HI constraints from the cross-correlation of eBOSS galaxies and Green
  Bank Telescope intensity maps
  We present the joint analysis of Neutral Hydrogen (HI) Intensity Mapping
observations with three galaxy samples: the Luminous Red Galaxy (LRG) and
Emission Line Galaxy (ELG) samples from the eBOSS survey, and the WiggleZ Dark
Energy Survey sample. The HI intensity maps are Green Bank Telescope
observations of the redshifted 21cm emission on 100deg2 covering the redshift
range $0.6<z<1.0$. We process the data by separating and removing the
foregrounds with FastICA, and construct a transfer function to correct for the
effects of foreground removal on the HI signal. We cross-correlate the cleaned
HI data with the galaxy samples and study the overall amplitude as well as the
scale-dependence of the power spectrum. We also qualitatively compare our
findings with the predictions by a semi-analytic galaxy evolution simulation.
The cross-correlations constrain the quantity $\Omega_{{HI}} b_{{HI}}
r_{{HI},{opt}}$ at an effective scale $k_{eff}$, where $\Omega_{HI}$ is the HI
density fraction, $b_{HI}$ is the HI bias, and $r_{{HI},{opt}}$ the
galaxy-hydrogen correlation coefficient, which is dependent on the HI content
of the optical galaxy sample. At $k_{eff}=0.31 \, h/{Mpc}$ we find
$\Omega_{{HI}} b_{{HI}} r_{{HI},{Wig}} = [0.58 \pm 0.09 \, {(stat) \pm 0.05 \,
{(sys)}}] \times 10^{-3}$ for GBT-WiggleZ, $\Omega_{{HI}} b_{{HI}}
r_{{HI,{ELG}}} = [0.40 \pm 0.09 \, {(stat) \pm 0.04 \, {(sys)}}] \times
10^{-3}$ for GBT-ELG, and $\Omega_{{HI}} b_{{HI}} r_{{HI},{LRG}} = [0.35 \pm
0.08 \, {(stat) \pm 0.03 \, {(sys)}}] \times 10^{-3}$ for GBT-LRG, at $z\simeq
0.8$. We also report results at $k_{eff}=0.24 \, h/{Mpc}$ and $k_{eff}=0.48 \,
h/{Mpc}$. With little information on HI parameters beyond our local Universe,
these are amongst the most precise constraints on neutral hydrogen density
fluctuations in an underexplored redshift range.
