are consistent with the VLBA HI
data observed in the directions of a number of sources, including 3C~147, but
are not consistent with our previous observations of the HI opacity structure
toward 3C~138
Are Compact High-Velocity Clouds Extragalactic Objects?
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the
Local Group model and would have HI volume densities of order 0.0003/cm^3.
Clouds with these volume densities and the observed neutral hydrogen column
densities will be largely ionized, even if exposed only to the extragalactic
ionizing radiation field. Here we examine the implications of this process for
models of CHVCs. We have modeled the ionization structure of spherical clouds
(with and without dark matter halos) for a large range of densities and sizes,
appropriate to CHVCs over the range of suggested distances, exposed to the
extragalactic ionizing photon flux. Constant-density cloud models in which the
CHVCs are at Local Group distances have total (ionized plus neutral) gas masses
roughly 20-30 times larger than the neutral gas masses, implying that the gas
mass alone of the observed population of CHVCs is about 40 billion solar
masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass
in such CHVCs is a significant fraction of the dynamical mass of the Local
Group, and their line widths would exceed the observed FWHM. Models with dark
matter halos fare even more poorly; they must lie within approximately 200 kpc
of the Galaxy. We show that exponential neutral hydrogen column density
profiles are a natural consequence of an external source of ionizing photons,
and argue that these profiles cannot be used to derive model-independent
distances to the CHVCs. These results argue strongly that the CHVCs are not
cosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa
A search for soft X-ray emission associated with prominent high-velocity-cloud complexes
We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI
survey, looking for soft X-ray signatures of prominent high-velocity-cloud
(HVC) complexes. We study the transfer of 1/4 keV photons through the
interstellar medium in order to distinguish variations in the soft X-ray
background (SXRB) intensity caused by photoelectric absorption effects from
those due to excess X-ray emission. The X-ray data are modelled as a
combination of emission from the Local Hot Bubble (LHB) and emission from a
distant plasma in the galactic halo and extragalactic sources. The X-ray
radiation intensity of the galactic halo and extragalactic X-ray background is
modulated by the photoelectric absorption of the intervening galactic
interstellar matter. We show that large- and small-scale intensity variations
of the 1/4 keV SXRB are caused by photoelectric absorption which is
predominantly traced by the total N(HI) distribution. The extensive coverage of
the two surveys supports evidence for a hot, X-ray emitting corona. We show
that this leads to a good representation of the SXRB observations. For four
large areas on the sky, we search for regions where the modelled and observed
X-ray emission differ. We find that there is excess X-ray emission towards
regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray
emission is positionally correlated with the high-velocity clouds. Some lines
of sight towards HVCs also pass through significant amounts of
intermediate-velocity gas, so we cannot constrain the possible role played by
IVC gas in these directions of HVC and IVC overlap, in determining the X-ray
excesses.Comment: 16 pages, 8 figures, accepted for publication in Astronomy &
Astrophysics main journa
VLA Detection of RRLs from the radio nucleus of NGC 253 : Ionization by a weak AGN, an obscured SSC or a compact SNR ?
