The CO-to-H2 conversion factor of molecular outflows

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; LIR = 1011.6 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-H2 conversion factor, αCO) of the outflowing gas. We find that the 12CO υ = 1−0 emission is optically thick for J < 4, while the 13CO 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 αCO 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 αCO factors (0.40 - 0.61) × 3.2×10-4/[CO/H2] M⊙ (K km s-1 pc2)-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 αCO is assumed. The reduced αCO 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 (Dn-PAHs).