======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@aoc.nrao.edu http://www.aoc.nrao.edu/~gcnews ======================================================================== Vol. 23, No. 3 Jan 12, 2006 Recently submitted papers: -------------------------- 1) A size of 1 AU for the radio source Sgr A* at the centre of the Milky Way (Shen et al., Nature) 2) Dynamical Cusp Regeneration (Merritt & Szell, ApJ) 3) Interstellar Extinction Law in the J, H, and K_S Bands, toward the Galactic Center (Nishiyama et al., ApJ) 4) The Arches Cluster - Evidence for a Truncated Mass Function? (Stolte et al., ApJL) 5) G359.95-0.04: An Energetic Pulsar Candidate Near Sgr A* (Wang et al., MNRAS) 6) Turbulent Origin of the Galactic-Center Magnetic Field: Nonthermal Radio Filaments (Yusef-Zadeh, ApJL) ------------------------------------------------------------------------ Email : llshang@shao.ac.cn Title : A size of 1 AU for the radio source Sgr A* at the centre of the Milky Way Author(s): Zhi-Qiang Shen, K. Y. Lo, M.-C. Liang, Paul T. P. Ho, J.-H. Zhao, Paper : Nature, 438(2005)62 EPrint : astro-ph/0512515 Abstract: Although it is widely accepted that most galaxies have supermassive black holes (SMBHs) at their centers, concrete proof has proved elusive. Sagittarius A* (Sgr A*), an extremely compact radio source at the center of our Galaxy, is the best candidate for proof, because it is the closest. Previous Very Long Baseline Interferometry (VLBI) observations (at 7mm) have detected that Sgr A* is 2 astronomical unit (AU) in size, but this is still larger than the "shadow" (a remarkably dim inner region encircled by a bright ring) arising from general relativistic effects near the event horizon. Moreover, the measured size is wavelength dependent. Here we report a radio image of Sgr A* at a wavelength of 3.5mm, demonstrating that its size is 1 AU. When combined with the lower limit on its mass, the lower limit on the mass density is 6.5*10^21 Msun pc^-3, which provides the most stringent evidence to date that Sgr A* is an SMBH. The power-law relationship between wavelength and intrinsic size (size proportional to wavelength^1.09), explicitly rules out explanations other than those emission models with stratified structure, which predict a smaller emitting region observed at a shorter radio wavelength. ------------------------------------------------------------------------ Email : merritt@astro.rit.edu Title : Dynamical Cusp Regeneration Author(s): David Merritt and Andras Szell Institute: (1) Department of Physics, Rochester Institute of Technology, Rochester, NY 14623 Paper : ApJ, 2006, in press EPrint : astro-ph/0510498 Abstract: After being destroyed by a binary supermassive black hole, a stellar density cusp can regrow at the center of a galaxy via energy exchange between stars moving in the gravitational field of the single, coalesced hole. We illustrate this process via high-accuracy N-body simulations. Regeneration requires roughly one relaxation time and the new cusp extends to a distance of roughly one-fifth the black hole's influence radius, with density \rho r^-7/4; the mass in the cusp is of order 10% the mass of the black hole. Growth of the cusp is preceded by a stage in which the stellar velocity dispersion evolves toward isotropy and away from the tangentially-anisotropic state induced by the binary. We show that density profiles similar to those observed at the center of the Milky Way and M32 can regenerate themselves in several Gyr following infall of a second black hole; the presence of density cusps at the centers of these galaxies can therefore not be used to infer that no merger has occurred. We argue that \rho r^-7/4 density cusps are ubiquitous in stellar spheroids fainter than M_V -18.5 that contain supermassive black holes, but the cusps have not been detected outside of the Local Group since their angular sizes are less than 0.1''. We show that the presence of a cusp implies a lower limit of 10^-4 yr^-1 on the rate of stellar tidal disruptions, and discuss the consequences of the cusps for gravitational lensing and the distribution of dark matter on sub-parsec scales. ------------------------------------------------------------------------ Email : shogo@z.phys.nagoya-u.ac.jp Title : Interstellar Extinction Law in the J, H, and K_S Bands, toward the Galactic Center Author(s): Shogo Nishiyama, Tetsuya Nagata, Nobuhiko Kusakabe, Noriyuki Matsunaga, Takahiro Naoi, Daisuke Kato, Chie Nagashima, Koji Sugitani, Motohide Tamura, Toshihiko Tanabe, and Shuji Sato Paper : ApJ, Feb 2006, in press EPrint : astro-ph/0601174 Abstract: We have determined the ratios of total to selective extinction in the near-infrared bands (J, H, K_S) toward the Galactic center from the observations of the region |l| < 2.0^o and 0.5^o < |b| < 1.0^o with the IRSF telescope and the SIRIUS camera. Using the positions of red clump stars in color-magnitude diagrams as a tracer of the extinction and reddening, we determine the average of the ratios of total to selective extinction to be A_K_S/E_H-K_S = 1.44+/-0.01, A_K_S/E_J-K_S = 0.494+/-0.006, and A_H/E_J-H =1.42+/-0.02, which are significantly smaller than those obtained in previous studies. >From these ratios, we estimate that A_J : A_H : A_K_S = 1 : 0.573 +/- 0.009 : 0.331 +/- 0.004 and E_J-H/E_H-K_S = 1.72 +/- 0.04, and