Major impact from a minor merger - The extraordinary hot molecular gas flow in the Eye of the NGC 4194 Medusa galaxy

摘要

Minor mergers are important processes contributing significantly to howgalaxies evolve across the age of the Universe. Their impact on supermassiveblack hole growth and star formation is profound. The detailed study of densemolecular gas in galaxies provides an important test of the validity of therelation between star formation rate and HCN luminosity on different galacticscales. We use observations of HCN, HCO+1-0 and CO3-2 to study the dense gasproperties in the Medusa merger. We calculate the brightness temperature ratiosand use them in conjunction with a non-LTE radiative line transfer model. TheHCN and HCO+1-0, and CO3-2 emission do not occupy the same structures as theless dense gas associated with the lower-J CO emission. The only emission fromdense gas is detected in a 200pc region within the "Eye of the Medusa". No HCNor HCO+ is detected for the extended starburst. The CO3-2/2-1 brightnesstemperature 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 andthe Medusa central region. The extreme conditions inside "the Eye" may be theresult 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 andfragment 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-starburstand/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 tracersto evidence of ongoing or imminent star formation.