Molecular clouds photoevaporation and FIR line emission

Abstract

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.