Infrared coronal emission lines and the possibility of their laser emission in Seyfert nuclei

Abstract

Infrared coronal emission lines are providing a new window for observation and analysis of highly ionized gas in such Galactic and extragalactic sources as Seyfert nuclei and classical novae shells. These lines are expected to be primary coolants in colliding galaxies, galaxy cluster cooling flows, cometary-compact H II regions, and supernova remnants. In this paper, we compile a complete list of infrared (λ > 1 micron) lines due to transitions within the ground configurations 2S^2^2p^k^ and 3S^2^3p^k^ (k = 1-5) or the first excited configurations 2s2p and 3s3p of highly ionized (χ >= 100 eV) astrophysically abundant [n(X)/n(H) >= 10^-6^] elements. Included are approximately 74 lines in ions of O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Fe, and Ni spanning a wavelength range of approximately 1-280 microns. We present results from detailed balance calculations, critical densities for collisional deexcitation, intrinsic photon rates, branching ratios, and excitation temperatures for the majority of the compiled transitions. The temperature and density parameter space for dominant cooling via infrared coronal lines is presented, and the relationship of infrared and optical coronal lines is discussed. We find that under physical conditions found in Seyfert nuclei, 14 of 70 transitions examined have significant population inversions in levels that give rise to infrared coronal lines. Laser gain lengths and corresponding column densities are calculated for dense [10^6^ <= n_e_ (cm^-3^) <= 10^9^] collisionally ionized plasmas. Application of these results to cooler plasmas photoionized by power-law continuum radiation fields is also discussed. We find that several infrared coronal line transitions have laser gain lengths that correspond to column densities of 10^24-25^ cm^-2^ which are modeled to exist in Seyfert nuclei. Observations that can reveal inverted level populations and laser gain in infrared coronal lines are also suggested.