======================================================================== 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. 7 May 1, 2009 Recently submitted papers: -------------------------- 1) Composition of the galactic center star cluster Population analysis from adaptive optics narrow band spectral energy distributions (Buchholz et al., A&A) 2) Dynamics of Ionized Gas at the Galactic Center:, VLA Observations of the 3D Velocity Field and Location of the Ionized Streams in Sagittarius A West (Zhao et al., ApJ) 3) Images of the radiatively inefficient accretion flow surrounding a Kerr black hole: application in Sgr A* (Yuan et al., ApJ) ------------------------------------------------------------------------ Email : buchholz@ph1.uni-koeln.de Title : Composition of the galactic center star cluster Population analysis from adaptive optics narrow band spectral energy distributions Author(s): R. M. Buchholz (1) and R. Schoedel (2,1) and A. Eckart (1,3) Institute: (1) I. Physikalisches Institut, Universitaet zu Koeln, Zuelpicher Str. 77, 50937 Koeln, Germany, (2) Instituto de Astrofisica de Andalucia (IAA)-CSIC, Camino Bajo de Huetor 50, E-18008 Granada, Spain, (3) Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, 53121 Bonn, Germany Paper : A&A, 2009, accepted EPrint : 0903.2135 Abstract: The goals of this work are to develop a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters, applying it to the central parsec of the GC, and conducting a population analysis of this area.We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR K-band. A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars.This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 15.5 mag in the K band in the central parsec. Compared to previous spectroscopic investigations that are limited to 13-14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time. Extremely red objects and foreground sources can also be reliably removed from the sample. Comparison to sources of known classification indicates that the method has an accuracy of better than 87%. We classify 312 stars as early type candidates out of a sample of 5914 sources. Several results, such as the shape of the KLF and the spatial distribution of both early and late type stars, confirm and extend previous works. The distribution of the early type stars can be fitted with a steep power law ( beta _1'' = -1.49 +/- 0.12), alternatively with a broken power law, beta _1-10'' = -1.08 +/- 0.12, beta _10-20'' = -3.46 +/- 0.58, since we find a drop in the early type density at 10''. We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type density function shows an inversion in the inner 6'', with a power-law slope of beta _R<6'' = 0.17 +/- 0.09. The late type KLF has a power-law slope of 0.30+/-0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of (0.14 +/- 0.02). Our results agree best with an in-situ star formation scenario. ------------------------------------------------------------------------ Email : jzhao@cfa.harvard.edu Title : Dynamics of Ionized Gas at the Galactic Center:, VLA Observations of the 3D Velocity Field and Location of the Ionized Streams in Sagittarius A West Author(s): Jun-Hui Zhao Mark R. Morris W. M. Goss Tao An Institute: (1) Harvard-Smithsonian CfA, 60 Garden St, Cambridge, MA 02138 (2) Department of Physics and Astronomy, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095 (3) NRAO, P. O. Box O, Socorro, NM 87801 (4) Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China Paper : ApJ, Apr 2009, in press EPrint : 0904.3133 Web : http://www.cfa.harvard.edu/~jzhao/papers/ApJ09aprGC.ps Abstract: We present new results based on high-resolution observations of Sgr A West at the Galactic center with the Very Large Array (VLA) at 1.3 cm. By combining recent observations with those made at earlier epochs with the VLA at wavelengths of 1.3 and 3.6 cm, we measured proper motions for 71 compact HII components in the central 80'' (3 parsecs, assuming D=8 pc). Using VLA archival data for the H92 alpha radio recombination line, we also investigated radial velocities in the LSR velocity range from +200 to -415 km/sec . Combining proper motion and radial velocity measurements, we have determined the 3D velocity distribution in Sgr A West. We find that the three ionized streams (Northern Arm, Eastern Arm, and Western Arc) in the central 3 parsecs can be modeled with three bundles of Keplerian orbits around Sgr A*. Assuming that each of the observed streams of ionized gas follows a single orbit, we determined the five orbital parameters (a, e, \Omega, omega , i) for each of them using LSQ fitting