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G C N E W S
* Newsflash *
- The Newsletter for Galactic Center Research -
gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews
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Vol. 29, No. 3 Jan 23, 2009
Recently submitted papers:
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1) Radio Recombination Lines toward the Galactic Center Lobe (Law et
al., ApJ)
2) Jet-lag in Sgr A*: What size and timing measurements tell us about
the central black hole in the Milky Way (Falcke et al., A&A)
3) On the number of young stellar discs in the Galactic Centre
(Loeckmann and Baumgardt, MNRAS)
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Email : claw@astro.berkeley.edu
Title : Radio Recombination Lines toward the Galactic Center Lobe
Author(s): C. J. Law(1,2,3), D. Backer(3), F. Yusef-Zadeh(1), R.
Maddalena(4)
Institute: (1) Department of Physics and Astronomy, Northwestern
University, Evanston, IL 60208, USA; (2) Astronomical
Institute ``Anton Pannekoek'', University of Amsterdam,
Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (3) Radio
Astronomy Lab, University of California, Berkeley, CA 94720,
USA; (4) National Radio Astronomy Observatory, Green Bank,
WV, 24944, USA;
Paper : ApJ accepted
EPrint : 09011480
Web : http://arxiv.org/abs/0901.1480
Abstract:
The Galactic Center lobe is a degree-tall shell seen in radio continuum
images of the Galactic center (GC) region. If it is actually located in
the GC region, formation models would require massive energy input
(e.g., starburst or jet) to create it. At present, observations have
not strongly constrained the location or physical conditions of the GC
lobe. This paper describes the analysis of new and archival single-dish
observations of radio recombination lines toward this enigmatic object.
The observations find that the ionized gas has a morphology similar to
the radio continuum emission, suggesting that they are associated. We
study averages of several transitions from H106 alpha to H191\epsilon
and find that the line ratios are most consistent with gas in local
thermodynamic equilibrium. The radio recombination line widths are
remarkably narrow, constraining the typical electron temperature to be
less than about 4000 K. These observations also find evidence of p!
ressure broadening in the higher electronic states, implying a gas
density of n_e=910^+310_-450 cm^-3. The electron temperature, gas
pressure, and morphology are all consistent with the idea that the GC
lobe is located in the GC region. If so, the ionized gas appears to
form a shell surrounding the central 100 parsecs of the galaxy with a
mass of roughly 10^5 \msol, similar to ionized outflows seen in dwarf
starbursts.
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Email : h.falcke@astro.ru.nl
Title : Jet-lag in Sgr A*: What size and timing measurements tell us
about the central black hole in the Milky Way
Author(s): Heino Falcke(1,2), Sera Markoff(3), Geoffrey C. Bower(4)
Institute: (1) Department of Astrophysics, Institute for Mathematics,
Astrophysics and Particle Physics, Radboud University, P.O.
Box 9010, 6500 GL Nijmegen, The Netherlands (2) ASTRON, Oude
Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands (3)
Astronomical Institute ``Anton Pannekoek'', University of
Amsterdam, Kruislaan 403, 1098SJ Amsterdam, The Netherlands
(4) UC Berkeley, 601 Campbell Hall, Astronomy Department \&
Radio Astronomy Lab, Berkeley, CA 94720, USA
Paper : A&A, in press
EPrint : 0901.3723
Web : http://www.astro.ru.nl/~falcke/publications.html#jetlag
Abstract:
The black hole at the Galactic Center, Sgr A*, is the prototype of a
galactic nucleus at a very low level of activity. Its radio through
submm-wave emission is known to come from a region close to the event
horizon, however, the source of the emission is still under debate. A
successful theory explaining the emission is based on a relativistic
jet model scaled down from powerful quasars. We want to test the
predictive power of this established jet model against newly available
measurements of wavelength-dependent time lags and the size-wavelength
structure in Sgr A*. Using all available closure amplitude VLBI data
from different groups, we again derived the intrinsic
wavelength-dependent size of Sgr A*. This allowed us to calculate the
expected frequency-dependent time lags of radio flares, assuming a
range of in- and outflow velocities. Moreover, we calculated the time
lags expected in the previously published pressure-driven jet model.
The predicted lags are then compared to radio monitoring observations
at 22, 43, and 350 GHz. The combination of time lags and size
measurementsfg imply a mildly relativistic outflow with bulk outflow
speeds of gamma beta 0.5-2. The newly measured time lags are reproduced
well by the jet model without any major fine tuning. The results
further strengthen the case for the cm-to-mm wave radio emission in Sgr
A* as coming from a mildly relativistic jet-like outflow. The
combination of radio time lag and VLBI closure amplitude measurements
is a powerful new tool for assessing the flow speed and direction in
Sgr A*. Future VLBI and time lag measurements over a range of
wavelengths will reveal more information about Sgr A*, such as the
existence of a jet nozzle, and measure the detailed velocity structure
of a relativistic jet near its launching point for the first time.
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Email : uloeck@astro.uni-bonn.de
Title : On the number of young stellar discs in the Galactic Centre
Author(s): Ulf Loeckmann and Holger Baumgardt
Paper : MNRAS, 2009, in press
EPrint : 0901.1660
Abstract:
Observations of the Galactic Centre show evidence of disc-like
structures of very young stars orbiting the central super-massive black
hole within a distance of a few 0.1 pc. While it is widely accepted
that about half of the stars form a relatively flat disc rotating
clockwise on the sky, there is a substantial ongoing debate on whether
there is a second, counter-clockwise disc of stars. By means of N-body
simulations using our BHINT code, we show that two highly inclined
stellar discs with the observed properties cannot be recognised as two
flat circular discs after 5 Myr of mutual interaction. Instead, our
calculations predict a significant warping of the two discs, which we
show to be apparent among the structures observed in the Galactic
Centre. While the high eccentricities of the observed counter-clockwise
orbits suggest an eccentric origin of this system, we show the
eccentricity distribution in the inner part of the more massive
clockwise disc to be perfectly consistent with an initially circular
disc in which stellar eccentricities increase due to both non-resonant
and resonant relaxation. We conclude that the relevant question to ask
is therefore not whether there are two discs of young stars, but
whether there were two such discs to begin with.
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Sera Markoff, Loránt Sjouwerman, Joseph Lazio,
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