Physics of ULIRGs with MUSE and ALMA: PUMA IV. No tight relation between cold molecular outflow rates and AGN luminosities

TL;DR

This study investigates molecular outflows in 25 nearby ULIRGs using ALMA, finding no tight correlation between outflow rates and AGN luminosities, but identifying a link between outflow velocity and star formation rate.

Contribution

It provides detailed measurements of molecular outflows in ULIRGs and challenges previous assumptions about their relation to AGN luminosity, highlighting the importance of star formation in driving outflows.

Findings

77% of nuclei show molecular outflows with high velocities

Outflow velocity correlates with star formation rate ($v_{out} \,\propto \, SFR^{0.25}$)

Outflows are likely momentum-driven rather than AGN luminosity-driven

Abstract

We study molecular outflows in a sample of 25 nearby (z< 0.17, d<750 Mpc) ULIRG systems (38 individual nuclei) as part of the "Physics of ULIRGs with MUSE and ALMA" (PUMA) survey, using ~400 pc (0.1-1.0" beam FWHM) resolution ALMA CO(2-1) observations. We used a spectro-astrometry analysis to identify high-velocity (> 300 km/s) molecular gas disconnected from the galaxy rotation, which we attribute to outflows. In 77% of the 26 nuclei with $\log L_{IR}/L_{\odot}>11.8$, we identifid molecular outflows with an average $v_{out}= 490$ km/s, outflow masses $1-35 \times 10^7$ $M_{\odot}$, mass outflow rates $\dot{M}_{out}=6-300$ $M_{\odot}$ yr$^{-1}$, mass-loading factors $\eta = \dot{M}_{out}/SFR = 0.1-1$, and an average outflow mass escape fraction of 45%. The majority of these outflows (18/20) are spatially resolved with radii of 0.2-0.9 kpc and have short dynamical times ($t_{dyn}=R_{out}/v_{out}$) in the range 0.5-2.8 Myr. The outflow detection rate is higher in nuclei dominated by starbursts (SBs, 14/15=93%) than in active galactic nuclei (AGN, 6/11=55%). Outflows perpendicular to the kinematic major axis are mainly found in interacting SBs. We also find that our sample does not follow the $\dot{M}_{out}$ versus AGN luminosity relation reported in previous works. In our analysis, we include a sample of nearby main-sequence galaxies (SFR = 0.3-17 $M_{\odot}$ yr$^{-1}$) with detected molecular outflows from the PHANGS-ALMA survey to increase the $L_{IR}$ dynamic range. Using these two samples, we find a correlation between the outflow velocity and the SFR, as traced by $L_{IR}$ ($v_{out} \propto SFR^{0.25\pm0.01})$, which is consistent with what was found for the atomic ionised and neutral phases. Using this correlation, and the relation between $M_{out}/R_{out}$ and $v_{out}$, we conclude that these outflows are likely momentum-driven.