Fast, Collimated Outflow in the Western Nucleus of Arp 220
  We present the first spatially and spectrally resolved image of the molecular
outflow in the western nucleus of Arp\,220. The outflow, seen in HCN~(1--0) by
ALMA, is compact and collimated, with extension $\lesssim$ 120\,pc. Bipolar
morphology emerges along the minor axis of the disk, with redshifted and
blueshifted components reaching maximum inclination-corrected velocity of
$\sim\,\pm$\,840\,km\,s$^{-1}$. The outflow is also seen in CO and continuum
emission, the latter implying that it carries significant dust. We estimate a
total mass in the outflow of $\geqslant$\,10$^{6}$\,M$_{\odot}$, a dynamical
time of $\sim$\,10$^{5}$\,yr, and mass outflow rates of
$\geqslant55$\,M$_{\odot}$\,yr$^{-1}$ and
$\geqslant\,15$\,M$_{\odot}$\,yr$^{-1}$ for the northern and southern lobes,
respectively. Possible driving mechanisms include supernovae energy and
momentum transfer, radiation pressure feedback, and a central AGN. The latter
could explain the collimated morphology of the HCN outflow, however we need
more complex theoretical models, including contribution from supernovae and
AGN, to pinpoint the driving mechanism of this outflow.
