We present new ALMA Band 7 ( \sim 340 GHz ) observations of the dense gas tracers HCN , HCO ^ { + } , and CS in the local , single-nucleus , ultraluminous infrared galaxy IRAS 13120–5453 .
We find centrally enhanced HCN ( 4–3 ) emission , relative to HCO ^ { + } ( 4–3 ) , but do not find evidence for radiative pumping of HCN .
Considering the size of the starburst ( 0.5 kpc ) and the estimated supernovae rate of \sim 1.2 yr ^ { -1 } , the high HCN/HCO ^ { + } ratio can be explained by an enhanced HCN abundance as a result of mechanical heating by the supernovae , though the active galactic nucleus and winds may also contribute additional mechanical heating .
The starburst size implies a high \Sigma _ { IR } of 4.7 \times 10 ^ { 12 } L _ { \odot } kpc ^ { -2 } , slightly below predictions of radiation-pressure limited starbursts .
The HCN line profile has low-level wings , which we tentatively interpret as evidence for outflowing dense molecular gas .
However , the dense molecular outflow seen in the HCN line wings is unlikely to escape the galaxy and is destined to return to the nucleus and fuel future star formation .
We also present modeling of Herschel observations of the H _ { 2 } O lines and find a nuclear dust temperature of \sim 40 K. IRAS 13120–5453 has a lower dust temperature and \Sigma _ { IR } than is inferred for the systems termed “ compact obscured nuclei ” ( such as Arp 220 and Mrk 231 ) .
If IRAS 13120–5453 has undergone a compact obscured nucleus phase , we are likely witnessing it at a time when the feedback has already inflated the nuclear ISM and diluted star formation in the starburst/AGN core .