Mind the gap: the power of combining photometric surveys with intensity
  mapping
  The long wavelength modes lost to bright foregrounds in the interferometric
21-cm surveys can partially be recovered using a forward modeling approach that
exploits the non-linear coupling between small and large scales induced by
gravitational evolution. In this work, we build upon this approach by
considering how adding external galaxy distribution data can help to fill in
these modes. We consider supplementing the 21-cm data at two different
redshifts with a spectroscopic sample (good radial resolution but low number
density) loosely modeled on DESI-ELG at $z=1$ and a photometric sample (high
number density but poor radial resolution) similar to LSST sample at $z=1$ and
$z=4$ respectively. We find that both the galaxy samples are able to
reconstruct the largest modes better than only using 21-cm data, with the
spectroscopic sample performing significantly better than the photometric
sample despite much lower number density. We demonstrate the synergies between
surveys by showing that the primordial initial density field is reconstructed
better with the combination of surveys than using either of them individually.
Methodologically, we also explore the importance of smoothing the density field
when using bias models to forward model these tracers for reconstruction.
