======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 28, No. 9 Sep 2, 2008 Recently submitted papers: -------------------------- 1) Low angular momentum flow model of Sgr A* activity (Czerny & Moscibrodzka, Proceedings) 2) Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre (Cuadra et al., MNRAS) 3) A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits (Lu et al., ApJ) 4) Exploring a New Population of Compact Objects: X-ray and IR Observations of the Galactic Centre (Bandyopadhyay et al., AIP) 5) IS THERE A SUPERMASSIVE BLACK HOLE AT THE CENTER OF THE MILKY WAY? (Reid, Modern Phys. D) 6) Comparison of 3.6 - 8.0 Micron Spitzer/IRAC Galactic Center Survey Point Sources with Chandra X-Ray Point Sources in the Central 40x40 Parsecs (Arendt et al., ApJ) 7) The Mid-Infrared Colors of the Interstellar Medium and Extended Sources at the Galactic Center (Arendt et al., ApJ) ------------------------------------------------------------------------ Email : bcz@camk.edu.pl Title : Low angular momentum flow model of Sgr A* activity Author(s): B. Czerny, M. Mo\' scibrodzka Institute: Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland Paper : Proceedings of AHAR2008 Conference, Bad Honeff Abstract: Sgr A* is the closest massive black hole and can be observed with the highest angular resolution. Nevertheless, our current understanding of the accretion process in this source is very poor. The inflow is almost certainly of low radiative efficiency and it is accompanied by a strong outflow and the flow is strongly variable but the details of the dynamics are unknown. Even the amount of angular momentum in the flow is an open question. Here we argue that low angular momentum scenario is better suited to explain the flow variability. We present a new hybrid model which describes such a flow and consists of an outer spherically symmetric Bondi flow and an inner axially symmetric flow described through MHD simulations. The assumed angular momentum of the matter is low, i.e. the corresponding circularization radius in the equatorial plane of the flow is just above the innermost stable circular orbit in pseudo-Newtonian potential. We compare the radiation spectrum from such a flow to the broad band observational data for Sgr A*. ------------------------------------------------------------------------ Email : jcuadra@cervantes.colorado.edu Title : Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre Author(s): Jorge Cuadra, Philip J. Armitage, Richard D. Alexander Paper : MNRAS 388, 64L EPrint : 0804.3596 Web : http://dx.doi.org/10.1111/j%2E1745-3933%2E2008%2E00500%2Ex Abstract: Observations of massive stars within the central parsec of the Galaxy show that, while most stars orbit within a well-defined disc, a significant fraction have large eccentricities and / or inclinations with respect to the disc plane. Here, we investigate whether this dynamically hot component could have arisen via scattering from an initially cold disc - the expected initial condition if the stars formed from the fragmentation of an accretion disc. Using N-body methods, we evolve a variety of flat, cold, stellar systems, and study the effects of initial disc eccentricity, primordial binaries, very massive stars and intermediate mass black holes. We find, consistent with previous results, that a circular disc does not become eccentric enough unless there is a significant population of undetected 100-1000 \msun objects. However, since fragmentation of an eccentric disc can readily yield eccentric stellar orbits, the strongest constraints come from inclinations. We show that none of our initial conditions yield the observed large inclinations, regardless of the initial disc eccentricity or the presence of massive objects. These results imply that the orbits of the young massive stars in the Galactic Centre are largely primordial, and that the stars are unlikely to have formed as a dynamically cold disc. ------------------------------------------------------------------------ Email : jlu@astro.ucla.edu Title : A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits Author(s): J. R. Lu(1), A. M. Ghez(1,2), S. D. Hornstein(1,3), M. R. Morris(1), E. E. Becklin(1), K. Matthews(4) Institute: (1) UCLA Department of Physics and Astronomy, Los Angeles, CA 90095-1562 (2) UCLA Institute of Geophysics