Time Evolution of Giant Molecular Cloud Mass Function
Nagoya U, Nagoya, JP
We formulate and compute the time evolution of giant molecular cloud mass
on galactic scales. In our model, the giant molecular cloud formation and
is driven by a network of expanding supernovae and HII regions.
Such a network provides (i) giant molecular cloud formation and self-growth
multiple episodes of warm neutral medium compression, (ii) giant molecular
cloud self-dispersal by radiation from massive stars that are born within
those clouds, and (iii) cloud-cloud collisions. Our formulation reflects
three phenomena by coarse-graining them typically over 10 Myrs.
We successfully reproduce the observed variation of giant molecular cloud mass functions between arm and inter-arm regions in nearby galaxies (e.g., M51). Our results indicate that the slope is controlled by the ratio of giant molecular cloud formation and dispersal timescales and also indicate that cloud-cloud collisions do not modify the evolution of giant molecular cloud mass function significantly. Therefore, cloud-cloud collisions may be rare although they could have a great impact on massive star and star cluster formation on galactic scales as suggested by recent observations in the Galaxy. If cloud-cloud collisions are ineffective, large radio observations may put unique constraints on the giant molecular cloud formation/dispersal timescales in different environment on galactic scales. Our results also suggest that some amount (typically a few per cent) of dispersed gas may be consumed to form a newer generation of giant molecular clouds. Therefore, we insist that, obtaining a complete picture of gas recycling processes in interstellar medium requires the understanding of the fate of dispersed gas such as CO-dark clouds and optically thick HI gas, which thus needs massive magnetohydrodynamics simulations.
S-I. Inutsuka, Nagoya U, JP
H. Kobayashi, Nagoya U, JP
K. Hasegawa, Nagoya U, JP
Suggested Session: Molecular Clouds