ALMA reveals a compact and massive molecular outflow driven by the young AGN in a nearby ULIRG

Abstract

The ultra luminous infrared galaxy (ULIRG) F13451+1232 is an excellent example of a galaxy merger in the early stages of active galactic nucleus (AGN) activity, a phase in which AGN-driven outflows are expected to be particularly important. However, previous observations have determined that the mass outflow rates of the warm ionised and neutral gas phases in F13451+1232 are relatively modest, and there has been no robust detection of molecular outflows. Using high spatial resolution ALMA CO(1-0) observations, we detect a kiloparsec-scale circumnuclear disk, as well as extended ($r\sim 440$ pc), intermediate-velocity (300<|$v$|<400 km s${}^{-1}$) cold molecular gas emission that cannot be explained by rotational disk motions. If interpreted as AGN-driven outflows, the mass outflow rates associated with this intermediate-velocity gas are relatively modest (${\dot{M}}_{\mathrm{out}}=22$-$27$ M${}_{\odot }$ yr${}^{-1}$); however, we also detect a compact ($r_{\mathrm{out}}$<120 pc), high velocity (400<$v$<680 km s${}^{-1}$) cold molecular outflow near the primary nucleus of F13451+1232, which carries an order of magnitude more mass (${\dot{M}}_{\mathrm{out}}\sim 230$ M${}_{\odot }$ yr${}^{-1}$) than (and several times the kinetic power of) the previously-detected warmer phases. Moreover, the similar spatial scales of this compact outflow and the radio structure indicate that it is likely accelerated by the small-scale ($r\sim 130$ pc) AGN jet in the primary nucleus of F13451+1232. Considering the compactness of the nuclear outflow and intermediate-velocity non-rotating gas that we detect, we argue that high spatial-resolution observations are necessary to properly quantify the properties of AGN-driven outflows and their impacts on host galaxies.

11 pages, 7 figures, 1 table; accepted for publication in MNRAS