and Planetary Physics, Los Angeles, CA 90095-1565 (3) Center for Astrophysics \& Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309 (4) Caltech Optical Observatories, California Institute of Technology, MS 320-47, Pasadena, CA 91125 Paper : ApJ, accepted EPrint : 0808.3818 Abstract: We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 3.''5 (0.14 pc) of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of only 0.07 mas yr^-1 (3 km s^-1), which is > 7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr^-2 (7 km s^-1 yr^-1). Using these measurements, line-of-sight velocities from the literature, and 3D velocities for additional young stars in the central parsec, we constrain the true orbit of each individual star and directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one of the previously proposed disks of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% of the young stars, is inclined by 115^o from the plane of the sky, and is oriented at a position angle of 100^o East of North. Additionally, the on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger projected radii as \propto r^-2. The disk has an out-of-the-disk velocity dispersion of 28 +/- 6 km s^-1, which corresponds to a half-opening angle of 7^o +/- 2^o, and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple, thin circular disk. ------------------------------------------------------------------------ Email : reba@astro.ufl.edu Title : Exploring a New Population of Compact Objects: X-ray and IR Observations of the Galactic Centre Author(s): Reba M. Bandyopadhyay Andrew J. Gosling Stephen E. Eikenberry Michael P. Muno Katherine M. Blundell Philipp Podsiadlowski Valerie J. Mikles Curtis DeWitt Paper : AIP, 2008, in press EPrint : 0808.0519 Abstract: I describe the IR and X-ray observational campaign we have undertaken for the purpose of determining the nature of the faint discrete X-ray source population discovered by Chandra in the Galactic Center (GC). Data obtained for this project includes a deep Chandra survey of the Galactic Bulge; deep, high resolution IR imaging from VLT/ISAAC, CTIO/ISPI, and the UKIDSS Galactic Plane Survey (GPS); and IR spectroscopy from VLT/ISAAC and IRTF/SpeX. By cross-correlating the GC X-ray imaging from Chandra with our IR surveys, we identify candidate counterparts to the X-ray sources via astrometry. Using a detailed IR extinction map, we are deriving magnitudes and colors for all the candidates. Having thus established a target list, we will use the multi-object IR spectrograph FLAMINGOS-2 on Gemini-South to carry out a spectroscopic survey of the candidate counterparts, to search for emission line signatures which are a hallmark of accreting binaries. By determining the nature of these X-ray sources, this FLAMINGOS-2 Galactic Center Survey will have a dramatic impact on our knowledge of the Galactic accreting binary population. ------------------------------------------------------------------------ Email : reid@cfa.harvard.edu Title : IS THERE A SUPERMASSIVE BLACK HOLE AT THE CENTER OF THE MILKY WAY? Author(s): Mark J. Reid Institute: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA, mreid@cfa.harvard.edu Paper : International Journal of Modern Physics D EPrint : 0808.2624 Abstract: This review outlines the observations that now provide an overwhelming scientific case that the center of our Milky Way Galaxy harbors a supermassive black hole. Observations at infrared wavelength trace stars that orbit about a common focal position and require a central mass (M) of 4*10^6 \Msun within a radius of 100 AU. Orbital speeds have been observed to exceed 5,000 km/sec . At the focal position there is an extremely compact radio source (\SgrA), whose apparent size is near the Schwarzschild radius (2GM/c^2). This radio source is motionless at the 1 km/sec level at the dynamical center of the Galaxy. The mass density required by these observations is now approaching the ultimate limit of a supermassive black hole within the last stable orbit for matter near the event horizon. ------------------------------------------------------------------------ Email : sellgren@astronomy.ohio-state.edu Title : Comparison of 3.6 - 8.0 Micron Spitzer/IRAC Galactic Center Survey Point Sources with Chandra X-Ray Point Sources in the Central 40x40 Parsecs