HIR4: Cosmological signatures imprinted on the cross correlation between
  21-cm map and galaxy clustering
  We explore the cosmological multitracer synergies between an emission line
galaxy distribution from the Dark Energy Spectroscopic Instrument and a Tianlai
Project 21-cm intensity map. We use simulated maps generated from a particle
simulation in the light-cone volume (Horizon Run 4), sky-trimmed and including
the effects of foreground contamination, its removal and instrument noise. We
first validate how the foreground residual affects the recovered 21-cm signal
by putting different levels of foreground contamination into the 21-cm maps. We
find that the contamination cannot be ignored in the angular autocorrelation
power spectra of HI even when it is small, but has no influence on the accuracy
of the angular cross-power spectra between HI and galaxies. In the
foreground-cleaned map case, as information is lost in the cleaning procedure,
there is also a bias in the cross-correlation power spectrum. However, we found
that the bias from the cross-correlation power spectrum is scale-independent,
which is easily parameterized as part of the model, while the offset in the HI
autocorrelation power spectrum is non-linear. In particular, we tested that the
cross-correlation power also benefits from the cancellation of the bias in the
power spectrum measurement that is induced by the instrument noise, which
changes the shape of the autocorrelation power spectra but leaves the
cross-correlation power unaffected. We then modelled the angular
cross-correlation power spectra to fit the baryon acoustic oscillation feature
in the broad-band shape of the angular cross-correlation power spectrum,
including contamination from the residual foreground and the effect of
instrument noise. We forecast a constraint on the angular diameter distance
$D_\mathrm{A}$ for the Tianlai Pathfinder redshift $0.775<z<1.03$, giving a
distance measurement with a precision of 2.7\% at that redshift.
