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

Abstract

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 μ m 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 K AB ≲ 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.