Surviving the hole

I. Spatially resolved chemistry around Sagittarius A^∗⋆

¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
e-mail: smartin@eso.org
² Harvard-Smithsonian Center for Astrophysics, 60 Garden St., 02138, Cambridge, MA, USA
³ Centro de Astrobiología (CSIC-INTA), Ctra. de Torrejón Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁴ Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23141, Taipei 106, Taiwan

Received: 17 May 2011
Accepted: 1 December 2011

Abstract

Context. The interstellar region within the few central parsecs around the super-massive black hole, Sgr A^∗, at the very Galactic center is composed of a number of overlapping molecular structures that are subject to one of the most hostile physical environments in the Galaxy.

Aims. Through the study of the morphology and kinematics of the emission from different molecular species as well as the variation of their line ratios we can attain a deeper understanding of the distribution and interaction between different gas structures and the energetic phenomena taking place in the surroundings of Sgr A^∗ and of the physical processes responsible for the heating in this region.

Methods. We performed high-resolution (4″ × 3″ ~ 0.16 × 0.11 pc) interferometric observations of CN, ¹³CN, H₂CO, SiO, c-C₃H₂ and HC₃N emission at 1.3 mm toward the central  ~4 pc of the Galactic center region.

Results. Strong differences are observed in the distribution of the different molecules. The UV resistant species CN, the only species tracing all previously identified circumnuclear disk (CND) structures, is mostly concentrated in optically thick clumps in the rotating filaments around Sgr A^∗. H₂CO emission traces a shell-like structure that we interpret as the expansion of Sgr A East against the 50 km s^-1 and 20 km s^-1 giant molecular clouds (GMCs). We derive isotopic ratios ¹²C/¹³C  ~ 15−45 across most of the CND region, except for the northeast arm, where the peak of H₂CO is observed and ratios  <10 are found. The densest molecular material, traced by SiO and HC₃N, is located in the southern CND, likely because of shocked gas infalling from the 20 km s^-1 GMC streamers, and the northeast arm, as a result of the expansion of Sgr A East or a connecting point of the 50 km s^-1 streamer east of the CND. The observed c-C₃H₂/HC₃N ratio observed in the region is more than an order of magnitude lower than in Galactic PDRs. Toward the central region only CN was detected in absorption. In addition to the known narrow line-of-sight absorptions, a 90 km s^-1 wide optically thick spectral feature is observed. We found evidence of an even wider (>100 km s^-1) absorption feature. About 70−75% of the gas mass, concentrated in just the 27% densest molecular clumps, is associated with rotating structures and show evidence of an association with each of the arcs of ionized gas in the mini-spiral structure.

Conclusions. These observations provide a combined chemical and kinematical picture of the very central region around Sgr A^∗. Chemical differentiation has been proven to be a powerful tool to distinguish the many overlapping molecular components in this crowded and heavily obscured region.