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To date the onset of large-scale star formation in galaxies and its link togravitational stability of the galactic disk have not been fully understood.The nearby face-on spiral galaxy M51 is an ideal target for studying thissubject. This paper combines CO, dust, HI, and stellar maps of M51 and itscompanion galaxy to study the H2/HI transition, the gas-to-dust ratios, and thestability of the disk against gravitational collapse. We combine maps of themolecular gas using 12CO 2--1 map HERA/IRAM-30m data and HI VLA data to studythe total gas surface density and the phase transition of atomic to moleculargas. The total gas surface density is compared to the dust surface density from850 micron SCUBA data. Taking into account the velocity dispersions of themolecular and atomic gas, and the stellar surface densities derived from the2MASS K-band survey, we derive the total Toomre Q parameter of the disk. Thegas surface density in the spiral arms is approximately 2-3 higher compared tothat of the interarm regions. The ratio of molecular to atomic surface densityshows a nearly power-law dependence on the hydrostatic pressure P_hydro. Thegas surface density distribution in M51 shows an underlying exponentialdistribution with a scale length of h_gas=7.6 kpc representing 55% of the totalgas mass, comparable to the properties of the exponential dust disk. Incontrast to the velocity widths observed in HI, the CO velocity dispersionshows enhanced line widths in the spiral arms compared to the interarm regions.The contribution of the stellar component in the Toomre Q-parameter analysis issignificant and lowers the combined Q-parameter Q_tot by up to 70% towards thethreshold for gravitational instability. The value of Q_tot varies from 1.5-3in radial averages. A map of Q_tot shows values around 1 on the spiral arms.

A complete 12CO 2--1 map of M51 with HERA: II. Total gas surface densities and gravitational stability