ALMA [C i]3P1–3P0 Observations of NGC 6240: A Puzzling Molecular Outflow, and the Role of Outflows in the Global αCO Factor of (U)LIRGs

Abstract

Abstract We present Atacama large millimeter/submillimeter array (ALMA) and compact array (ACA) [C i] ([C i](1–0)) observations of NGC 6240, which we combine with ALMA CO(2–1) and IRAM Plateau de Bure Interferometer CO(1–0) data to study the physical properties of the massive molecular (H2) outflow. We discover that the receding and approaching sides of the H2 outflow, aligned east–west, exceed 10 kpc in their total extent. High resolution ( ) [C i](1–0) line images surprisingly reveal that the outflow emission peaks between the two active galactic nuclei (AGNs), rather than on either of the two, and that it dominates the velocity field in this nuclear region. We combine the [C i](1–0) and CO(1–0) data to constrain the CO-to-H2 conversion factor ( ) in the outflow, which is on average . We estimate that 60 ± 20% of the total H2 gas reservoir of NGC 6240 is entrained in the outflow, for a resulting mass-loss rate of SFR. These energetics rule 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 , which is at least twice the value commonly employed for (ultra) luminous infrared galaxies ((U)LIRGs). We observe a tentative trend of increasing ratios with velocity dispersion and measure r 21 > 1 in the outflow, whereas r 21 ≃ 1 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 and increasing their r 21 values.