An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: Identifying Candidate z ∼ 4.5 [C II] Emitters

Abstract

We report the results of a search for serendipitous [C ii ] 157.74 μ m emitters at z  ≃ 4.4–4.7 using the Atacama Large Millimeter/submillimeter Array (ALMA). The search exploits the AS2UDS continuum survey, which covers ∼50 arcmin 2 of the sky toward 695 luminous ( S 870  ≳ 1 mJy) submillimeter galaxies (SMGs), selected from the SCUBA-2 Cosmology Legacy Survey 0.96 deg 2 Ultra Deep Survey (UDS) field. We detect 10 candidate line emitters, with an expected false detection rate of 10%. All of these line emitters correspond to 870 μ m continuum-detected sources in AS2UDS. The emission lines in two emitters appear to be high- J CO, but the remainder have multi-wavelength properties consistent with [C ii ] from z  ≃ 4.5 galaxies. Using our sample, we place a lower limit of  on the space density of luminous ( L IR  ≃ 10 13 ) SMGs at z  = 4.40–4.66, suggesting % of SMGs with mJy lie at 4 <  z  < 5. From stacking the high-resolution (∼0.″15 full-width half maximum) ALMA 870 μ m imaging, we show that the [C ii ] line emission is more extended than the continuum dust emission, with an average effective radius for the [C ii ] of kpc, compared to r e  = 1.0 ± 0.1 kpc for the continuum (rest-frame 160 μ m). By fitting the far-infrared photometry for these galaxies from 100 to 870 μ m, we show that SMGs at z  ∼ 4.5 have a median dust temperature of T d  = 55 ± 4 K. This is systematically warmer than 870 μ m selected SMGs at z  ≃ 2, which typically have temperatures around 35 K. These z  ≃ 4.5 SMGs display a steeper trend in the luminosity-temperature plane than z  ≤ 2 SMGs. We discuss the implications of this result in terms of the selection biases of high-redshift starbursts in far-infrared/submillimeter surveys.