======================================================================== G C N E W S * Newsflash * - The Newsletter for Galactic Center Research - gcnews@mpifr-bonn.mpg.de http://www.mpifr-bonn.mpg.de/gcnews ======================================================================== Vol. 10, No. 5 Jul 5, 1999 Recently submitted papers: -------------------------- 1) Discovery of a Non-Thermal Galactic Center Filament (G358.85+0.47) Parallel to the Galactic Plane (Lang et al., ApJL) 2) A Radio Polarimetric Study of the Galactic Center Threads (Lang et al., ApJ) 3) High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection (Figer et al., ApJ) ------------------------------------------------------------------------ Email : clang@zia.aoc.NRAO.EDU Title : Discovery of a Non-Thermal Galactic Center Filament (G358.85+0.47) Parallel to the Galactic Plane Author(s): Cornelia C. Lang(1,2),K.R. Anantharamaiah(1,3), N.E. Kassim(4), T.J.W. Lazio(4) Institute: (1) National Radio Astronomy Observatory, Box 0, Socorro, NM 87801, email: clang@nrao.edu (2) Division of Astronomy, 8371 Math Sciences Building, Box 951562, University of California at Los Angeles, LA, CA 90095-1562 (3) Raman Research Institute, Bangalore 560 080, India (4) Code 7213, Naval Research Laboratory, Washington D.C. 20375-5351 Paper : to appear in ApJ Letters EPrint : astro-ph/9906285 Abstract: We report the discovery of a new non-thermal filament, G358.85+0.47, the ``Pelican'', located \ab225 pc in projection from SgrA, and oriented parallel to the Galactic plane. VLA continuum observations at lambda 20 cm reveal that this 7' (17.5 pc) structure bends at its northern extension and is comprised of parallel strands, most apparent at its ends. Observations at lambda 6 and lambda 3.6 cm reveal that the Pelican is a synchrotron-emitting source and is strongly linearly polarized over much of its extent. The spectral index of the filament changes from alpha_20/6=-0.8 to alpha_6/3.6=-1.5. The rotation measures exhibit a smooth gradient, with values ranging from -1000 \radms to +500 \radms. The intrinsic magnetic field is well-aligned along the length of the filament. Based on these properties, we classify the Pelican as one of the non-thermal filaments unique to the Galactic center. Since these filaments (most of which are oriented perpendicular to the Galactic plane) are believed to trace the overall magnetic field in the inner Galaxy, the Pelican is the first detection of a component of this field parallel to the plane. The Pelican may thus mark a transition region of the magnetic field orientation in the inner kiloparsec of the Galaxy. ------------------------------------------------------------------------ Email : clang@zia.aoc.NRAO.EDU Title : A Radio Polarimetric Study of the Galactic Center Threads Author(s): Cornelia C. Lang(1,2), Mark Morris(2), and Luis Echevarria(3) Institute: (1) National Radio Astronomy Observatory, Box 0, Socorro, NM 87801; email: clang@nrao.edu (2) Division of Astronomy, 8371 Math Sciences Building, Box 951562, University of California at Los Angeles, LA, CA 90095-1562 (3) Department of Physics \& Astronomy, Arizona State University, Tempe, AZ Paper : to appear in the Astrophysical Journal EPrint : astro-ph/9906336 Abstract: Multi-frequency, polarimetric VLA observations of the non-thermal filaments (NTF's), G0.08+0.15, and G359.96+0.09, also known as the Northern and Southern Threads are presented at 20, 6, 3.6 and 2 cm, with high enough spatial resolution to be resolved for the first time at 6 and 3.6 cm. The 20 cm image reveals a wealth of new detail in the radio sources lying within the inner 60 pc of the Galaxy. The Southern Thread has a prominent split along its length, similar to splitting at the ends of previously studied NTF's. With resolutions as fine as 2'', the 3.6 and 6 cm images reveal a high degree of continuity and little substructure internal to the filament. The spectral index of the Northern Thread has been determined over a broad range of frequencies. Its flux density falls with frequency, alpha=-0.5 between 90 and 6 cm, and becomes much steeper (alpha=-2.0) between 6 and 2 cm. The spectral index does not vary significantly along the length of the Northern Thread, which implies either that the diffusion timescale for the emitting electrons is less