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
(~< 10 pc) global (~ 20 kpc) simulations of isolated Milky Way-type galaxy
disks. The clouds are formed through gravitational fragmentation and structures
with densities n_H>=100cm^-3 are identified as GMCs. Between 1000-1500 clouds
are created in the simulations with masses M > 10^5 Msolar and 180-240 with
masses M > 10^6 Msolar 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 ISM, 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, slowing the conversation of gas into stars. We
therefore conclude that the interstellar environment plays an important role in
the GMCs evolution. Our clouds live between 0-20 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 ~1/4 of the orbital period. It is not clear whether such collisions
trigger or suppress star formation at our current resolution. Our star
formation rate is a factor of 10 higher than observations in local galaxies.
This is likely due to the absence of localized feedback in our models.
