Major impact from a minor merger - The extraordinary hot molecular gas
  flow in the Eye of the NGC 4194 Medusa galaxy
  Minor mergers are important processes contributing significantly to how
galaxies evolve across the age of the Universe. Their impact on supermassive
black hole growth and star formation is profound. The detailed study of dense
molecular gas in galaxies provides an important test of the validity of the
relation between star formation rate and HCN luminosity on different galactic
scales. We use observations of HCN, HCO+1-0 and CO3-2 to study the dense gas
properties in the Medusa merger. We calculate the brightness temperature ratios
and use them in conjunction with a non-LTE radiative line transfer model. The
HCN and HCO+1-0, and CO3-2 emission do not occupy the same structures as the
less dense gas associated with the lower-J CO emission. The only emission from
dense gas is detected in a 200pc region within the "Eye of the Medusa". No HCN
or HCO+ is detected for the extended starburst. The CO3-2/2-1 brightness
temperature ratio inside "the Eye" is ~2.5 - the highest ratio found so far.
The line ratios reveal an extreme, fragmented molecular cloud population inside
"the Eye" with large temperatures (>300K) and high gas densities (>10^4 cm^-3).
"The Eye" is found at an interface between a large-scale minor axis inflow and
the Medusa central region. The extreme conditions inside "the Eye" may be the
result of the radiative and mechanical feedback from a deeply embedded, young,
massive super star cluster, formed due to the gas pile-up at the intersection.
Alternatively, shocks from the inflowing gas may be strong enough to shock and
fragment the gas. For both scenarios, however, it appears that the HCN and HCO+
dense gas tracers are not probing star formation, but instead a post-starburst
and/or shocked ISM that is too hot and fragmented to form new stars. Thus,
caution is advised in linking the detection of emission from dense gas tracers
to evidence of ongoing or imminent star formation.
