2018ApJ...861..100C

We report the results of a search for serendipitous [C II] 157.74 µm 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 arcmin² of the sky toward 695 luminous (S₈₇₀ >= 1 mJy) submillimeter galaxies (SMGs), selected from the SCUBA-2 Cosmology Legacy Survey 0.96 deg² 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 µm 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 > 5×10^–6Mpc^–3 on the space density of luminous (L_IR ≃ 10¹³ L_☉) SMGs at z = 4.40-4.66, suggesting ≥ 7% of SMGs with S_870µm_≳1 mJy lie at 4 < z < 5. From stacking the high-resolution (∼0.''15 full-width half maximum) ALMA 870 µm 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 r_e=1.7_–0.2^+0.1 kpc, compared to r_e = 1.0 ± 0.1 kpc for the continuum (rest-frame 160 µm). By fitting the far-infrared photometry for these galaxies from 100 to 870 µm, 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 µm 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.