STAR FORMATION IN DISK GALAXIES. II. THE EFFECT OF STAR FORMATION AND PHOTOELECTRIC HEATING ON THE FORMATION AND EVOLUTION OF GIANT MOLECULAR CLOUDS

We investigate the effect of star formation and diffuse photoelectric heating on the properties of giant molecular clouds (GMCs) formed in high-resolution (less than or similar to 10 pc) global (similar to 20 kpc) simulations of isolated Milky-Way-type galaxy disks. The clouds are formed through gravitational fragmentation, and structures with densities n(H,c) > 100 cm(-3) are identified as GMCs. Between 1000 and 1500 clouds are created in the simulations with masses M > 10(5) M-circle dot and 180-240 with masses M > 10(6) M-circle dot in agreement with estimates of the Milky Way's population. We find that the effect of photoelectric heating is to suppress the fragmentation of the interstellar medium, resulting in a filamentary structure in the warm gas surrounding clouds. This environment suppresses the formation of a retrograde rotating cloud population, with 88% of the clouds rotating prograde with respect to the galaxy after 300 Myr. The diffuse heating also reduces the initial star formation rate (SFR), slowing the conversation of gas into stars. We therefore conclude that the interstellar environment plays an important role in the GMC evolution. Our clouds live between 0 and 20 Myr with a high infant mortality (t' < 3 Myr) due to cloud mergers and star formation. Other properties, including distributions of mass, size, and surface density, agree well with observations. Collisions between our clouds are common, occurring at a rate of similar to 1/4 of the orbital period. It is not clear whether such collisions trigger or suppress star formation at our current resolution. Our SFR is a factor of 10 higher than observations in local galaxies. This is likely due to the absence of localized feedback in our models.