Fast, Collimated Outflow in the Western Nucleus of Arp 220

TL;DR

This paper presents the first detailed spatial and spectral imaging of a highly collimated molecular outflow in Arp 220's western nucleus, revealing its morphology, velocity, and potential driving mechanisms.

Contribution

It provides the first resolved image of a collimated molecular outflow in Arp 220, with detailed measurements of its properties and discussion of possible driving forces.

Findings

Outflow extends up to 120 pc with velocities up to ±840 km/s.

Mass of the outflow is at least 10^6 solar masses.

Mass outflow rates are at least 55 and 15 solar masses per year for the two lobes.

Abstract

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.