ALMA [CI]$^{3}P_{1}-^{3}P_{0}$ observations of NGC6240: a puzzling molecular outflow, and the role of outflows in the global $\alpha_{\rm CO}$ factor of (U)LIRGs

TL;DR

This study uses ALMA and ACA observations of NGC6240 to analyze its molecular outflow, revealing unexpected outflow morphology, constraining the CO-to-H2 conversion factor, and exploring outflow effects on galaxy properties.

Contribution

It provides new high-resolution [CI] observations of NGC6240's molecular outflow, constrains the CO-to-H2 conversion factor in outflows, and links outflow properties to changes in the global $ m extalpha_{CO}$ factor.

Findings

Over 10 kpc outflow extent with emission peaking between two AGNs.

The outflow's $ m extalpha_{CO}$ is approximately 2.1, with 60% of H2 in outflow.

Outflow-driven molecular gas constitutes about 50% of the galaxy's total H2 mass.

Abstract

We present ALMA and ACA [CI]$^{3}P_{1}-^{3}P_{0}$ ([CI](1-0)) observations of NGC6240, which we combine with ALMA CO(2-1) and IRAM PdBI CO(1-0) data to study the physical properties of the massive molecular (H$_2$) outflow. We discover that the receding and approaching sides of the H$_2$ outflow, aligned east-west, exceed 10 kpc in their total extent. High resolution ($0.24"$) [CI](1-0) line images surprisingly reveal that the outflow emission peaks between the two AGNs, rather than on either of the two, and that it dominates the velocity field in this nuclear region. We combine the [CI](1-0) and CO(1-0) data to constrain the CO-to-H$_2$ conversion factor ($\alpha_{\rm CO}$) in the outflow, which is on average $2.1\pm1.2~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$. We estimate that $60\pm20$ % of the total H$_2$ gas reservoir of NGC6240 is entrained in the outflow, for a resulting mass-loss rate of $\dot{M}_{\rm out}=2500\pm1200~M_{\odot}~yr^{-1}$ $\equiv 50\pm30$ SFR. This energetics rules out a solely star formation-driven wind, but the puzzling morphology challenges a classic radiative-mode AGN feedback scenario. For the quiescent gas we compute $\langle\alpha_{\rm CO}\rangle = 3.2\pm1.8~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$, which is at least twice the value commonly employed for (U)LIRGs. We observe a tentative trend of increasing $r_{21}\equiv L^{\prime}_{\rm CO(2-1)}/L^{\prime}_{\rm CO(1-0)}$ ratios with velocity dispersion and measure $r_{21}>1$ in the outflow, whereas $r_{21}\simeq1$ in the quiescent gas. We propose that molecular outflows are the location of the warmer, strongly unbound phase that partially reduces the opacity of the CO lines in (U)LIRGs, hence driving down their global $\alpha_{\rm CO}$ and increasing their $r_{21}$ values.