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

Abstract

We report the results of a search for serendipitous [CII] 157.74$\mu$m emitters at $z\sim4.4$-$4.7$ 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 ($S_{870}\gtrsim1$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 $z\simeq4.5$ galaxies. Using our sample, we place a lower limit of $>5\times10^{-6}$Mpc$^{-3}$ on the space density of luminous ($L_{\rm IR} \simeq 10^{13}$Lsun) SMGs at $z=4.40$-$4.66$, suggesting $\ge7$percent of SMGs with $S_{870\mu{\rm m}}\gtrsim1$mJy lie at $4<z<5$. From stacking the high-resolution ($\sim0.15"$ full-width half maximum) 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 $r_{\rm e} = 1.7^{+0.1}_{-0.2}$kpc compared to $r_{\rm e} = 1.0\pm0.1$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 $z\sim4.5$ have a median dust temperature of $T_{\rm d}=55\pm4$K. This is systematically warmer than $870\mu$m-selected SMGs at $z\simeq2$, which typically have temperatures around $35$K. These $z\simeq4.5$ SMGs display a steeper trend in the luminosity-temperature plane than $z\le2$ SMGs. We discuss the implications of this result in terms of the selection biases of high redshift starbursts in far-infrared/submillimeter surveys.