A Foreground Masking Strategy for [C II] Intensity Mapping Experiments Using Galaxies Selected by Stellar Mass and Redshift

Intensity mapping provides a unique means to probe the epoch of reionization (EoR), when the neutral intergalactic medium was ionized by energetic photons emitted from the first galaxies. The [C II] 158 μm fine-structure line is typically one of the brightest emission lines of star-forming galaxies and thus a promising tracer of the global EoR star formation activity. However, [C II] intensity maps at 6 ≾ z ≾ 8 are contaminated by interloping CO rotational line emission (3 ⩽ J_(upp) ⩽ 6) from lower-redshift galaxies. Here we present a strategy to remove the foreground contamination in upcoming [C II] intensity mapping experiments, guided by a model of CO emission from foreground galaxies. The model is based on empirical measurements of the mean and scatter of the total infrared luminosities of galaxies at z < 3 and with stellar masses M* > 10^8 M⊙ selected in the K-band from the COSMOS/UltraVISTA survey, which can be converted to CO line strengths. For a mock field of the Tomographic Ionized-carbon Mapping Experiment, we find that masking out the "voxels" (spectral–spatial elements) containing foreground galaxies identified using an optimized CO flux threshold results in a z-dependent criterion m^(AB)_K ≾ 22 (or M* ≳ 10^9 M⊙)) at z < 1 and makes a [C II]/CO_(tot) power ratio of ≳10 at k = 0.1 h/Mpc achievable, at the cost of a moderate ≾ 8% loss of total survey volume.