Abstract
abridged While observations of Local Group galaxies show a very simple,
local star formation law in which the star formation rate per unit area in each
patch of a galaxy scales linearly with the molecular gas surface density in
that patch, recent observations of both Milky Way molecular clouds and high
redshift galaxies apparently show a more complicated relationship, in which
regions of equal molecular gas surface density can form stars at quite
different rates. These data have been interpreted as implying either that
different star formation laws may apply in different circumstances, that the
star formation law is sensitive to large-scale galaxy properties rather than
local properties, or that there are high density thresholds for star formation.
Here we collate observations of the relationship between gas and star formation
rate from resolved observations of Milky Way molecular clouds, from kpc-scale
observations of Local Group galaxies, and from unresolved observations of both
disk and starburst galaxies in the local universe and at high redshift. We show
that all of these data are in fact consistent with a simple, local, volumetric
star formation law. The apparent variations stem from the fact that the
observed objects have a wide variety of scale heights, so even at fixed gas
column density the regions being observed can have wildly varying volume
densities. Once this projection effect is removed, all the data, from small
Solar neighborhood clouds with masses ~10^3 Msun to sub-mm galaxies with masses
~10^11 Msun, fall on a single star formation law in which the star formation
rate is simply ~1% of the molecular gas mass per local free-fall time. In
contrast, proposed star formation laws in which the star formation timescale is
set by the galactic rotation period or is linearly proportional to the gas mass
above some density threshold fail to match the data.
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