Forecasting the cross-correlation of the CSST galaxy survey with the FAST H I Intensity Map

The cross-correlation of optical galaxies with the neutral hydrogen (III) radiation intensity can enhance the signal-to-noise ratio (SNR) of the HI intensity measurement. In this paper, we investigate the cross-correlation of the galaxy samples obtained by the spectroscopic survey of the China Space Station Telescope (CSST) with the H I Intensity mapping (IM) survey of the Five-hundred-meter Aperture Spherical Telescope (FAST). Using the IllusitrisTNG simulation result at redshift 0.2 similar to 0.3, we generate mock data of the CSST survey and a FAST L-band drift scan survey. The CSST spectroscopic survey can yield a sample of galaxies with a high comoving number density of 10(-2) ( Mpc h(-1))(-3) at z similar to 0.3. We cross-correlate the foreground-removed radio intensity with the CSST galaxies, including both the whole sample, and red and blue galaxy subsamples separately. We find that in all cases the H I and optical galaxies are well correlated. The total H I abundance can be measured with a high precision from this correlation. A relative error of similar to 0.6 per cent for Omega(HI) could be achieved at z similar to 0.3 for an overlapping survey area of 10 000 deg(2).