THE ALMA SPECTROSCOPIC SURVEY IN THE HUBBLE ULTRA DEEP FIELD: SEARCH FOR [ ] LINE AND DUST EMISSION IN 6 < z < 8 GALAXIES

Abstract

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) × 10 8 L ⊙ . We identify 14 [C ii ] line emitting candidates in this redshift range with significances >4.5 σ , 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 μ Jy. This implies a dust-obscured star-formation rate (SFR) of (3 ± 1) M ⊙ 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.