ALMA uncovers the [CII] emission and warm dust continuum in a z = 8.31 Lyman break galaxy

Abstract

We report on the detection of the [CII] 157.7 $\hat{I}¼$m emission from the Lyman break galaxy (LBG) MACS0416_Y1 at z = 8.3113, by using the Atacama Large Millimeter/submillimeter Array (ALMA). The luminosity ratio of [OIII] 88 $\hat{I}¼$m (from previous campaigns) to [CII] is 9.31 $\pm$ 2.6, indicative of hard interstellar radiation fields and/or a low covering fraction of photo-dissociation regions. The emission of [CII] is cospatial to the 850 $\hat{I}¼$m dust emission (90 $\hat{I}¼$m rest-frame, from previous campaigns), however the peak [CII] emission does not agree with the peak [OIII] emission, suggesting that the lines originate from different conditions in the interstellar medium. We fail to detect continuum emission at 1.5 mm (160 $\hat{I}¼$m rest-frame) down to 18 $\hat{I}¼$Jy (3$\ddot{I}ƒ$). This nondetection places a strong limit on the dust spectrum, considering the 137 $\pm$ 26 $\hat{I}¼$Jy continuum emission at 850 $\hat{I}¼$m. This suggests an unusually warm dust component (T $>$ 80 K, 90% confidence limit), and/or a steep dust-emissivity index (${\hat{I}}_{\mathrm{dust}}^2$ $>$ 2), compared to galaxy-wide dust emission found at lower redshifts (typically T $\sim$ 30 - 50 K, ${\hat{I}}_{\mathrm{dust}}^2$ $\sim$ 1 - 2). If such temperatures are common, this would reduce the required dust mass and relax the dust production problem at the highest redshifts. We therefore warn against the use of only single-wavelength information to derive physical properties, recommend a more thorough examination of dust temperatures in the early Universe, and stress the need for instrumentation that probes the peak of warm dust in the Epoch of Reionization.

15 pages, 8 figures, Accepted by MNRAS