Abstract
Of the almost 40 star forming galaxies at z>~5 (not counting QSOs) observed
in CII to date, nearly half are either very faint in CII, or not detected
at all, and fall well below expectations based on locally derived relations
between star formation rate (SFR) and CII luminosity. Combining cosmological
zoom simulations of galaxies with SIGAME (SImulator of GAlaxy
Millimeter/submillimeter Emission) we have modeled the multi-phased
interstellar medium (ISM) and its emission in CII, OI and OIII, from 30
main sequence galaxies at z~6 with star formation rates ~3-23Msun/yr, stellar
masses ~(0.7-8)x10^9Msun, and metallicities ~(0.1-0.4)xZsun. The simulations
are able to reproduce the aforementioned CII-faintness at z>5, match two of
the three existing z>~5 detections of OIII, and are furthermore roughly
consistent with the OI and OIII luminosity relations with SFR observed for
local starburst galaxies. We find that the CII emission is dominated by the
diffuse ionized gas phase and molecular clouds, which on average contribute
~66% and ~27%, respectively. The molecular gas, which constitutes only ~10% of
the total gas mass is thus a more efficient emitter of CII than the ionized
gas making up ~85% of the total gas mass. A principal component analysis shows
that the CII luminosity correlates with the star formation activity as well
as average metallicity. The low metallicities of our simulations together with
their low molecular gas mass fractions can account for their CII-faintness,
and we suggest these factors may also be responsible for the CII-faint normal
galaxies observed at these early epochs.
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