Molecular clouds photoevaporation and FIR line emission
  With the aim of improving predictions on far infrared (FIR) line emission
from Giant Molecular Clouds (GMC), we study the effects of photoevaporation
(PE) produced by external far-ultraviolet (FUV) and ionizing
(extreme-ultraviolet, EUV) radiation on GMC structure. We consider three
different GMCs with mass in the range $M_{\rm GMC} = 10^{3-6}\,\rm{M_{\odot}}$.
Our model includes: (i) an observationally-based inhomogeneous GMC density
field, and (ii) its time evolution during the PE process. In the fiducial case
($M_{\rm GMC}\approx10^5 M_{\odot}$), the photoevaporation time ($t_{pe}$)
increases from 1 Myr to 30 Myr for gas metallicity $Z=0.05-1\,\rm Z_{\odot}$,
respectively. Next, we compute the time-dependent luminosity of key FIR lines
tracing the neutral and ionized gas layers of the GMCs, ([CII] at $158\,\rm{\mu
m}$, [OIII] at $88\,\rm \mu m$) as a function of $G_0$, and $Z$ until complete
photoevaporation at $t_{pe}$. We find that the specific [CII] luminosity is
almost independent on the GMC model within the survival time of the cloud.
Stronger FUV fluxes produce higher [CII] and [OIII] luminosities, however
lasting for progressively shorter times. At $Z=Z_{\odot}$ the [CII] emission is
maximized ($L_{\rm CII}\approx 10^4\,\rm{L_{\odot}}$ for the fiducial model)
for $t<1\,\rm{Myr}$ and $\log G_0\geq 3$. Noticeably, and consistently with the
recent detection by Inoue et al. (2016) of a galaxy at redshift $z\approx 7.2$,
for $Z\leq 0.2\,\rm{Z_{\odot}}$ the [OIII] line might outshine [CII] emission
by up to $\approx 1000$ times. We conclude that the [OIII] line is a key
diagnostic of low metallicity ISM, especially in galaxies with very young
stellar populations.
