Giant Molecular Clouds in M64

(abridged) We investigate the properties of Giant Molecular Clouds (GMCs) inthe molecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density ofmolecular gas is 2N(H_2) ~ 10^22 over a 2 kpc region, equal to the surfacedensities of individual GMCs in the Milky Way. We observed the J=1->0transitions of CO, 13CO, and HCN. The line ratio W_CO/W_13CO for 200 pc < R_gal< 800 pc is comparable to that found in the Milky Way and increasessignificantly outside this region, in part due to a large contribution to theCO emission from diffuse gas, which composes 25% of the molecular mass in thegalaxy. We developed a modified CLUMPFIND algorithm to decompose the 13COemission into 25 resolved clouds. The clouds have a luminosity--linewidthrelationship L ~ DV^{2.2+/-0.4}, substantially different from the Milky Waytrend reported by Solomon et al. (1987): L ~ DV^5. Similarly, the clouds have alinewidth--size relationship of DV ~ R_e^{1.0 +/- 0.3} compared to DV ~ R_e^0.5in the Milky Way. Estimates of the kinetic and binding energies of the cloudssuggest that the clouds are self-gravitating and significantly overpressuredwith respect to the remainder of the ISM in M64. The M64 clouds have a meansurface density at least 2.5 times larger than observed in Local Group GMCs,and the surface density is not independent of mass as it is in the Local Group:\Sigma_H2 ~ M^{0.7+/-0.2}. The clouds are correlated with the recombinationemission from the galaxy, implying that they are star forming; the rate iscomparable to that in other galaxies despite the increased densities of theclouds. We note that the internal pressures of clouds in several galaxiesscales with the external pressure exerted on the clouds by the ambient ISM:P_int ~ P_ext^{0.75+/-0.05}.Comment: Accepted for publication in ApJ, 22 pages, 21 figure