The CO-to-H₂ conversion factor of molecular outflows

Rovibrational CO emission in NGC 3256-S resolved by JWST/NIRSpec

¹ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
e-mail: miguel.pereira@iff.csic.es
² Universidad de Alcalá, Departamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
³ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁴ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain

Received: 11 September 2023
Accepted: 27 October 2023

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-H₂ conversion factor, α_CO) 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 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/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 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).