Diagnosing the interstellar medium of galaxies with far-infrared
  emission lines I. The [C II] 158 microns line at z~0
  Atomic fine structure lines have been detected in the local Universe and at
high redshifts over the past decades. The [C II] emission line at 158 $\mu$m is
an important observable as it provides constraints on the interstellar medium
(ISM) cooling processes. We develop a physically motivated framework to
simulate the production of far-infrared line emission from galaxies in a
cosmological context. This first paper sets out our methodology and describes
its first application, simulating the [C II] 158 $\mu$m line emission in the
local Universe. We combine the output from EAGLE cosmological hydrodynamical
simulations with a multi-phase model of the ISM. Gas particles are divided into
three phases: dense molecular gas, neutral atomic gas and diffuse ionised gas
(DIG). We estimate the [C II] line emission from the three phases using a set
of Cloudy cooling tables. Our results agree with previous findings regarding
the contribution of these three ISM phases to the [C II] emission. Our model
shows good agreement with the observed ${\rm L_{[C II]}}$-star formation rate
(SFR) relation in the local Universe within 0.4 dex scatter. The fractional
contribution to the [C II] line from different ISM phases depends on the total
SFR and metallicity. The neutral gas phase dominates the [C II] emission in
galaxies with $\rm{SFR}\sim0.01$-$1\,\rm{M_{\odot}}\,\rm{yr^{-1}}$, but the
ionised phase dominates at lower SFRs. Galaxies above solar metallicity exhibit
lower ${\rm L_{[C II]}}$/SFR ratios for the neutral phase. In comparison, the
${\rm L_{[C II]}}$/SFR ratio in the DIG is stable when metallicity varies. We
suggest that the reduced size of the neutral clouds, caused by increased SFRs,
is the likely cause for the ${\rm L_{[C II]}}$ deficit at high infrared
luminosities, although EAGLE simulations do not reach these luminosities at
$z=0$.
