Abstract
With a goal toward deriving the physical conditions in external galaxies, we have conducted a survey and subsequent high spatial resolution imaging of formaldehyde (H2CO) and ammonia (NH3) emission and absorption in a sample of starburst galaxies. In this article we present the results from a subset of this survey which focuses on high spatial resolution measurements of volume density- and kinetic temperature-sensitive transitions of the H2CO molecule. The volume density structure toward the nuclear region of NGC 253 has been derived from theta similar or equal to 4 arcsec NRAO Very Large Array (VLA) measurements of the 1(10) - 1(11) and 2(11)- 2(12) K-doublet transitions of H2CO. The kinetic temperature structure toward NGC 253 and NGC 4945 has been derived from theta similar or equal to 0.5-1.0 arcsec measurements of the H2CO 3(K-1K+1) - 2(K-1K+1) (near 218 GHz) and 5(K-1K+1) 4(K-1K+1) (near 365 GHz) transitions acquired using the Atacama Large Millimeter/submillimeter Array (ALMA). These measurements have allowed us to characterize the dense gas and kinetic temperature structure within these star forming galaxies, which is a first step toward associating dense star-forming gas and the heating processes at work within galaxies.