to the locii of the streams. The degeneracy in the orbital solutions for both the direction of flow and the two mirror images can be further resolved using the information obtained from the velocity measurements. Our results confirm earlier results on the streams in the Western Arc and the Northern Arm to be in Keplerian orbits, suggesting that the stream in the Eastern Arm is also consistent with an elliptical orbit. All three are confined within the central three parsecs. Both the Northern and Eastern Arm streams have high eccentricities (e=0.83+/-0.10 and 0.82+/-0.05, respectively), while the Western Arc stream is nearly circular, with e=0.2+/-0.15. All three streams orbit around Sgr A* in a counterclockwise sense (viewed from the Earth) and have orbital periods in the range 4-8*10^4 y. To verify the fit, the distributions of radial and transverse velocity vectors in Sgr A West were also computed using the Keplerian model and they show good agreement with both the proper motion and radial velocity data. In addition, the computed orbits suggest that the Northern and Eastern Arm streams may collide in the ``Bar'' region (a few arcsec south of Sgr A*) and that most of the orbiting ionized gas in the ``Bar'' region is located behind Sgr A*. We also report an ionized nebula associated with IRS 8, including a bow shock in radio continuum emission which shows excellent agreement with near IR observations. From the H92 alpha line data, we find evidence for substantial interaction between the IRS 8 nebula and the Northern Arm stream occuring in the bow-shock region. Other new morphological features revealed in our high-resolution image include: 1) a helical structure in the Northern Arm, suggesting that MHD plays an important role in the motion of the ionized gas, in addition to the dynamics determined by the central gravitational field and 2) a linear feature in the IRS 16 region, suggesting the compressed edge of the Northern Arm may result from the collective winds and radiation pressure from the high mass stars in the IRS16 cluster. ------------------------------------------------------------------------ Email : yfyuan@ustc.edu.cn Title : Images of the radiatively inefficient accretion flow surrounding a Kerr black hole: application in Sgr A* Author(s): Ye-Fei Yuan(1), Xinwu Cao(2), Lei Huang(1,2,3), Zhi-Qiang Shen(2) Institute: (1) Key Laboratory for Research in Galaxies and Cosmology, University of Sciences and Technology of China, Chinese Academy of Sciences, Hefei 230026, China (2) Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China (3) Academia Sinica, Institute of Astronomy and Astrophysics, Taipei 106, Taiwan Paper : ApJ, April 2009, accepted Abstract: In fully general relativity, we calculate the images of the radiatively inefficient accretion flow (RIAF) surrounding a Kerr black hole with arbitrary spins, inclination angles, and observational wavelengths. For the same initial conditions, such as the fixed accretion rate, it is found that the intrinsic size and radiation intensity of the images become larger, but the images become more compact in the inner region, while the size of the black hole shadow decreases with the increase of the black hole spin. With the increase of the inclination angles, the shapes of the black hole shadows change and become smaller, even disappear at all due to the obscuration by the thick disks. For median inclination angles, the radial velocity observed at infinity is larger because of both the rotation and radial motion of the fluid in the disk, which results in the luminous part of the images is much brighter. For larger inclination angles, such as the disk is edge on, the emission becomes dimmer at longer observational wavelengths (such as at 7.0mm and 3.5mm wavelengths), or brighter at shorter observational wavelengths (such as at 1.3 mm wavelength) than that of the face on case, except for the high spin and high inclination images. These complex behaviors are due to the combination of the Lorentz boosting effect and the radiative absorption in the disk. We hope our results are helpful to determine the spin parameter of the black hole in low luminosity sources, such as the Galactic center. A primary analysis by comparison with the observed sizes of Sgr A* at millimeters strongly suggests that the disk around the central black hole at Sgr A* is highly inclined or the central black hole is rotating fast. ------------------------------------------------------------------------ (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 ========================================================================