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 non-linear coupling between small and large scales induced by gravitational evolution. In this work, we build upon considering how adding external galaxy distribution data help fill these modes. We consider supplementing at two different redshifts with spectroscopic sample (good radial resolution but low number density) loosely modeled on DESI-ELG z = 1 photometric (high density poor resolution) similar LSST 4 respectively. find both samples are able reconstruct largest better than only data, performing significantly despite much lower density. demonstrate synergies showing primordial initial field is reconstructed combination of either them individually. Methodologically, also explore importance smoothing when bias models model tracers for reconstruction.