507 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
Simulations of the Origin and Fate of the Galactic Center Cloud G2
We investigate the origin and fate of the recently discovered gas cloud G2
close to the Galactic Center. Our hydrodynamical simulations focussing on the
dynamical evolution of the cloud in combination with currently available
observations favor two scenarios: a Compact Cloud which started around the year
1995 and a Spherical Shell of gas, with an apocenter distance within the
disk(s) of young stars and a radius of a few times the size of the Compact
Cloud. The former is able to explain the detected signal of G2 in the
position-velocity diagram of the Br gamma emission of the year 2008.5 and
2011.5 data. The latter can account for both, G2's signal as well as the
fainter extended tail-like structure G2t seen at larger distances from the
black hole and smaller velocities. In contrast, gas stripped from a compact
cloud by hydrodynamical interactions is not able to explain the location of the
detected G2t emission in the observed position-velocity diagrams. This favors
the Spherical Shell Scenario and might be a severe problem for the Compact
Cloud as well as the so-called Compact Source Scenario. From these first
idealized simulations we expect a roughly constant feeding of the supermassive
black hole through a nozzle-like structure over a long period, starting shortly
after the closest approach in 2013.51 for the Compact Cloud. If the matter
accretes in the hot accretion mode, we do not expect a significant boost of the
current activity of Sgr A* for the Compact Cloud model, but a boost of the
average infrared and X-ray luminosity by roughly a factor of 80 for the
Spherical Shell scenario with order of magnitude variations on a timescale of a
few months. The near-future evolution of the cloud will be a sensitive probe of
the conditions of the gas distribution in the milli-parsec environment of the
massive black hole in the Galactic Center.Comment: 16 pages, 16 figures, accepted by Ap
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
Detecting Pulsars with Interstellar Scintillation in Variance Images
Pulsars are the only cosmic radio sources known to be sufficiently compact to
show diffractive interstellar scintillations. Images of the variance of radio
signals in both time and frequency can be used to detect pulsars in large-scale
continuum surveys using the next generation of synthesis radio telescopes. This
technique allows a search over the full field of view while avoiding the need
for expensive pixel-by-pixel high time resolution searches. We investigate the
sensitivity of detecting pulsars in variance images. We show that variance
images are most sensitive to pulsars whose scintillation time-scales and
bandwidths are close to the subintegration time and channel bandwidth.
Therefore, in order to maximise the detection of pulsars for a given radio
continuum survey, it is essential to retain a high time and frequency
resolution, allowing us to make variance images sensitive to pulsars with
different scintillation properties. We demonstrate the technique with
Murchision Widefield Array data and show that variance images can indeed lead
to the detection of pulsars by distinguishing them from other radio sources.Comment: 8 papes, 9 figures, accepted for publication in MNRA
Radio jet interactions in the radio galaxy PKS 2152-699
We present radio observations of the radio galaxy PKS 2152-699 obtained with
the Australia Telescope Compact Array (ATCA). The much higher resolution and
s/n of the new radio maps reveals the presence of a bright radio component
about 10 arcsec NE of the nucleus. This lies close to the highly ionized cloud
previously studied in the optical and here shown in a broadband red snapshot
image with the HST PC2. It suggests that PKS 2152-699 may be a jet/cloud
interaction similar to 3C277.3. This could cause the change in the position
angle (of ~20 deg) of the radio emission from the inner to the outer regions.
On the large scale, the source has Fanaroff & Riley type II morphology although
the presence of the two hot-spots in the centres of the lobes is unusual. The
northern lobe shows a particularly relaxed structure while the southern one has
an edge-brightened, arc-like structure.Comment: 7 pages, 5 encapsulated figures, 1 JPEG figure, accepted for MNRA
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