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

Claudia Cicone; Paola Severgnini; Padelis P. Papadopoulos; Roberto Maiolino; Chiara Feruglio; Ezequiel Treister; George C. Privon; Zhi-yu Zhang; Roberto Della Ceca; Fabrizio Fiore; Kevin Schawinski; Jeff Wagg

doi:10.3847/1538-4357/aad32a

paper

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

arXiv:1807.06015 · doi:10.3847/1538-4357/aad32a

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 ( $α_{CO}$ ) in the outflow, which is on average $2.1 \pm 1.2 M_{⊙} (K km s^{- 1} p c^{2})^{- 1}$ . We estimate that $60 \pm 20$ % of the total H $_{2}$ gas reservoir of NGC6240 is entrained in the outflow, for a resulting mass-loss rate of $M ˙_{out} = 2500 \pm 1200 M_{⊙} y r^{- 1}$ $\equiv 50 \pm 30$ 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 $⟨ α_{CO} ⟩ = 3.2 \pm 1.8 M_{⊙} (K km s^{- 1} p c^{2})^{- 1}$ , which is at least twice the value commonly employed for (U)LIRGs. We observe a tentative trend of increasing $r_{21} \equiv L_{CO (2 - 1)}^{'} / L_{CO (1 - 0)}^{'}$ 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 $α_{CO}$ and increasing their $r_{21}$ values.

arXiv commentAccepted for publication in ApJ

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

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ALMA [CI]3P1​−3P0​ observations of NGC6240: a puzzling molecular outflow, and the role of outflows in the global αCO​ factor of (U)LIRGs

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