2025/06/20 by Speranza, G., Pereira-Santaella, M., Agúndez, M. +7
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2506.17390
We analyse the ro-vibrational absorption bands of various molecular cations (HCO+, HCNH+, and N2H+) and neutral species (HCN, HNC, and HC3N) detected in the James Webb Space Telescope/Mid-Infrared Instrument Medium Resolution Spectrometer spectrum (4.9--27.9 \upmum) of the local ultra luminous infrared galaxy IRAS~07251-0248. We find that the molecular absorptions are blueshifted by 160 km s-1 relative to the systemic velocity of the target. Using local thermal equilibrium (LTE) excitation models, we derive rotational temperatures (T\rm rot) from 42 to 185 K for these absorption bands. This range of measured T\rm rot can be explained by infrared (IR) radiative pumping as a by--product of the strength, effective critical density, and opacity of each molecular band. Thus, these results suggest that these absorptions originate in a warm expanding gas shell (M∼90--330 M_\odot yr-1), which might be the base of the larger scale cold molecular outflow detected in this source. Finally, the elevated abundance of molecular cations can be explained by a high cosmic ray ionization rate, with log(ζH2/n\rm H [cm3 s-1]) in the range of -18.2 (from H3+) to -19.1 (inferred from HCO+ and N2H+, which are likely tracing denser gas), consistent with a cosmic ray dominated chemistry as predicted by chemical models.