122 research outputs found
Disk mass densities in edge-on spiral galaxies
Very Large Array (VLA) observations of the neutral hydrogen (HI) gas in two nearby edge-on spirals (NGC 4565 and NGC 891) successfully resolve the thickness of the gas layers in both disks over a wide range in radii. The combination of B, C, and D array data produces a 4 arcsec (approx. 200 pc) beam and 21 km s(exp -1) velocity resolution, combined with sensitivity to structures as large as 18 arcmin (approx. 54 kpc). These observations directly constrain the mid-plane disk mass densities, under the assumption of an equilibrium between the thermal pressure of the gas and the gravitational attraction of the disk. The results of a preliminary analysis are given regarding the z-velocity dispersion of the gas, the mass-to-light ratio of the disk in NGC 4565, and the roles of atomic and molecular gases. The data also allow a detailed study of the HI in these galaxies; in general their brightness temperature distributions seem similar to that in the Milky Way. Both galaxies show asymmetric HI extensions beyond the optical disk. In NGC 4565 the extension is a surprisingly abrupt warp, which may bend back to parallel the galactic plane; the velocity structure implies the warp is continuous around the disk
The EVLA: Prospects for HI
To continue the unparalleled success of the Very Large Array (VLA) for radio
astronomy, the facility is currently being converted to become the 'Expanded
VLA' (EVLA). The EVLA will radically improve the VLA in order to cover the full
0.93-50 GHz radio wavelength range without gaps, provide up to an order of
magnitude better sensitivity, and to allow observations at much larger
bandwidths and spectral resolution as currently possible. For observations of
the 21 cm line of atomic neutral hydrogen (HI), the EVLA offers thousands of
km/s velocity coverage at sub-km/s resolution for targeted observations as well
as an improved spectral baseline stability. In addition, every L-band (21 cm)
continuum or targeted HI observation can be set-up to simultaneously observe a
full z=0-0.53 HI redshift survey at a velocity resolution of a few km/s. In
turn, every HI observation will also yield deep radio continuum images of the
field. These synergies will deliver a wealth of data which opens up a wide
'discovery space' to study the details of galaxy evolution and cosmology.Comment: to appear in the proceedings to the conference: "The Evolution of
Galaxies through the Neutral Hydrogen Window", Arecibo, PR, US
First Detection of Thermal Radio Emission from Solar-Type Stars with the Karl G. Jansky Very Large Array
We present the first detections of thermal radio emission from the
atmospheres of solar-type stars {\tau} Cet, {\eta} Cas A, and 40 Eri A. These
stars all resemble the Sun in age and level of magnetic activity, as indicated
by X-ray luminosity and chromospheric emission in calcium-II H and K lines. We
observed these stars with the Karl G. Jansky Very Large Array with
sensitivities of a few {\mu}Jy at combinations of 10.0, 15.0, and 34.5 GHz.
{\tau} Cet, {\eta} Cas A, and 40 Eri A are all detected at 34.5 GHz with
signal-to-noise ratios of 6.5, 5.2, and 4.5, respectively. 15.0-GHz upper
limits imply a rising spectral index greater than 1.0 for {\tau} Cet and 1.6
for {\eta} Cas A, at the 95% confidence level. The measured 34.5-GHz flux
densities correspond to stellar disk-averaged brightness temperatures of
roughly 10,000 K, similar to the solar brightness temperature at the same
frequency. We explain this emission as optically- thick thermal free-free
emission from the chromosphere, with possible contributions from coronal
gyroresonance emission above active regions and coronal free-free emission.
These and similar quality data on other nearby solar-type stars, when combined
with ALMA observations, will enable the construction of temperature profiles of
their chromospheres and lower transition regions.Comment: 9 pages, 2 figures. Accepted for publication in Ap
High-Resolution Images of Diffuse Neutral Clouds in the Milky Way. I. Observations, Imaging, and Basic Cloud Properties
A set of diffuse interstellar clouds in the inner Galaxy within a few hundred
pc of the Galactic plane has been observed at an angular resolution of ~1
arcmin combining data from the NRAO Green Bank Telescope and the Very Large
Array. At the distance of the clouds the linear resolution ranges from ~1.9 pc
to ~2.8 pc. These clouds have been selected to be somewhat out of the Galactic
plane and are thus not confused with unrelated emission, but in other respects
they are a Galactic population. They are located near the tangent points in the
inner Galaxy, and thus at a quantifiable distance: kpc
from the Galactic Center, and pc from the Galactic
plane. These are the first images of the diffuse neutral HI clouds that may
constitute a considerable fraction of the ISM. Peak HI column densities range
from cm. Cloud diameters vary between
about 10 and 100 pc, and their HI mass spans the range from less than a hundred
to a few thousands Msun. The clouds show no morphological consistency of any
kind except that their shapes are highly irregular. One cloud may lie within
the hot wind from the nucleus of the Galaxy, and some clouds show evidence of
two distinct thermal phases as would be expected from equilibrium models of the
interstellar medium.Comment: 81 pages, 42 figures, accepted for publication in the Astrophysical
Journal Supplement Serie
The ruff of equatorial emission around the SS433 jets: its spectral index and origin
We present unique radio observations of SS433, using MERLIN, the VLBA, and
the VLA, which allow us to, for the first time, properly image and derive a
meaningful spectral index for the `ruff' of equatorial emission which surrounds
SS433's jet. We interpret this smooth ruff as a wind-like outflow from the
binary.Comment: 4 pages, 4 figures, to appear in Proceedings of the 4th Microquasar
Workshop, eds. Ph Durouchoux, Y. Fuchs and J. Rodrigue
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