2005ApJ...623..826R

We investigate the properties of giant molecular clouds (GMCs) in the molecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density of molecular gas is 2N(H₂)∼10²²/cm2 over a 2 kpc region, equal to the surface densities of individual GMCs in the Milky Way. We observed the J=1⟶0 transitions of CO, ¹³CO, and HCN. The line ratio W_CO/W_13COfor 200pc<R_gal<800pc is comparable to that found in the Milky Way and increases significantly outside this region, in part because of a large contribution to the CO emission from diffuse gas, which composes 25% of the molecular mass in the galaxy. We developed a modified CLUMPFIND algorithm to decompose the ¹³CO emission into 25 resolved clouds. The clouds have a luminosity-line width relationship L∝ΔV^2.2±0.4, substantially different from the Milky Way trend, L∝ΔV⁵. Similarly, the clouds have a line width-size relationship of ΔV∝R^1.0±0.3_e, compared to ΔV∝R^0.5_e in the Milky Way. Estimates of the kinetic and binding energies of the clouds suggest that the clouds are self-gravitating and significantly overpressured with respect to the remainder of the ISM in M64. The ¹³CO-to-H₂conversion factor is comparable to what is seen in the Galaxy. The M64 clouds have a mean surface 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: Σ_H2∝M^0.7±0.2. The clouds are correlated with the recombination emission from the galaxy, implying that they are star forming; the rate is comparable to that in other galaxies despite the increased densities of the clouds. The gas-to-dust ratio is similar to the Galactic value, but the low extinction in the visual band requires that the molecular gas be clumpy on small scales. We note that the internal pressures of clouds in several galaxies scale with the external pressure exerted on the clouds by the ambient ISM: P_int∝P^0.75±0.05_ext. We attribute the differences between M64 molecular clouds and those in the Local Group to the high ambient pressures and large molecular gas content found in M64.