Evolutionary Description of Giant Molecular Cloud Mass Functions on
  Galactic Disks
  Recent radio observations show that the giant molecular cloud (GMC) mass
functions noticeably vary across galactic disks. High-resolution
magnetohydrodynamics simulations show that multiple episodes of compression are
required for creating a molecular cloud in the magnetized interstellar medium.
In this article, we formulate the evolution equation for the GMC mass function
to reproduce the observed profiles, for which multiple compression are driven
by the network of expanding shells due to HII regions and supernova remnants.
We introduce the cloud-cloud collision (CCC) terms in the evolution equation in
contrast to the previous work (Inutsuka et al. 2015). The computed time
evolution suggests that the GMC mass function slope is governed by the ratio of
GMC formation timescale to its dispersal timescale, and that the CCC effect is
limited only in the massive-end of the mass function. In addition, we identify
a gas resurrection channel that allows the gas dispersed by massive stars to
regenerate GMC populations or to accrete onto the pre-existing GMCs. Our
results show that almost all of the dispersed gas contribute to the mass growth
of pre-existing GMCs in arm regions whereas less than 60 per cent in inter-arm
regions. Our results also predict that GMC mass functions have a single
power-law exponent in the mass range < 10^5.5 Msun (where Msun represents the
solar mass), which is well characterized by GMC self-growth and dispersal
timescales. Measurement of the GMC mass function slope provides a powerful
method to constrain those GMC timescales and the gas resurrecting factor in
various environment across galactic disks.
