======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 29, No. 3 Jan 23, 2009 Recently submitted papers: -------------------------- 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) ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Sera Markoff, Loránt Sjouwerman, Joseph Lazio, Cornelia Lang, Rainer Schödel, Masaaki Sakano, Feng Yuan - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================