The CO-to-H<sub>2</sub> conversion factor of molecular outflows
M. Pereira-Santaella, E. González-Alfonso, I. García-Bernete, S. García‐Burillo, D. Rigopoulou
Abstract
We analyze JWST/NIRSpec observations of the CO rovibrational υ = 1−0 band at ~4.67 µm around the dust-embedded southern active galactic nucleus (AGN) of NGC 3256 ( d = 40 Mpc; L IR = 10 11.6 L ⊙ ). We classify the CO υ = 1−0 spectra into three categories based on the behavior of P- and R-branches of the band: (a) both branches in absorption toward the nucleus; (b) P-R asymmetry (P-branch in emission and R-branch in absorption) along the disk of the galaxy; and (c) both branches in emission in the outflow region above and below the disk. In this paper, we focus on the outflow. The CO υ = 1−0 emission can be explained by the vibrational excitation of CO in the molecular outflow by the bright mid-IR ~4.7 µm continuum from the AGN up to r ~ 250 pc. We model the ratios between the P( J +2) and R( J ) transitions of the band to derive the physical properties (column density, kinetic temperature, and CO-to-H 2 conversion factor, α CO ) of the outflowing gas. We find that the 12 CO υ = 1−0 emission is optically thick for J < 4, while the 13 CO υ = 1−0 emission remains optically thin. From the P(2)/R(0) ratio, we identify a temperature gradient in the outflow from >40 K in the central 100 pc to <15 K at 250 pc, sampling the cooling of the molecular gas in the outflow. We used three methods to derive α CO in eight 100 pc (0″.5) apertures in the outflow by fitting the P( J +2)/ R ( J ) ratios with nonlocal thermodynamic equilibrium (NLTE) models. We obtain low median α CO factors (0.40 - 0.61) × 3.2×10 -4 /[CO/H 2 ] M ⊙ (K km s -1 pc 2 ) -1 in the outflow regions. This implies that outflow rates and energetics might be overestimated if a 1.3−2 times larger ultraluminous infrared galaxy (ULIRG) like α CO is assumed. The reduced α CO can be explained if the outflowing molecular clouds are not virialized. We also report the first extragalactic detection of a broad ( σ = 0.0091 µm) spectral feature at 4.645 µm associated with aliphatic deuterium on polycyclic aromatic hydrocarbons (D n -PAHs).