The ALMA Spectroscopic Survey in the HUDF: A Search for [C ii] Emitters at 6 ≤ z ≤ 8

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 arcmin(2) area. By selecting on known Ly alpha emitters (LAEs) and photometric dropout galaxies in the field, we perform targeted searches down to a 5 sigma [C ii] luminosity depth L ([C II]) similar to 2.0 x 10(8) L (circle dot), corresponding roughly to star formation rates (SFRs) of 10-20 M (circle dot) 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 10(8) L (circle dot) (5 sigma) 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 similar to 0 by comparing the ASPECS measurement to literature results at lower redshift.