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

Abstract

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 × 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 × 10⁸ L_⊙ (5σ) to be less than 1.8 × 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.