than their synchrotron lifetime, or that the emitting electrons are reaccelerated continuously at multiple positions along the filament. Because of the lack of spectral index variation, we have not located the source of relativistic electrons. Polarization observations at 6 and 3.6 cm confirm the non-thermal nature of the emission from the Northern Thread. The fractional polarization in the Northern Thread reaches 70 rotation measures ( RM > 2000 rad/m^2) have been observed with irregular variations across the filament.The intrinsic magnetic field in the Northern Thread is predominantly aligned along its long axis. ------------------------------------------------------------------------ Email : figer@astro.ucla.edu Title : High Resolution Infrared Imaging and Spectroscopy of the Pistol Nebula: Evidence for Ejection Author(s): Donald F. Figer(1,2), Mark Morris(3,7), T. R. Geballe(3), R. Michael Rich(1), Eugene Serabyn(4), Ian S. McLean(1), R. C. Puetter(5), Amos Yahil(6) Institute: (1) Division of Astronomy, Department of Physics \& Astronomy, University of California, Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095-1562; figer@astro.ucla.edu, morris@astro.ucla.edu, rmr@astro.ucla.edu, mclean@astro.ucla.edu (2) Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (3) Gemini Observatory, 670 N. A'ohoku Place, Hilo, HI 96720; tgeballe@gemini.edu (4) JPL 171-113, 4800 Oak Grove Dr., Pasadena, CA 91109; eserabyn@huey.jpl.nasa.gov (5) Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0111; rpuetter@ucsd.edu (6) Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY 11794-3800; Amos.Yahil@sunysb.edu (7) Institut d'Astrophysique de Paris, 98 bis Blvd Arago, 75014 Paris, France Paper : to appear in the Astrophysical Journal EPrint : astro-ph/9906479 Abstract: We present new infrared images, obtained with the Hubble Space Telescope (HST) Near-infrared Camera and Multi-object Spectrometer (NICMOS), and Br- alpha (4.05 micron ) spectroscopy, obtained using CGS4 on UKIRT, of the Pistol Star and its associated nebula. We find strong evidence to support the hypothesis that the Pistol Nebula was ejected from the Pistol Star. The Pa- alpha (1.87 micron ) NICMOS image shows that the nebula completely surrounds the Pistol Star, although the line intensity is much stronger on its northern and western edges. The Br- alpha CGS4 spectra show the classical ring-like signature of quasi-spherical expansion. The blueshifted emission (V_max ~ -60 km/sec ) is much weaker than the redshifted emission (V_max ~ +10 km/sec ), where the velocities are with respect to the velocity of the Pistol Star; further, the redshifted emission spans a very narrow range of velocities, i.e., it appears ``flattened'' in the position-velocity diagram. These data suggest that the nebula was ejected from the star several thousand years ago, with a velocity between the current terminal velocity of the stellar wind (95 km/sec ) and the present expansion velocity of gas in the outer shell of the nebula (60 km/sec ). The Pa- alpha image reveals several emission-line stars in the region, including two newly-identified emission-line stars north of the Pistol Star, both of which are likely to be the hottest known stars in the Galactic center with spectral types earlier than WC8 and T_eff > 50,000 K). The presence of these stars, the morphology of the Pa-alpha emission, and the velocity field in the gas suggest that the side of the nebula furthest from us is approaching, and being ionized by, the hot stars of the Quintuplet, and that the highest velocity redshifted gas has been decelerated by winds from the Quintuplet stars. We also discuss the possibility that the nebular gas might be magnetically confined by the ambient magnetic field delineated by the nearby nonthermal filaments. ------------------------------------------------------------------------ (Older versions of the Newsflash can be found at the gcnews web-page) ======================================================================== Edited by Angela Cotera Heino Falcke (cotera@ipac.caltech.edu) (hfalcke@mpifr-bonn.mpg.de) - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - For Abstract submission please send the (La)Tex file of your paper to gcnews@mpifr-bonn.mpg.de ========================================================================