Exploring the CO/CN line ratio in nearby galaxies with the ALMA archive
  We describe an archival project using Cycle 0 data from the Atacama Large
Millimeter/submilleter Array to survey the CO/CN line ratio in 17 nearby
galaxies. CN is an interesting molecule that traces dense gas exposed to
ultraviolet radiation and its N=1-0 lines can be observed simultaneously with
the CO J=1-0 line. We identify 8 galaxies with distances < 200 Mpc for which
both lines are detected. Signal-to-noise matched CO/CN ratios range from as low
as 7 to as high as 65, while ratios using the total detected flux range from 20
to 140. Spatial variations greater than a factor of 3 are seen in several
galaxies. These line ratio changes are likely due to changes in the [CN]/[H2]
abundance ratio and/or the CN excitation. Additional measurements of the warm
gas pressure and the CN excitation should help to distinguish between these two
possibilities. 3 of the 4 active galactic nuclei in our sample show CO/CN line
ratios that are roughly a factor of 2-3 larger than those seen in
starburst-dominated regions, which may be in conflict with models of molecular
abundances in X-ray dominated regions.
