Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21721601m&link_type=abstract
American Astronomical Society, AAS Meeting #217, #216.01; Bulletin of the American Astronomical Society, Vol. 43, 2011
Astronomy and Astrophysics
Astronomy
Scientific paper
Observations of spiral galaxies show a strong linear correlation between midplane pressure and the ratio of molecular to atomic hydrogen surface density R. The suggestion has been made that this occurs because of the equilibrium balance between radiative dissociation of molecular hydrogen and formation on the surfaces of dust grains. We use a 3D numerical model of magnetized turbulence including a simplified chemical network and treatment of the propagation of dissociating radiation to examine this question. We find that the formation timescale for molecular hydrogen is sufficiently long that equilibrium is not reached within the lifetimes of molecular clouds of 20-30 Myr. However, if we assume that the effective temperature in galactic disks is roughly constant, the correlation of R with pressure corresponds to a correlation with local gas density. We find that if we examine the value of R in our local (5-20 pc box) models after a free-fall time at the average density, the observational ratio is reproduced well over two orders of magnitude in density. This supports the suggestion that the formation of molecular clouds and of stars in galaxies proceeds from large-scale gravitational instability, and so that the observed linear correlation between the star formation rate and molecular hydrogen surface densities occurs because both have a common cause.
Glover Simon C. O.
Mac Low Mordecai-Mark
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