SIMBAD references

The mechanisms governing the star formation rate in spiral galaxies are not yet clear. The nearby, almost face-on, and interacting galaxy M51 offers an excellent opportunity to study at high spatial resolutions the local star formation laws. In this first paper, we investigate the correlation of H₂, HI, and total gas surface densities with the star forming activity, derived from the radio continuum (RC), along radial averages out to radii of 12kpc. We have created a complete map of M51 in ¹²CO 2-1 at a resolution of 450pc using HERA at the IRAM-30 m telescope. These data are combined with maps of HI and the radio-continuum at 20cm 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 Σ_SFR∝Σ_gasⁿ. The velocity dispersion from CO is used to study the critical surface density and the gravitational stability of the disk. The total mass of molecular material derived from the integrated ¹²CO 2-1 intensities is 2x10⁹M_☉. The 3σ detection limit corresponds to a mass of 1.7x10⁵M_☉. The global star formation rate is 2.56M_☉/yr and the global gas depletion time is 0.8Gyr. HI 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 Σ_gas drops by a factor of ∼20 from 70M_☉/pc2 at the center to 3M_☉/pc2 in the outskirts at radii of 12kpc. The fraction of atomic gas gradually increases with radius. The ratio of HI over H₂ surface densities, Σ_ HI_/ΣH2_, increases from ∼0.1 near the center to ∼20 in the outskirts without following a simple power-law. Σ_HI starts to exceed Σ_H2 at a radius of ∼4kpc. The star formation rate per unit area drops from ∼400M_☉/pc²/Gyr in the starburst center to ∼2M_☉/pc2/Gyr in the outskirts. The gas depletion time varies between 0.1Gyr in the center and 1Gyr in the outskirts, and is shorter than in other non-interacting normal galaxies. Neither the HI surface densities nor the H₂ surface densities show a simple power-law dependence on the star formation rate per unit area. In contrast, Σ_gas and Σ_SFR are well characterized by a local Schmidt law with a power-law index of n=1.4±0.6. The index equals the global Schmidt law derived from disk-averaged values of Σ_gas and Σ_SFR 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 M51 using the Toomre criterion, vary with radius between 1.7 and 6.8km/s. Observed radially averaged dispersions derived from the CO data vary between 28km/s in the center and ∼8km/s at radii of 7 to 9kpc. 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.