Dense gas in the Galactic central molecular zone is warm and heated by turbulence^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: Adam.Ginsburg@eso.org
² Max Planck Institute for Radio Astronomy, auf dem Hugel, 53121 Bonn, Germany
³ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁵ Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
⁶ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁷ CASA, University of Colorado, 389-UCB, Boulder, CO 80309, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ Department of Earth and Space Sciences, Chalmers University of Technology, 41258 Gôteborg, Sweden
¹⁰ Department of Astrophysics, The Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹¹ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹² Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹³ University Observatory Munich, Scheinerstr. 1, 81679 München, Germany

Received: 15 March 2015
Accepted: 16 August 2015

Abstract

Context. The Galactic center is the closest region where we can study star formation under extreme physical conditions like those in high-redshift galaxies.

Aims. We measure the temperature of the dense gas in the central molecular zone (CMZ) and examine what drives it.

Methods. We mapped the inner 300 pc of the CMZ in the temperature-sensitive J = 3–2 para-formaldehyde (p - H₂CO) transitions. We used the 3_2,1−2_2,0/ 3_0,3−2_0,2 line ratio to determine the gas temperature in n ~ 10⁴−10⁵ cm^-3 gas. We have produced temperature maps and cubes with 30′′ and 1 km s^-1 resolution and published all data in FITS form.

Results. Dense gas temperatures in the Galactic center range from ~60 K to >100 K in selected regions. The highest gas temperatures T_G> 100 K are observed around the Sgr B2 cores, in the extended Sgr B2 cloud, the 20 km s^-1 and 50 km s^-1 clouds, and in “The Brick” (G0.253+0.016). We infer an upper limit on the cosmic ray ionization rate ζ_CR< 10^-14s^-1.

Conclusions. The dense molecular gas temperature of the region around our Galactic center is similar to values found in the central regions of other galaxies, in particular starburst systems. The gas temperature is uniformly higher than the dust temperature, confirming that dust is a coolant in the dense gas. Turbulent heating can readily explain the observed temperatures given the observed line widths. Cosmic rays cannot explain the observed variation in gas temperatures, so CMZ dense gas temperatures are not dominated by cosmic ray heating. The gas temperatures previously observed to be high in the inner ~75 pc are confirmed to be high in the entire CMZ.