The THESAN project: predictions for multi-tracer line intensity mapping
  in the Epoch of Reionization
  Line intensity mapping (LIM) is rapidly emerging as a powerful technique to
study galaxy formation and cosmology in the high-redshift Universe. We present
LIM estimates of select spectral lines originating from the interstellar medium
(ISM) of galaxies and 21 cm emission from neutral hydrogen gas in the Universe
using the large volume, high resolution THESAN reionization simulations. A
combination of sub-resolution photo-ionization modelling for HII regions and
Monte Carlo radiative transfer calculations is employed to estimate the
dust-attenuated spectral energy distributions (SEDs) of high-redshift galaxies
($z\gtrsim5.5$). We show that the derived photometric properties such as the
ultraviolet (UV) luminosity function and the UV continuum slopes match
observationally inferred values, demonstrating the accuracy of the SED
modelling. We provide fits to the luminosity--star formation rate relation
(L-SFR) for the brightest emission lines and find that important differences
exist between the derived scaling relations and the widely used low-$z$ ones
because the interstellar medium of reionization era galaxies is generally less
metal-enriched than in their low redshift counterparts. We use these relations
to construct line intensity maps of nebular emission lines and cross correlate
with the 21 cm emission. Interestingly, the wavenumber at which the correlation
switches sign ($k_\mathrm{transition}$) depends heavily on the reionization
model and to a lesser extent on the targeted emission line, which is consistent
with the picture that $k_\mathrm{transition}$ probes the typical sizes of
ionized regions. The derived scaling relations and intensity maps represent a
timely state-of-the-art framework for forecasting and interpreting results from
current and upcoming LIM experiments.