Author(s): R. G. Arendt(1,2), D. Y. Gezari(3), S. R. Stolovy(4), K. Sellgren(5), R. Smith(3), S. V. Ramirez(6), F. Yusef-Zadeh(7), C. J. Law(7,8), H. A. Smith(9), A. S. Cotera(10), S. H. Moseley(3) Institute: (1) CRESST/UMBC/GSFC; (2) Science Systems and Applications, Inc. (3) NASA/Goddard Space Flight Center (4) Spitzer Science Center, California Institute of Technology (5) Ohio State University (6) IPAC, California Institute of Technology (7) Northwestern University (8) Sterrenkundig Instituut ``Anton Pannekoek'', Universiteit van Amsterdam (9) Harvard-Smithsonian Center for Astrophysics (10) SETI Institute Paper : ApJ, 2008, in press EPrint : 0806.1880 Abstract: We have studied the correlation between 2357 Chandra X-ray point sources in a 40 * 40 parsec field and 20,000 infrared sources we observed in the corresponding subset of our 2\arcdeg * 1.4\arcdeg Spitzer/IRAC Galactic Center Survey at 3.6-8.0 micron , using various spatial and X-ray hardness thresholds. The correlation was determined for source separations of less than 0.''5, 1'' or 2''. Only the soft X-ray sources show any correlation with infrared point sources on these scales, and that correlation is very weak. The upper limit on hard X-ray sources that have infrared counterparts is <1.7% (3 sigma ). However, because of the confusion limit of the IR catalog, we only detect IR sources with absolute magnitudes <= 1. As a result, a stronger correlation with fainter sources cannot be ruled out. Only one compact infrared source, IRS 13, coincides with any of the dozen prominent X-ray emission features in the 3 * 3 parsec region centered on Sgr A*, and the diffuse X-ray and infrared emission around Sgr A* seems to be anti-correlated on a few-arcsecond scale. We compare our results with previous identifications of near-infrared companions to Chandra X-ray sources. ------------------------------------------------------------------------ Email : sellgren@astronomy.ohio-state.edu Title : The Mid-Infrared Colors of the Interstellar Medium and Extended Sources at the Galactic Center Author(s): R. G. Arendt(1,2), S. R. Stolovy(3), S. V. Ramirez(4), K. Sellgren(5), A. S. Cotera(6), C. J. Law(7,8), F. Yusef-Zadeh(7), H. A. Smith(9), D. Y. Gezari(10) Institute: (1) CRESST/UMBC/GSFC (2) Science Systems and Applications, Inc. (3) Spitzer Science Center (4) IPAC, California Institute of Technology (5) Ohio State University (6) SETI Institute (7) Northwestern University (8) Astronomical Institute ``Anton Pannekoek'', University of Amsterdam (9) Harvard-Smithsonian Center for Astrophysics (10) NASA/Goddard Space Flight Center Paper : ApJ, 2008, 682, 384 EPrint : 0804.4491 Abstract: A mid-infrared (3.6 - 8 micron ) survey of the Galactic Center has been carried out with the IRAC instrument on the Spitzer Space Telescope. This survey covers the central 2\arcdeg *1.^o4 ( 280* 200 pc) of the Galaxy. At 3.6 and 4.5 micron the emission is dominated by stellar sources, the fainter ones merging into an unresolved background. At 5.8 and 8 micron the stellar sources are fainter, and large-scale diffuse emission from the ISM of the Galaxy's central molecular zone becomes prominent. The survey reveals that the 8 to 5.8 micron color of the ISM emission is highly uniform across the surveyed region. This uniform color is consistent with a flat extinction law and emission from polycyclic aromatic hydrocarbons (PAHs). Models indicate that this broadband color should not be expected to change if the incident radiation field heating the dust and PAHs is < 10^4 times that of the solar neighborhood. The few regions with unusually red emission are areas where the PAHs are underabundant and the radiation field is locally strong enough to heat large dust grains to produce significant 8 micron emission. These red regions include compact H II regions, Sgr B1, and wider regions around the Arches and Quintuplet Clusters. In these regions the radiation field is > 10^4 times that of the solar neighborhood. Other regions of very red emission indicate cases where thick dust clouds obscure deeply embedded objects or very early stages of star formation. ------------------------------------------------------------------------ (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 ========================================================================