An ALMA survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS field: Identifying candidate z~4.5 [CII] emitters

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 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 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 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 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 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.