[CII] line intensity mapping the epoch of reionization with the Prime-Cam on FYST. II. CO foreground masking based on an external catalog

The Fred Young Submillimeter Telescope (FYST) line intensity mapping (LIM) survey will measure the power spectrum (PS) of the singly ionized carbon 158 $ fine-structure line CII to trace the appearance of the first galaxies that emerged during and right after the epoch of reionization (EoR, $6<z<9$). We aim to quantify the contamination of the (post-)EoR CII LIM signal by foreground carbon monoxide (CO) line emission ($3 < J_ up < 12$) and assess the efficiency to retrieve this CII LIM signal by the targeted masking of bright CO emitters. Using the IllustrisTNG300 simulation, we produced mock CO intensity tomographies based on empirical star formation rate-to-CO luminosity relations. Combining these predictions with the CII PS predictions of the first paper of this series, we evaluated a masking technique where the interlopers are identified and masked using an external catalog whose properties are equivalent to those of a deep Euclid survey. Prior to masking, our CII PS forecast is an order of magnitude lower than the predicted CO contamination in the 225 GHz ( CII emitted at $z=6.8-8.3$) band of the FYST LIM survey, at the same level in its 280 GHz ( CII emitted at $z=5.3-6.3$) and 350 GHz ( CII emitted at $z=4.1-4.8$) bands, and an order of magnitude higher in its 410 GHz ( CII emitted at $z=3.4-3.9$) band. For our fiducial model, the optimal masking depth is reached when less than 10<!PCT!> of the survey volume is masked at 350 and 410 GHz but around 40<!PCT!> at 280 GHz and 60 <!PCT!> at 225 GHz. At these masking depths we anticipate a detection of the CII PS at 350 and 410 GHz, a tentative detection at 280 GHz, whereas at 225 GHz the CO signal still dominates our model. In the last case, alternative decontamination techniques will be needed.