An ALMA Survey of the SCUBA-2 Cosmology Legacy Survey UKIDSS/UDS Field: Identifying Candidate z ∼ 4.5 [C II] Emitters
Thomson, A. P.; Scott, D.; Smail, Ian; Farrah, D.; An, Fang Xia; Michałowski, M. J.; Swinbank, A. M.; Wardlow, J. L.; van der Werf, P.; Chen, Chian-Chou; Simpson, J. M.; Conselice, C. J.; Blain, A. W.; Chapman, S. C.; Gullberg, B.; Almaini, O.; Coppin, K. E. K.; Simpson, C. J.; Cooke, E. A.; Stach, S. M.; Maltby, D. T.
United Kingdom, China, United States, Canada, Germany, Poland, Taiwan, Netherlands
Abstract
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 (S 870 ≳ 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}-6 {Mpc}}-3 on the space density of luminous (L IR ≃ 1013 {L}⊙ ) SMGs at z = 4.40-4.66, suggesting ≥slant 7% of SMGs with {S}870μ {{m}}≳ 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.