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We present new ALMA Band 7 ($\sim340$ GHz) observations of the dense gastracers HCN, HCO$^+$, and CS in the local, single-nucleus, ultraluminousinfrared 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 ofHCN. Considering the size of the starburst (0.5 kpc) and the estimatedsupernovae rate of $\sim1.2$ yr$^{-1}$, the high HCN/HCO$^+$ ratio can beexplained by an enhanced HCN abundance as a result of mechanical heating by thesupernovae, though the active galactic nucleus and winds may also contributeadditional mechanical heating. The starburst size implies a high $\Sigma_{IR}$of $4.7\times10^{12}$ $L_{\odot}$ kpc$^{-2}$, slightly below predictions ofradiation-pressure limited starbursts. The HCN line profile has low-levelwings, which we tentatively interpret as evidence for outflowing densemolecular gas. However, the dense molecular outflow seen in the HCN line wingsis unlikely to escape the galaxy and is destined to return to the nucleus andfuel future star formation. We also present modeling of Herschel observationsof the H$_2$O lines and find a nuclear dust temperature of $\sim40$ K. IRAS13120-5453 has a lower dust temperature and $\Sigma_{IR}$ than is inferred forthe systems termed "compact obscured nuclei" (such as Arp 220 and Mrk 231). IfIRAS 13120-5453 has undergone a compact obscured nucleus phase, we are likelywitnessing it at a time when the feedback has already inflated the nuclear ISMand diluted star formation in the starburst/AGN core.

The Dense Molecular Gas and Nuclear Activity in the ULIRG IRAS 13120-5453