We have imaged the H92alpha and H75alpha radio recombination line (RRL)
emissions from the starburst galaxy NGC 253 with a resolution of ~4 pc. The
peak of the RRL emission at both frequencies coincides with the unresolved
radio nucleus. Both lines observed towards the nucleus are extremely wide, with
FWHM of ~200 km /s. Modeling the RRL and radio continuum data for the radio
nucleus shows that the lines arise in gas whose density is ~10^4 \cc and mass
is few thousand Msun, which requires an ionizing flux of (6-20)x10^{51} photons
/s. We consider a SNR expanding in a dense medium, a star cluster and also an
AGN as potential ionizing sources. Based on dynamical arguments, we rule out an
SNR as a viable ionizing source. A star cluster model was considered and the
dynamics of the ionized gas in a stellar-wind driven structure was
investigated. Such a model is consistent with the properties of the ionized gas
only for a cluster younger than ~10^5 years. The existence of such a young
cluster at the nucleus seems improbable. The third model assumes the ionizing
source to be an AGN at the nucleus. In this model, it was shown that the
observed X-ray flux is too weak to account for the required ionizing photon
flux. However, the ionization requirement can be explained if the accretion
disk is assumed to have a Big Blue Bump in its spectrum. Hence, we favor an AGN
at the nucleus as the source responsible for ionizing the observed RRLs. A
hybrid model consisting of a inner ADAF disk and an outer thin disk is
suggested, which could explain the radio, UV and the X-ray luminosities of the
nucleus.Comment: 9 pages, 4 figures, 4 tables. Accepted for publication in Ap
Multi-density model of the ionised gas in NGC 253 using radio recombination lines
We have imaged the H92alpha (8.3 GHz), H75alpha (15 GHz), and H166alpha (1.4
GHz) Radio Recombination Lines (RRLs) from NGC 253 at resolutions of 4.5 pc
(0.4"), 2.5 pc (0.2") and 53 pc (4.5") respectively. The H92alpha line arises
from individual compact sources, most of which possess radio continuum
counterparts. The line widths range from ~200 km/s for the sources near the
radio nucleus to 70-100 km/s for the extranuclear ones. These lines are emitted
by gas at a density ~10000 /cc. The remainder of the cm-wave RRLs arise in
lower density gas (~500 /cc) with a higher area filling factor and with ten
times highermass. A third component of higher density gas (>10000 /cc) is
required to explain the mm-wave RRLs.Comment: Accepted by A&A; Changed to fit all figures within pag
Detection of Cold Atomic Clouds in the Magellanic Bridge
We report a detection of cold atomic hydrogen in the Magellanic Bridge using
21-cm absorption spectroscopy toward the radio source B0312-770. With a column
density of N_HI=1.2E20 cm^-2, a maximum absorption optical depth of tau=0.10
and a maximum 21-cm emission brightness temperature of 1.4 K, this line of
sight yields a spin temperature, T_s, between 20 K and 40 K. H I 21-cm
absorption and emission spectroscopy toward 7 other low column density
sightlines on the periphery of the LMC and SMC reveal absorption toward one
additional background radio source behind the SMC with tau=0.03. The data have
typical sensitivities of sigma_tau=0.005 to 0.070 in absorption and
sigma_{T_B}=0.03 K in emission. These data demonstrate the presence of a cold
atomic phase which is probably accompanied by molecular condensations in the
tenuous interstellar medium of the Bridge region. Young OB stars observed in
the Magellanic Bridge could form "in situ" from these cold condensations rather
than migrate from regions of active star formation in the main body of the SMC.
The existence of cold condensations and star formation in the Magellanic Bridge
might be understood as a small scale version of the mechanism that produces
star formation in the tidal tails of interacting galaxies.Comment: 25 pages, uses AASTeX and psfig; Accepted for Publication in the
Astronomical Journa
The Parkes HI Survey of the Magellanic System
We present the first fully and uniformly sampled, spatially complete HI
survey of the entire Magellanic System with high velocity resolution, performed
with the Parkes Telescope. The final data-cubes have an rms noise of sigma ~
0.05 K and an effective angular resolution of 16 arcmin. The Large Magellanic
Cloud (LMC) and the Small Magellanic Cloud (SMC) are associated with huge
gaseous features with a total HI mass of M(HI) = 4.87 10^8 M_sun [d/55 kpc]^2,
if all HI gas is at the same distance of 55 kpc. Approximately two thirds of
this HI gas is located close to the Magellanic Clouds (Magellanic Bridge and
Interface Region), and 25% of the HI gas is associated with the Magellanic
Stream. The Leading Arm has a four times lower HI mass than the Magellanic
Stream, corresponding to 6% of the total HI mass of the gaseous features. We
have analyzed the velocity field of the Magellanic Clouds and their
neighborhood introducing a LMC-standard-of-rest frame. The HI in the Magellanic
Bridge shows low velocities relative to the Magellanic Clouds suggesting an
almost parallel motion, while the gas in the Interface Region has significantly
higher relative velocities indicating that this gas is leaving the Magellanic
Bridge building up a new section of the Magellanic Stream. The clouds in the
Magellanic Stream and the Leading Arm show significant differences, both in the
column density distribution and in the shapes of the line profiles. The HI gas
in the Magellanic Stream is more smoothly distributed than the gas in the
Leading Arm. These morphological differences can be explained if the Leading
Arm is at considerably lower z-heights and embedded in a higher pressure
ambient medium.Comment: 23 pages, 18 figures, accepted for publication in A&
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