X-Ray--irradiated Molecular Gas. I. Physical Processes and General Results

We have modeled the physical and chemical state of dense (n = 10³-10⁵ cm^-3) neutral gas exposed to intense X-ray fluxes and the resultant infrared and submillimeter emission from the irradiated gas. The dominant parameter controlling the state of the gas in these X-ray dissociation regions (XDRs) is the ratio of local X-ray energy deposition rate to gas density; this can be expressed in terms of an effective ionization parameter. The parameter space we model ranges over 5 orders of magnitude in this ionization parameter; the gas physical conditions vary from warm, atomic, and partially ionized (T ∼ 10⁴ K, molecular hydrogen abundance X_H2 ≤ 10^-4, electron abundance X_e 0.1) to cold, molecular, and neutral (T ∼20 K, X_H2 ≈ 0.5, X_e ∼10^-6). We thus cover the entire range of parameter space in which X-ray ionization and heating is important and the gas is largely neutral. Although we assume a power law for the incident X-ray flux, most of our results are independent of this assumption and are of general applicability. A wide range of diagnostic atomic and molecular line emission is produced by XDRs, which are luminous sources of infrared and submillimeter lines. This is a consequence of the large column densities (N > 10²² cm^-2) that hard (E > 1 keV) photons are capable of penetrating before being absorbed. Strong emission lines include [Fe n] 1.26 and 1.64 μm, [O I] 63 μm, [C II] 158 μm, [Si II] 35 μm, and the 2 μm vibration-rotation lines of H₂, such as the V = 1-0 5(1) line at 2.12 μm. We discuss diagnostic line ratios for discriminating XDRs from shocks and photodissociation regions. One strong signature of emission from XDRs is the large flux ratio (∼0.1) of the major coolant line fluxes (e.g., [O I] 63 μm) to the bolometric continuum flux. XDRs are likely to be the dominant sources of emission in a range of astrophysical environments, such as molecular clouds within roughly 1 kpc of typical active galactic nuclei.