======================================================================== 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. 12 Aug 6, 2009 Recently submitted papers: -------------------------- 1) First Spectroscopic Identification of Massive Young Stellar Objects, in the Galactic Center (An et al., ApJL) 2) An analysis of infrared emission spectra from the regions near the Galactic Center (Contini, MNRAS) 3) Near-Infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center I. Statistics and a Catalog of Candidates (Mauerhan et al., ApJ) 4) Near-infrared Polarimetry of flares from Sgr A* with Subaru/CIAO (Nishiyama et al., ApJ) 5) High angular resolution integral-field spectroscopy of the Galaxy's nuclear cluster: a missing stellar cusp? (Do et al., ApJ) ------------------------------------------------------------------------ Email : deokkeun@ipac.caltech.edu Title : First Spectroscopic Identification of Massive Young Stellar Objects, in the Galactic Center Author(s): Deokkeun An(1), Solange V. Ramirez(1), Kris Sellgren(2), Richard G. Arendt(3,4), A. C. Adwin Boogert(1), Mathias Schultheis(5,6), Susan R. Stolovy(7), Angela S. Cotera(8), Thomas P. Robitaille(9,10), and Howard A. Smith(9) Institute: (1) Infrared Processing and Analysis Center, California Institute of Technology, Mail Stop 100-22, Pasadena, CA 91125 (2) Department of Astronomy, Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (3) CRESST/UMBC/GSFC, Code 665, NASA/Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771 (4) Science Systems and Applications, Inc. (5) Observatoire de Besan\ccon, 41bis, avenue de l'Observatoire, F-25000 Besan\ccon, France (6) Institut d'Astrophysique de Paris, CNRS, 98bis Bd Arago, F-75014 Paris, France (7) Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125 (8) SETI Institute, 515 North Whisman Road, Mountain View, CA 94043 (9) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (10) Spitzer Postdoctoral Fellow Paper : ApJL, 2009, in press EPrint : 0907.4752 Abstract: We report the detection of several molecular gas-phase and ice absorption features in three photometrically-selected young stellar object (YSO) candidates in the central 280 pc of the Milky Way. Our spectra, obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, reveal gas-phase absorption from CO_2 (15.0 micron ), C_2H_2 (13.7 micron ) and HCN (14.0 micron ). We attribute this absorption to warm, dense gas in massive YSOs. We also detect strong and broad 15 micron CO_2 ice absorption features, with a remarkable double-peaked structure. The prominent long-wavelength peak is due to CH_3OH-rich ice grains, and is similar to those found in other known massive YSOs. Our IRS observations demonstrate the youth of these objects, and provide the first spectroscopic identification of massive YSOs in the Galactic Center. ------------------------------------------------------------------------ Email : marcel@wise.tau.ac.il Title : An analysis of infrared emission spectra from the regions near the Galactic Center Author(s): Marcella Contini Institute: School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel Paper : MNRAS, July 2009, in press EPrint : 0907.2014 Abstract: We present consistent modelling of line and continuum IR spectra in the region close to the Galactic center. The models account for the coupled effect of shocks and photoionization from an external source. The results show that the shock velocities range between 65 and 80 km/sec , the pre-shock densities between 1cm3 in the ISM to 200cm3 in the filamentary structures. The pre-shock magnetic field increases from 5. 10^-6 gauss in the surrounding ISM to 8 10^-5 gauss in the Arched Filaments. The stellar temperatures are 38000 K in the Quintuplet cluster and 27000 K in the Arches Cluster. The ionization parameter is relatively low (< 0.01 ) with the highest values near the clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the gaseous phase of Si is found throughout the whole observed region, indicating the presence of silicate dust. Grains including iron, are concentrated throughout the Arched Filaments. The modelling of the continuum SED in the IR range, indicates that a component of dust at temperatures of 100-200 K is present in the central region of the Galaxy. Radio emission appears to be thermal bremsstrahlung in the E2-W1 filaments crossing strip, however a synchrotron component is not excluded. More data are necessary to resolve this questions. ------------------------------------------------------------------------ Email : mauerhan@ipac.caltech.edu Title : Near-Infrared Counterparts to Chandra X-ray Sources Toward the Galactic Center I. Statistics and a Catalog of Candidates Author(s): J. C. Mauerhan(1, 2), M. P. Muno(3), M. R. Morris(2), F. E. Bauer(4), S. Nishiyama(5), T. Nagata(5) Institute: (1) Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA (2) Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA (3) Space Radiation Laboroatory, California Institute of Technology, Pasadena, CA 91125, USA (4) Columbia Astrophysics Laboratory, Columbia University, Pupin Laboratories, 550 West 120th Street, New York, NY 10027, USA (5) Department of Astronomy, Kyoto University, Kyoto 606-8502, Japan Paper : ApJ, July 2009, accepted EPrint : 0907.1935 Abstract: We present a catalog of 5184 candidate infrared counterparts to X-ray sources detected towards the Galactic center. The X-ray sample contains 9017 point sources detected in this region by the Chandra X-ray Observatory during the past decade, including data from a recent deep survey of the central 2^o*0.