A complete ¹²CO 2–1 map of M 51 with HERA

II. Total gas surface densities and gravitational stability

¹ KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany e-mail: hitschfeld@ph1.uni-koeln.de
² IRAM, Avenida Divina Pastora 7, Ncleo Central, 18012 Granada, Spain
³ IRAM, 300 rue de la Piscine, 38406 St Martin d'Hères, France
⁴ Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

Received: 2 September 2008
Accepted: 22 December 2008

Abstract

Context. To date the onset of large-scale star formation in galaxies and its link to gravitational stability of the galactic disk have not been fully understood. The nearby face-on spiral galaxy M 51 is an ideal target for studying this subject.

Aims. This paper combines CO, dust, H I, and stellar maps of M 51 and its companion galaxy to study the H₂/H I transition, the gas-to-dust ratios, and the stability of the disk against gravitational collapse.

Methods. We combine maps of the molecular gas using ¹²CO 2-1 map HERA/IRAM-30 m data and H I VLA data to study the total gas surface density and the phase transition of atomic to molecular gas. The total gas surface density is compared to the dust surface density from 850 μm SCUBA data. Taking into account the velocity dispersions of the molecular and atomic gas, and the stellar surface densities derived from the 2MASS K-band survey, we derive the total Toomre Q parameter of the disk.

Results. The gas surface density in the spiral arms is ~ higher compared to that of the interarm regions. The ratio of molecular to atomic surface density shows a nearly power-law dependence on the hydrostatic pressure P_hydro. The distribution in M 51 shows an underlying exponential distribution with a scale length of h_gas = 7.6 kpc representing 55% of the total gas mass, comparable to the properties of the exponential dust disk. In contrast to the velocity widths observed in H I, the CO velocity dispersion shows enhanced line widths in the spiral arms compared to the interarm regions. The contribution of the stellar component in the Toomre Q-parameter analysis is significant and lowers the combined Q-parameter Q_tot by up to 70% towards the threshold for gravitational instability. The value of Q_tot varies from 1.5-3 in radial averages. A map of Q_tot shows values around 1 on the spiral arms indicating self-regulation at play.