2021ApJ...912...67U

The Atacama Large Millimeter/submillimeter Array (ALMA) Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS) Band 6 scan (212-272 GHz) covers potential [C II] emission in galaxies at 6 <= z <= 8 throughout a 2.9 arcmin² area. By selecting on known Lyα emitters (LAEs) and photometric dropout galaxies in the field, we perform targeted searches down to a 5σ [C II] luminosity depth L_[C II]_ ∼ 2.0 x 10⁸ L_☉, corresponding roughly to star formation rates (SFRs) of 10-20 M_☉ yr^–1 when applying a locally calibrated conversion for star-forming galaxies, yielding zero detections. While the majority of galaxies in this sample are characterized by lower SFRs, the resulting upper limits on [C II] luminosity in these sources are consistent with the current literature sample of targeted ALMA observations of z = 6-7 LAEs and Lyman-break galaxies (LBGs), as well as the locally calibrated relations between L_[C II]_ and SFR-with the exception of a single [C II]-deficient, UV-luminous LBG. We also perform a blind search for [C II]-bright galaxies that may have been missed by optical selections, resulting in an upper limit on the cumulative number density of [C II] sources with L_[C II]_ > 2.0 x 10⁸ L_☉ (5σ) to be less than 1.8 x 10^–4 Mpc^–3 (90% confidence level). At this luminosity depth and volume coverage, we present an observed evolution of the [C II] luminosity function from z = 6-8 to z ∼ 0 by comparing the ASPECS measurement to literature results at lower redshift.