  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 M51 is an ideal target for studying this
subject. This paper combines CO, dust, HI, and stellar maps of M51 and its
companion galaxy to study the H2/HI transition, the gas-to-dust ratios, and the
stability of the disk against gravitational collapse. We combine maps of the
molecular gas using 12CO 2--1 map HERA/IRAM-30m data and HI 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 micron 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. The
gas surface density in the spiral arms is approximately 2-3 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
gas surface density distribution in M51 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 HI, 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.
