The Galactic centre mini-spiral in the mm-regime

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: devaky@astro.cas.cz
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Astronomical Institute, Academy of Sciences, Boční II 1401, 14100 Prague, Czech Republic
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA
⁵ Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Str., Toronto ON M5S 3H4, Canada
⁶ Instituto de Astrofísica de Andalucía – CSIC, Glorieta de la Astronomía S/N, 18008, Spain
⁷ LATT, Université de Toulouse, CNRS, 14 Avenue Edouard Belin, 31400 Toulouse, France
⁸ NASA Goddard Space Flight Ctr., Greenbelt, MD 20771, USA
⁹ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 9 July 2011
Accepted: 2 December 2011

Abstract

Context. The mini-spiral is a feature of the interstellar medium in the central  ~2 pc of the Galactic center. It is composed of several streamers of dust and ionised and atomic gas with temperatures between a few 100 K to 10⁴ K. There is evidence that these streamers are related to the so-called circumnuclear disk of molecular gas and are ionized by photons from massive, hot stars in the central parsec.

Aims. We attempt to constrain the emission mechanisms and physical properties of the ionized gas and dust of the mini-spiral region with the help of our multiwavelength data sets.

Methods. Our observations were carried out at 1.3 mm and 3 mm with the mm interferometric array CARMA in California in March and April 2009, with the MIR instrument VISIR at ESO’s VLT in June 2006, and the NIR Brγ with VLT NACO in August 2009.

Results. We present high resolution maps of the mini-spiral, and obtain a spectral index of 0.5  ±  0.25 for Sgr A*, indicating an inverted synchrotron spectrum. We find electron densities within the range 0.8–1.5  ×  10⁴ cm^-3 for the mini-spiral from the radio continuum maps, along with a dust mass contribution of  ~0.25 M_⊙ from the MIR dust continuum, and extinctions ranging from 1.8–3 at 2.16 μm in the Brγ line.

Conclusions. We observe a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm observations of the extended emission of the mini-spiral, which we interpret as evidence that there are a range of contributions to the thermal free-free emission by the ionized gas emission and by dust at 1.3 mm.