THE ALMA SPECTROSCOPIC SURVEY in the HUBBLE ULTRA DEEP FIELD: SEARCH for [CII] LINE and DUST EMISSION in 6 < z < 8 GALAXIES

We present a search for [C ii] line and dust continuum emission from optical dropout galaxies at z > 6 using ASPECS, our Atacama Large Millimeter submillimeter Array Spectroscopic Survey in the Hubble Ultra-deep Field (UDF). Our observations, which cover the frequency range of 212-272 GHz, encompass approximately the range of 6 < z < 8 for [C ii] line emission and reach a limiting luminosity of L _{[C ii]} ∼ (1.6-2.5) ×10⁸ L _o. We identify 14 [C ii] line emitting candidates in this redshift range with significances >4.5σ, two of which correspond to blind detections with no optical counterparts. At this significance level, our statistical analysis shows that about 60% of our candidates are expected to be spurious. For one of our blindly selected [C ii] line candidates, we tentatively detect the CO(6-5) line in our parallel 3 mm line scan. None of the line candidates are individually detected in the 1.2 mm continuum. A stack of all [C ii] candidates results in a tentative detection with S _{1.2 mm} = 14 ±5 μJy. This implies a dust-obscured star-formation rate (SFR) of (3 ± 1) M _o yr^-1. We find that the two highest-SFR objects have candidate [C ii] lines with luminosities that are consistent with the low-redshift L _{[C ii]} versus SFR relation. The other candidates have significantly higher [C ii] luminosities than expected from their UV-based SFR. At the current sensitivity, it is unclear whether the majority of these sources are intrinsically bright [C ii] emitters, or spurious sources. If only one of our line candidates was real (a scenario greatly favored by our statistical analysis), we find a source density for [C ii] emitters at 6 < z < 8 that is significantly higher than predicted by current models and some extrapolations from galaxies in the local universe.