The Galactic Center Molecular Cloud Survey

II. A lack of dense gas and cloud evolution along Galactic center orbits

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: jeans.kauffmann@gmail.com
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁴ School of Astronomy and Space Science, Nanjing University, 22 Hankou Road, 210093 Nanjing, PR China
⁵ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Santiago, Chile
⁶ I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany

Received: 7 January 2016
Accepted: 7 February 2017

Abstract

We present the first systematic study of the density structure of clouds found in a complete sample covering all major molecular clouds in the central molecular zone (CMZ; inner ~200 pc) of the Milky Way. This is made possible with data from the Galactic Center Molecular Cloud Survey (GCMS), which is the first study resolving all major molecular clouds in the CMZ at interferometer angular resolution. We find that many CMZ molecular clouds have unusually shallow density gradients compared to regions elsewhere in the Milky Way. This is possibly a consequence of weak gravitational binding of the clouds. The resulting relative absence of dense gas on spatial scales ~0.1 pc is probably one of the reasons why star formation (SF) in dense gas of the CMZ is suppressed by a factor ~10, compared to solar neighborhood clouds. Another factor suppressing star formation is the high SF density threshold that likely results from the observed gas kinematics. Further, it is possible but not certain that the star formation activity and cloud density structure evolve systematically as clouds orbit the CMZ.