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 [CII] 157.74$\mu$m emitters at $z\sim 4.4$-$4.7$ using the Atacama Large Millimeter/submillimeter Array (ALMA). The search exploits the AS2UDS continuum survey, which covers ~50 arcmin${}^2$ of the sky towards 695 luminous ($S_{870}\gtrsim 1$mJy) submillimeter galaxies (SMGs), selected from the SCUBA-2 Cosmology Legacy Survey (S2CLS) 0.96deg${}^2$ Ultra Deep Survey (UDS) field. We detect ten candidate line emitters, with an expected false detection rate of ten percent. All of these line emitters correspond to 870$\mu$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 [CII] from $z\simeq 4.5$ galaxies. Using our sample, we place a lower limit of $>5\times {10}^{-6}$Mpc${}^{-3}$ on the space density of luminous ($L_{\mathrm{I}\mathrm{R}}\simeq {10}^{13}$Lsun) SMGs at $z=4.40$-$4.66$, suggesting $\ge 7$percent of SMGs with $S_{870\mu{\rm m}}\gtrsim1$mJylieat$4<z<5$\mathrm{.}Fromstackingthehigh-resolution($\sim0.15"$full-widthhalfmaximum)ALMA$870\mu$m imaging, we show that the [CII] line emission is more extended than the continuum dust emission, with an average effective radius for the [CII] of $r_{\mathrm{e}}=1.7_{-0.2}^{+0.1}$kpc compared to $r_{\mathrm{e}}=1.0\pm 0.1$kpc for the continuum (rest-frame $160\mu$m). By fitting the far-infrared photometry for these galaxies from $100$-$870\mu$m, we show that SMGs at $z\sim 4.5$ have a median dust temperature of $T_{\rm d}=55\pm4$K\mathrm{.}Thisissystematicallywarmerthan$870\mu$m-selectedSMGsat$z\simeq2$,whichtypicallyhavetemperaturesaround$35$K\mathrm{.}These$z\simeq4.5$SMGsdisplayasteepertrendintheluminosity-temperatureplanethan$z\le2$ SMGs. We discuss the implications of this result in terms of the selection biases of high redshift starbursts in far-infrared/submillimeter surveys.