^o8 of the Galactic plane. A total of 6760 of these sources have hard X-ray colors, and the majority of them lie near the Galactic center; while most of the remaining 2257 soft X-ray sources lie in the foreground. We cross-correlated the X-ray source positions with the 2MASS and SIRIUS near-infrared catalogs, which collectively contain stars with a 10 sigma limiting flux of K_s<=15.6 mag. In order to distinguish absorbed infrared sources near the Galactic center from those in the foreground, we defined red and blue sources as those which have H-K_s>=0.9 and <0.9 mag, respectively. We find that 5.8+/-1.5% (2 sigma ) of the hard X-ray sources have real infrared counterparts, of which 228+/-99 are red and 166+/-27 are blue. The red counterparts are probably comprised of Wolf-Rayet and O stars, high-mass X-ray binaries, and symbiotic binaries located near the Galactic center. Foreground X-ray binaries suffering intrinsic X-ray absorption could be included in the sample of blue infrared counterparts to hard X-ray sources. We also find that 39.4+/-1.0% of the soft X-ray sources have blue infrared counterparts; most of these are probably coronally-active dwarfs in the foreground. There is a noteworthy collection of 20 red counterparts to hard X-ray sources near the Sagittarius B H ii region, which are probably massive binaries that have formed within the last several Myr. For each of the infrared matches to X-ray sources in our catalog we derived the probability that the association is real, based on the source properties and the results of the cross-correlation analysis. These data are included in our catalog and will serve spectroscopic surveys to identify infrared counterparts to X-ray sources near the Galactic center. ------------------------------------------------------------------------ Email : shogo@kusastro.kyoto-u.ac.jp Title : Near-infrared Polarimetry of flares from Sgr A* with Subaru/CIAO Author(s): Shogo Nishiyama(1,2), Motohide Tamura(3), Hirofumi Hatano(4), Tetsuya Nagata(1), Tomoyuki Kudo(3), Miki Ishii(5), Rainer Schoedel(6), and Andreas Eckart(7,8) Institute: (1) Department of Astronomy, Kyoto University, Kyoto, 606-8502 Japan (2) Research Fellow of the Japan Society for the Promotion of Science (JSPS) (3) National Astronomical Observatory of Japan, Mitaka, Tokyo, 181-8588 Japan (4) Department of Astrophysics, Nagoya University, Nagoya, 464-8602 Japan (5) Subaru Telescope, National Astronomical Observatory of Japan, 650 North A`ohoku Place, Hilo, HI 96720 (6) Instituto de Astrofisica de Andalucia (IAA)-CSIC, Camino Bajo de Huetor 50, E-18008 Granada, Spain (7) I.Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany (8) Max Planck Institute fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany Paper : ApJ, accepted EPrint : 0907.5466 Web : http://www.kusastro.kyoto-u.ac.jp/~shogo/paper/ApJ/SgrAS08/09 NishiyamaSgrASNIRPol.pdf Abstract: We have performed near-infrared monitoring observations of Sgr A*, the Galactic center radio source associated with a super-massive black hole, with the near-infrared camera CIAO and the 36-element AO system on the Subaru telescope. We observed three flares in the Ks band (2.15micron) during 220 min monitoring on 2008 May 28, and confirmed the flare emission is highly polarized, supporting the synchrotron radiation nature of the near-infrared emission. Clear variations in the degree and position angle of polarization were also detected: an increase of the degree of polarization of about 20 percent, and a swing of the position angle of about 60 - 70 degrees in the declining phase of the flares. The correlation between the flux and the degree of polarization can be well explained by the flare emission coming from hotspot(s) orbiting Sgr A*. Comparison with calculations in the literature gives a constraint to the inclination angle i of the orbit of the hotspot around Sgr A*, as 45 < i < 90 degrees (close to edge-on). ------------------------------------------------------------------------ Email : tdo@astro.ucla.edu Title : High angular resolution integral-field spectroscopy of the Galaxy's nuclear cluster: a missing stellar cusp? Author(s): T. Do(1), A. M. Ghez(1), M. R. Morris(1), J. R. Lu(2), K. Matthews(2), S. Yelda(1), J. Larkin(1) Institute: (1) Physics and Astronomy Department, University of California, Los Angeles, CA (2) California Institute of Technology, Pasadena, CA Paper : ApJ, accepted Abstract: We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type ( 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young ( 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. In the survey region, we classified 60 stars as early-type (23 newly identified) and 74 stars as late-type (61 newly identified). We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K' < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, SIGMA (R) \propto R^-\Gamma, is flat, with \Gamma = -0.27+/-0.19. Monte Carlo simulations of the possible de-projected volume density profile, n(r) \propto r^- gamma , show that gamma is less than 1.0 at the 99.73 % confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey - stellar collisions, mass segregation from stellar remnants, or a recent merger event - mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions. ------------------------------------------------------------------------ (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 ========================================================================