======================================================================== 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. 17 Jan 19, 2010 Recently submitted papers: -------------------------- 1) Constraining the initial mass function of stars in the Galactic Centre (Loeckmann et al., MNRAS) 2) HST/NICMOS Paschen- alpha Survey of the Galactic Center: Overview (Wang et al., unknown) 3) A Large Scale Survey of X-Ray Filaments in the Galactic Center (Johnson et al., 2009MNRAS.399.1429) ------------------------------------------------------------------------ Email : holger@astro.uni-bonn.de Title : Constraining the initial mass function of stars in the Galactic Centre Author(s): U. Loeckmann, H. Baumgardt, & P. Kroupa Institute: Argelander Institute for Astronomy, University of Bonn, Auf dem Huegel 71, 53121 Bonn, Germany Paper : MNRAS, Okt 2009, in press Web : http://de.arxiv.org/abs/0910.4960 Abstract: For half a century, evidence has been growing that the formation of stars follows a universal distribution of stellar masses. In fact, no stellar population has been found showing a systematic deviation from the canonical initial mass function (IMF) found for example for the stars in the solar neighbourhood. The only exception may be the young stellar discs in the Galactic Centre, which have been argued to exhibit a top-heavy IMF. Here we discuss the question whether the extreme circumstances in the centre of the Milky Way may be the reason for a significant variation of the IMF. By means of stellar evolution models using different codes, we show that the observed luminosity in the central parsec is too high to be explained by a long-standing top-heavy IMF as suggested by other authors, considering the limited amount of mass inferred from stellar kinematics in this region. In contrast, continuous star formation over the Galaxy's lifetime following a canonical IMF results in a mass-to-light ratio and a total mass of stellar black holes (SBHs) consistent with the observations. Furthermore, these SBHs migrate towards the centre due to dynamical friction, turning the cusp of visible stars into a core as observed in the Galactic Centre. For the first time here we explain the luminosity and dynamical mass of the central cluster and both the presence and extent of the observed core, since the number of SBHs expected from a canonical IMF is just enough to make up for the missing luminous mass. We conclude that observations of the Galactic Centre are well consistent with continuous star formation following the canonical IMF and do not suggest a systematic variation as a result of the region's properties such as high density, metallicity, strong tidal field etc. If the young stellar discs prove to follow a top-heavy IMF, the circumstances that led to their formation must be very rare, since these have not affected most of the central cluster. ------------------------------------------------------------------------ Email : wqd@astro.umass.edu Title : HST/NICMOS Paschen- alpha Survey of the Galactic Center: Overview Author(s): Q. D. Wang^1, H. Dong^1, A. Cotera,^2 S. Stolovy,^3 M. Morris,^4 C. C. Lang,^5 M. P. Muno,^6 G. Schneider,^7 and D. Calzetti^1 Institute: (1)Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA, (2)SETI Institute, 515 North Whisman Road, Mountain View, CA 94043, USA, (3)Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA, (4)Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA, (5)Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245, USA, (6)Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, USA, (7)Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA Paper : unknown Abstract: We have recently carried out the first wide-field hydrogen Paschen- alpha line imaging survey of the Galactic Center (GC), using the NICMOS instrument aboard the Hubble Space Telescope. The survey maps out a region of 2253 \rm pc^2 (416 \rm arcmin^2) around the central supermassive black hole (Sgr A*) in the 1.87 and 1.90 micron narrow bands with a spatial resolution of 0.01 pc (0.''2 FWHM) at a distance of 8 kpc. Here we present an overview of the observations, data reduction, preliminary results, and potential scientific implications, as well as a description of the rationale and design of the survey. We have produced mosaic maps of the Paschen- alpha line and continuum emission, giving an unprecedentedly high resolution and high sensitivity panoramic view of stars and photo-ionized gas in the nuclear environment of the Galaxy. We detect a significant number of previously undetected stars with Paschen- alpha in emission. They are most likely massive stars with strong winds, as confirmed by our initial follow-up spectroscopic observations. About half of the newly detected massive stars are found outside the known clusters (Arches, Quintuplet, and Central). the clusters, while many of the others are apparently embedded in distinct HII regions, probably representing massive stellar clusters/groups in formation. Many previously known diffuse thermal features are now resolved into arrays of intriguingly fine linear filaments indicating a profound role of magnetic fields in sculpting the gas. The bright spiral-like Paschen- alpha emission around Sgr A* is seen to be well confined within the known dusty torus. In the directions roughly perpendicular to it, we further detect faint, diffuse Paschen- alpha emission features, which, like earlier radio images, suggest an outflow from the structure. In addition, we detect various compact Paschen- alpha nebulae, probably tracing the accretion and/or ejection of stars at various evolutionary stages. Multi-wavelength comparisons together with follow-up observations are helping us to address such questions as where and how massive stars form, how stellar clusters are disrupted, how massive stars shape and heat the surrounding medium, how various phases of this medium are interspersed, and how the supermassive black hole interacts with its environment. ------------------------------------------------------------------------ Email : wqd@astro.umass.edu Title : A Large Scale Survey of X-Ray Filaments in the Galactic Center Author(s): S. P. Johnson, H. Dong, and Q. D. Wang Institute: (1) Department of Astronomy, University of Massachusetts Amherst, Amherst, MA Paper : 2009MNRAS.399.1429 Abstract: We present a catalog of 17 filamentary X-ray features located within a 68'x34' view centered on the Galactic Center region from images taken by \textitChandra. These features are described by their morphological and spectral properties. Many of the X-ray features have non-thermal spectra that are well fit by an absorbed power-law. Of the 17 features, we find 6 that have not been previously detected, 4 of which are outside the immediate 20'*20' area centered on the GC. 8 of the 17 identified filaments have morphological and spectral properties expected for pulsar wind nebulae with X-ray luminosities of 10^32-10^33 ergs s^-1 in the 2.0-10.0 keV band and photon indexes of \Gamma=1-2. In one feature, we suggest the strong neutral Fe K alpha emission line to be a possible indicator for past activity of Sgr A*. For G359.942-0.03, a particular filament of interest, we propose the model of a ram pressure confined stellar wind bubble from a massive star to account for the morphology, spectral shape and 6.7 keV He-like Fe emission detected. We also present a piecewise spectral analysis on two features of interest, G0.13-0.11 and G359.89-0.08, to further examine their physical interpretations. This analysis favors the PWN scenario for these features. ------------------------------------------------------------------------ (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 ========================================================================