Infrared Response of H 2 to X‐Rays in Dense Clouds

Abstract

The excitation by X-rays and cosmic rays of molecular hydrogen in interstellar clouds is analyzed. We carried out detailed calculations of entry efficiencies in rovibrational levels of H-2 following impact with fast electrons produced by X-ray ionization of the gas. The competing effect of collisional excitation, and quenching, by the ambient gas is examined in detail. Up to date values for H-H-2 collisional rate coefficients are adopted, and some derivations of H-2-H-2 rovibrational rate coefficients from existing literature data are proposed. Several models as a function of temperature, density, and ionization rate are presented. We found that H-2 infrared emission in X-ray dominated regions (XDR) is potentially observable for temperatures and ionization rates lower than certain critical values (typically T < 1000 K and zeta/n(H) < 10(-15) cm(3) s(-1), where zeta is the ionization rate). At higher temperatures, collisional excitation by the ambient gas dominates the population of low vibrational levels, and at higher values of zeta/n(H), the abundance of H-2 is negligible. If such conditions are satisfied, the resulting infrared emission spectrum can be used as a diagnostic of nearby X-ray sources such as in cooling flows in galaxy clusters, quasars, Seyfert galaxies and supernova remnants. The intensity ratio of the 2-1S(1) and 1-0S(1) lines measured for the Seyfert galaxy NGC 1275 is consistent with X-ray pumping.