  We forecast astrophysical and cosmological parameter constraints from
synergies between 21 cm intensity mapping and wide field optical galaxy surveys
(both spectroscopic and photometric) over $z \sim 0-3$. We focus on the
following survey combinations in this work: (i) a CHIME-like and DESI-like
survey in the northern hemisphere, (ii) an LSST-like and SKA I MID-like survey
and (ii) a MeerKAT-like and DES-like survey in the southern hemisphere. We work
with the $\Lambda$CDM cosmological model having parameters $\{h, \Omega_m, n_s,
\Omega_b, \sigma_8\}$, parameters $v_{c,0}$ and $\beta$ representing the cutoff
and slope of the HI-halo mass relation in the previously developed HI halo
model framework, and a parameter $Q$ that represents the scale dependence of
the optical galaxy bias. Using a Fisher forecasting framework, we explore (i)
the effects of the HI and galaxy astrophysical uncertainties on the
cosmological parameter constraints, assuming priors from the present knowledge
of the astrophysics, (ii) the improvements on astrophysical constraints over
their current priors in the three configurations considered, (ii) the
tightening of the constraints on the parameters relative to the corresponding
HI auto-correlation surveys alone.
