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 $zsimeq 0.8$. 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.
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