  Star formation laws, like i.e. the Schmidt law relating star formation rate
and total gas density, have been studied in several spiral galaxies but the
underlying physics are not yet well understood. M51, as a nearby face-on, grand
design spiral galaxy studied in many line transitions, is an ideal target to
study the connection between physical conditions of the gas and star formation
activity. In this contribution we combine molecular, atomic, total gas and
stellar surface densities and study the gravitational stability of the gas
(Schuster et al.2007, Hitschfeld et al. in prep.). From our IRAM-30m 12 CO2-1
map and complementary HI-, Radio Continuum- and ACS-HST B-band-data we derive
maps of the total gas density and the stellar surface density to study the
gravitational stability of the gas via the Toomre Q parameter.
