A complete CO 2-1 map of M 51 with HERA

I. Radial averages of CO, H I, and radio continuum

¹ IRAM, 300 rue de la Piscine, 38406 St. Martin d'Hères, France e-mail: schuster@iram.es
² KOSMA, I. Physikalisches Institut, Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
³ Centro Astronomico de Yebes, IGN, 19080 Guadalajara, Spain
⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 8 May 2006
Accepted: 20 September 2006

Abstract

Context. The mechanisms governing the star formation rate in spiral galaxies are not yet clear. The nearby, almost face-on, and interacting galaxy M 51 offers an excellent opportunity to study at high spatial resolutions the local star formation laws.

Aims.In this first paper, we investigate the correlation of H₂, , and total gas surface densities with the star forming activity, derived from the radio continuum (RC), along radial averages out to radii of 12 kpc.

Methods.We have created a complete map of M 51 in ¹²CO 2-1 at a resolution of 450 pc using HERA at the IRAM-30 m telescope. These data are combined with maps of  and the radio-continuum at 20 cm wavelength. The latter is used to estimate the star formation rate (SFR), thus allowing to study the star formation efficiency and the local Schmidt law . The velocity dispersion from CO is used to study the critical surface density and the gravitational stability of the disk.

Results. The total mass of molecular material derived from the integrated ¹²CO 2-1 intensities is  . The detection limit corresponds to a mass of  . The global star formation rate is 2.56  yr^-1 and the global gas depletion time is 0.8 Gyr.  and RC emission are found to peak on the concave, downstream side of the outer south-western CO arm, outside the corotation radius. The total gas surface density drops by a factor of ~20 from 70  pc^-2 at the center to 3  pc^-2 in the outskirts at radii of 12 kpc. The fraction of atomic gas gradually increases with radius. The ratio of  over H₂ surface densities, , increases from ~0.1 near the center to ~20 in the outskirts without following a simple power-law. starts to exceed at a radius of ~4 kpc. The star formation rate per unit area drops from ~400  pc^-2 Gyr^-1 in the starburst center to ~2  pc^-2 Gyr^-1 in the outskirts. The gas depletion time varies between 0.1 Gyr in the center and 1 Gyr in the outskirts, and is shorter than in other non-interacting normal galaxies. Neither the  surface densities nor the H₂ surface densities show a simple power-law dependence on the star formation rate per unit area. In contrast, and are well characterized by a local Schmidt law with a power-law index of . The index equals the global Schmidt law derived from disk-averaged values of and of large samples of normal and starburst galaxies. The critical gas velocity dispersions needed to stabilize the gas against gravitational collapse in the differentially rotating disk of M 51 using the Toomre criterion, vary with radius between 1.7 and 6.8 km s^-1. Observed radially averaged dispersions derived from the CO data vary between 28 km s^-1 in the center and ~8 km s^-1 at radii of 7 to 9 kpc. They exceed the critical dispersions by factors Q_gas of 1 to 5. We speculate that the gravitational potential of stars leads to a critically stable disk.