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 $z\sim4.4$-$4.7$ 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.
