VALES: II. The physical conditions of interstellar gas in normal
  star-forming galaxies up to z=0.2 revealed by ALMA
  We use new Band-3 CO(1-0) observations taken with the Atacama Large
Millimeter/submillimeter Array (ALMA) to study the physical conditions in the
interstellar gas of a sample of 27 dusty main-sequence star-forming galaxies at
0.03<$z$<0.2 present in the Valpara\'iso ALMA Line Emission Survey (VALES). The
sample is drawn from far-IR bright galaxies over $\sim$160 deg$^{2}$ in the
Herschel Astrophysical Terahertz Large Area Survey (HATLAS), which is covered
by Herschel [CII] 158 $\mu$m spectroscopy and far-infrared (FIR) photometry.
The [CII] and CO lines are both detected at >5$\sigma$ in 26 sources. We find
an average [CII] to CO(1-0) luminosity ratio of 3500$\pm$1200 for our sample
that is consistent with previous studies. Using the [CII], CO and FIR
measurements as diagnostics of the physical conditions of the interstellar
medium, we compare these observations to the predictions of a photodissociation
region (PDR) model to determine the gas density, surface temperature, pressure,
and the strength of the incident far-ultraviolet (FUV) radiation field,
$G_{0}$, normalised to the Habing Field. The majority of our sample exhibit
hydrogen densities of 4 < $\log n/\mathrm{cm}^{3}$ < 5.5 and experience an
incident FUV radiation field with strengths of 2 < $\log G_0$ < 3 when adopting
standard adjustments. A comparison to galaxy samples at different redshifts
indicates that the average strength of the FUV radiation field appears constant
up to redshift $z\sim$6.4, yet the neutral gas density increases with redshift
by a factor of $\sim$100, that persists regardless of various adjustments to
our observable quantities. This evolution could provide an explanation for the
observed evolution of the star formation rate density with cosmic time, yet
could arise from a combination of observational biases when using different
suites of emission lines as diagnostic tracers of PDR gas.
