359 research outputs found
Proper Motions of Ionized Gas at the Galactic Center: Evidence for Unbound Orbiting Gas
We present radio continuum observations of the spiral-shaped ionized feature
(Sgr A West) within the inner pc of the Galactic center at three epochs
spanning 1986 to 1995. The VLA A-configuration was used at 2cm
(resolution of 0\dasec10\dasec2). We detect proper motions of a number
of features in the Northern and Eastern Arms of Sgr A West including the
ionized gas associated with IRS 13 with V(RA)= 113 \pm 10, V(Dec)=150 \pm15
km/s, IRS 2 with V(RA)= 122 \pm 11, V(Dec)=24 \pm 34 km/s and the Norther Arm
V(RA)= 126 \pm 30, V(Dec)=--207 \pm 58 km/s. We also report the detection of
features having transverse velocities > 1000 km/s including a head-tail radio
structure, the ``Bullet'', northwest of Sgr A with V(RA)= 722
\pm 156, V(Dec)=832 \pm 203 km/s, exceeding the escape velocity at the Galactic
center.
The proper motion measurements when combined with previous H92 radio
recombination line data suggest an unambiguous direction of the flow of ionized
gas orbiting the Galactic center. The measured velocity distribution suggests
that the ionized gas in the Northern Arm is not bound to the Galactic center
assuming a 2.5 million solar mass of dark matter residing at the Galactic
center. This implies that the stellar and ionized gas systems are not
dynamically coupled, thus, supporting a picture in which the gas features in
the Northern Arm and its extensions are the result of an energetic phenomenon
that has externally driven a cloud of gas cloud into the Galactic center.Comment: 11 pages, three figures (one color) and one table. Astrophysical
Journal Letters in pres
The properties of extragalactic radio sources selected at 20 GHz
We present some first results on the variability, polarization and general
properties of radio sources selected in a blind survey at 20 GHz, the highest
frequency at which a sensitive radio survey has been carried out over a large
area of sky. Sources with flux densities above 100 mJy in the AT20G Pilot
Survey at declination -60 to -70 were observed at up to three epochs during
2002-4, including near-simultaneous measurements at 5, 8 and 18 GHz in 2003. Of
the 173 sources detected, 65% are candidate QSOs, BL Lac objects or blazars,
20% galaxies and 15% faint (b > 22 mag) optical objects or blank fields.
On a 1-2 year timescale, the general level of variability at 20 GHz appears
to be low. For the 108 sources with good-quality measurements in both 2003 and
2004, the median variability index at 20 GHz was 6.9% and only five sources
varied by more than 30% in flux density.
Most sources in our sample show low levels of linear polarization (typically
1-5%), with a median fractional polarization of 2.3% at 20 GHz. There is a
trend for fainter sources to show higher fractional polarization.
At least 40% of sources selected at 20GHz have strong spectral curvature over
the frequency range 1-20 GHz. We use a radio `two-colour diagram' to
characterize the radio spectra of our sample, and confirm that the radio-source
population at 20 GHz (which is also the foreground point-source population for
CMB anisotropy experiments like WMAP and Planck) cannot be reliably predicted
by extrapolating the results of surveys at lower frequencies. As a result,
direct selection at 20 GHz appears to be a more efficient way of identifying 90
GHz phase calibrators for ALMA than the currently-proposed technique of
extrapolation from all-sky surveys at 1-5 GHz.Comment: 14-page paper plus 5-page data table. Replaced with published versio
LUNASKA simultaneous neutrino searches with multiple telescopes
The most sensitive method for detecting neutrinos at the very highest
energies is the lunar Cherenkov technique, which employs the Moon as a target
volume, using conventional radio telescopes to monitor it for nanosecond-scale
pulses of Cherenkov radiation from particle cascades in its regolith.
Multiple-antenna radio telescopes are difficult to effectively combine into a
single detector for this purpose, while single antennas are more susceptible to
false events from radio interference, which must be reliably excluded for a
credible detection to be made. We describe our progress in excluding such
interference in our observations with the single-antenna Parkes radio
telescope, and our most recent experiment (taking place the week before the
ICRC) using it in conjunction with the Australia Telescope Compact Array,
exploiting the advantages of both types of telescope.Comment: 4 pages, 4 figures, in Proceedings of the 32nd International Cosmic
Ray Conference (Beijing 2011
The Australia Telescope 20GHz (AT20G) Survey: analysis of the extragalactic source sample
The Australia Telescope 20 GHz (AT20G) survey is a blind survey of the whole
Southern sky at 20 GHz with follow-up observations at 4.8, 8.6, and 20 GHz
carried out with the Australia Telescope Compact Array (ATCA). In this paper we
present an analysis of radio spectral properties in total intensity and
polarisation, sizes, optical identifications, and redshifts of the sample of
the 5808 extragalactic sources in the survey catalogue of confirmed sources
over the whole Southern sky excluding the strip at Galactic latitude
|b|<1.5deg. The sample has a flux density limit of 40 mJy. Completeness has
been measured as a function of scan region and flux density. Averaging over the
whole survey area the follow-up survey is 78% complete above 50mJy and 93%
complete above 100mJy. 3332 sources with declination <-15deg have good quality
almost simultaneous observations at 4.8, 8.6, and 20GHz. The spectral analysis
shows that the sample is dominated by flat-spectrum sources. The fraction of
flat-spectrum sources decreases from 81% for 20GHz flux densities S>500mJy, to
60% for S<100mJy. There is also a clear spectral steepening at higher
frequencies with the median spectral index decreasing from -0.16 between 4.8
and 8.6GHz to -0.28 between 8.6 and 20GHz. Simultaneous observations in
polarisation are available for all the sources at all the frequencies. 768
sources have a good quality detection of polarised flux density at 20GHz; 467
of them were also detected in polarisation at 4.8 and/or at 8.6GHz so that it
has been possible to compare the spectral behaviour in total intensity and
polarisation. We have found that the polarised fraction increases slightly with
frequency and decreases with flux density. Cross matches and comparisons have
been made with other catalogues at lower radio frequencies, and in the optical,
X-ray and gamma-ray bands. Redshift estimates are available for 825 sources.Comment: 15 pages, 16 figures, accepted for publication in MNRA
Where Do Cooling Flows Cool?
Although only about 5 percent of the total baryonic mass in luminous
elliptical galaxies is in the form of cooled interstellar gas, it is
concentrated within the optical effective radius r_e where it influences the
local dynamical mass. The mass of cooled gas must be spatially distributed
since it greatly exceeds the masses of central black holes. We explore here the
proposition that a population of low mass, optically dark stars is created from
the cooled gas. We consider a wide variety of radial distributions for the
interstellar cooling, but only a few are consistent with observed X-ray surface
brightness profiles. In a region of concentrated interstellar cooling, the
X-ray emission can exceed that observed, suggesting the presence of additional
support by magnetic stresses or non-thermal pressure. In general we find that
the mass of cooled gas contributes significantly to stellar dynamical mass to
light ratios which vary with galactic radius. If the stars formed from cooled
interstellar gas are optically luminous, their influence on the the mass to
light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to
the rate that old stars lose mass, which is nearly independent of the initial
mass function of the old stellar population.Comment: 18 pages with 6 figures; accepted by Astrophysical Journa
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