we find that the power law A_ lambda proportional to lambda ^-1.99 +/- 0.02 is a good approximation over these wavelengths. Moreover, we find a small variation in A_K_S/E_H-K_S across our survey. This suggests that the infrared extinction law changes from one line of sight to another, and the so-called ``universality'' does not necessarily hold in the infrared wavelengths. ------------------------------------------------------------------------ Email : stolte@astro.ufl.edu Title : The Arches Cluster - Evidence for a Truncated Mass Function? Author(s): Andrea Stolte, Wolfgang Brandner, Eva K. Grebel, Rainer Lenzen, Anne-Marie Lagrange Paper : ApJL, Aug 2005 EPrint : astro-ph/0506575 Abstract: We have analyzed high-resolution, adaptive optics (AO) HK observations of the Arches cluster obtained with NAOS/CONICA. With a spatial resolution of 84 mas, the cluster center is uniquely resolved. From these data the present-day mass function (MF) of Arches is derived down to 4 M_o. The integrated MF as well as the core and 2^nd annulus MFs are consistent with a turn-over at 6-7 M_o. This turn-over indicates severe depletion of intermediate and low-mass stars in the Arches cluster, possibly caused by its evolution in the Galactic Center environment. The Arches MF represents the first resolved observation of a starburst cluster exhibiting a low-mass truncated MF. This finding has severe implications for stellar population synthesis modelling of extragalactic starbursts, the derivation of integrated properties such as the total mass of star clusters in dense environments, the survival of low-mass remnants from starburst populations, and chemical enrichment during starburst phases. ------------------------------------------------------------------------ Email : wqd@astro.umass.edu Title : G359.95-0.04: An Energetic Pulsar Candidate Near Sgr A* Author(s): Q.D. Wang, F.J. Lu, and E.V. Gotthelf Institute: (1) Astronomy Department, University of Massachusetts, Amherst, MA 01003 (2) Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540 (3) Laboratory of Particle Astrophysics, Institute of High Energy Physics, CAS, Beijing 100039, P.R. China (4) Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027 Paper : MNRAS, accepted EPrint : astro-ph/0512643 Abstract: We report the discovery of a prominent nonthermal X-ray feature located near the Galactic center that we identify as an energetic pulsar wind nebula. This feature, G359.95-0.04, lies 1 lyr north of Sgr A* (in projection), is comet-like in shape, and has a power law spectrum that steepens with increasing distance from the putative pulsar. The distinct spectral and spatial X-ray characteristics of the feature are similar to those belonging the rare class of ram-pressure confined pulsar wind nebulae. The luminosity of the nebula at the distance of Sgr A*, consistent with the inferred X-ray absorptions, is L_x 1 * 10^34 ergs s^-1 in the 2-10 keV energy band. The cometary tail extends back to a region centered at the massive stellar complex IRS 13 and surrounded by enhanced diffuse X-ray emission, which may represent an associated supernova remnant. Furthermore, the inverse Compton scattering of the strong ambient radiation by the nebula consistently explains the observed TeV emission from the Galactic center. We also briefly discuss plausible connections of G359.95-0.04 to other high-energy sources in the region, such as the young stellar complex IRS 13 and SNR Sgr A East. ------------------------------------------------------------------------ Email : zadeh@northwestern.edu Title : Turbulent Origin of the Galactic-Center Magnetic Field: Nonthermal Radio Filaments Author(s): Stanislav Boldyrev and Farhad Yusef-Zadeh Paper : ApJL in press Abstract: A great deal of study has been carried out over the last twenty years on the origin of the magnetic activity in the Galactic center. One of the most popular hypotheses assumes milli-Gauss magnetic field with poloidal geometry, pervading the inner few hundred parsecs of the Galactic-center region. However, there is a growing observational evidence for the large-scale distribution of a much weaker field of B less than or equal than 10 micro G in this region. Here, we propose that the Galactic-center magnetic field originates from turbulent activity that is known to be extreme in the central hundred parsecs. In this picture the spatial distribution of the magnetic field energy is highly intermittent, and the regions of strong field have filamentary structures. We propose that the observed nonthermal radio filaments appear in (or, possibly, may be identified with) such strongly magnetized regions. At the same time, the large-scale diffuse magnetic field is weak. Both results of our model can explain the magnetic field measurements of the the Galactic-center region. In addition, we discuss the role of ionized outflow from stellar clusters in producing the long magnetized filaments perpendicular to the Galactic plane. ------------------------------------------------------------------------ (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, Robin Herrnstein - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please follow the instructions which are at http://www.aoc.nrao.edu/~gcnews/home/submission.shtml ========================================================================