Molecular cloud photoevaporation and far-infrared line emission

With the aim of improving predictions on far-infrared (FIR) line emission from Giant Molecular Clouds (GMCs), we study the effects of photoevaporation (PE) produced by external far-ultraviolet (FUV) and ionizing (extreme-ultraviolet) radiation on GMC structure. We consider three different GMCs with mass in the range M_GMC = 10³-10⁶ M_⊙. Our model includes (I) an observationally based inhomogeneous GMC density field, and (II) its time evolution during the PE process. In the fiducial case (M_GMC ≈ 10⁵ M_⊙), the PE time (t_pe) increases from 1 to 30 Myr for gas metallicity Z = 0.05-1 Z_⊙, respectively. Next, we compute the time-dependent luminosity of key FIR lines tracing the neutral and ionized gas layers of the GMCs, ([C II] at 158 μm, [O III] at 88 μm) as a function of G₀, and Z until complete PE at t_pe. We find that the specific [C II] luminosity is almost independent of the GMC model within the survival time of the cloud. Stronger FUV fluxes produce higher [C II] and [O III] luminosities, however, lasting for progressively shorter times. At Z = Z_⊙, the [C II] emission is maximized (L_{C II} ≈ 10^4 {L_{{⊙}}} for the fiducial model) for t < 1 Myr and log G₀ ≥ 3. Noticeably, and consistently with the recent detection by Inoue et al. of a galaxy at redshift z ≈ 7.2, for Z ≤ 0.2 Z_⊙, the [O III] line might outshine [C II] emission by up to ≈1000 times. We conclude that the [O III] line is a key diagnostic of low-metallicity interstellar medium, especially in galaxies with very young stellar populations.