The Dense Molecular Gas and Nuclear Activity in the ULIRG IRAS
  13120-5453
  We present new ALMA Band 7 ($\sim340$ 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 $\sim1.2$ 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 $4.7\times10^{12}$ $L_{\odot}$ 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 $\sim40$ 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.
