MULTI-TRANSITION STUDY OF M51'S MOLECULAR GAS SPIRAL ARMS

Two selected regions in the molecular gas spiral arms in M51 were mapped with the Owens Valley Radio Observatory (OVRO) mm-interferometer in the (CO)-C-12(2-1), (CO)-C-13(1-0), (CO)-O-18(1-0), HCN(1-0), and HCO+(1-0) emission lines. The CO data have been combined with the (CO)-C-12(1-0) data from Aalto et al. covering the central 3.5 kpc to study the physical properties of the molecular gas. All CO data cubes were short spacing corrected using IRAM 30 m ((CO)-C-12(1-0): NRO 45 m) single-dish data. A large velocity gradient analysis finds that the giant molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180 pc (120 pc) resolution with an average kinetic temperature of T-kin = 20(16) K and H-2 density of n(H-2) = 120(240) cm(-3) when assuming virialized clouds (a constant velocity gradient dv/dr). The associated conversion factor between H-2 mass and CO luminosity is close to the Galactic value for most regions analyzed. Our findings suggest that the GMC population in the spiral arms of M51 is similar to those of the Milky Way and therefore the strong star formation occurring in the spiral arms has no strong impact on the molecular gas in the spiral arms. Extinction inferred from the derived H-2 column density is very high (A(V) about 15-30 mag), about a factor of 5-10 higher than the average value derived toward HII regions. Thus, a significant fraction of the ongoing star formation could be hidden inside the dust lanes of the spiral arms. A comparison of MIPS 24 mu m and H alpha data, however, suggests that this is not the case and most of the GMCs studied here are not (yet) forming stars. We also present low (4 ''.5) resolution OVRO maps of the HCN(1-0) and HCO+(1-0) emission at the location of the brightest (CO)-C-12(1-0) peak.