Densitometry and Thermometry of Starburst Galaxies
  With a goal toward deriving the physical conditions in external galaxies, we
present a survey of formaldehyde (H2CO) and ammonia (NH3) emission and
absorption in a sample of starburst galaxies using the Green Bank Telescope. By
extending well-established techniques used to derive the spatial density in
star formation regions in our own Galaxy, we show how the relative intensity of
the 1(10)-1(11) and 2(11)-2(12) K-doublet transitions of H2CO can provide an
accurate densitometer for the active star formation environments found in
starburst galaxies (c.f. Mangum et al. 2008). Similarly, we employ the
well-established technique of using the relative intensities of the (1,1),
(2,2), and (4,4) transitions of NH3 to derive the kinetic temperature in
starburst galaxies. Our measurements of the kinetic temperature constrained
spatial density in our starburst galaxy sample represent the first mean density
measurements made toward starburst galaxies. We note a disparity between
kinetic temperature measurements derived assuming direct coupling to dust and
those derived from our NH3 measurements which points to the absolute need for
direct gas kinetic temperature measurements using an appropriate molecular
probe. Finally, our spatial density measurements point to a rough constancy to
the spatial density (10^{4.5} to 10^{5.5} cm^{-3}) in our starburst galaxy
sample. This implies that the Schmidt-Kennicutt relation between L_{IR} and
M_{dense}: (1) Is a measure of the dense gas mass reservoir available to form
stars, and (2) Is not directly dependent upon a higher average density driving
the star formation process in the most luminous starburst galaxies.
