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 MGMC = 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 (MGMC ≈ 105 M⊙), the PE time (tpe) 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 tpe. 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 (⁠LCII≈104L⊙ 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.