The ALMA REBELS Survey. Epoch of Reionization giants: properties of dusty galaxies at $z\approx 7$

Abstract

We analyse FIR dust continuum measurements for 14 galaxies ($z\approx 7$) in the ALMA REBELS LP to derive their physical properties. Our model uses three input data: (a) the UV spectral slope, ${\hat{I}}^2$, (b) the observed UV continuum flux at $1500$A, $F_{\mathrm{UV}}$, (c) the observed continuum flux at $\approx 158\hat{I}¼$m, $F_{158}$, and considers Milky Way (MW) and SMC extinction curves, along with different dust geometries. We find that REBELS galaxies have (28-90.5)% of their star formation obscured; the total (UV+IR) star formation rates are in the range $31.5<\mathrm{SFR}/(M_{\odot }{\mathrm{yr}}^{-1})<129.5$. The sample-averaged dust mass and temperature are $(1.3\pm 1.1)\times 10^7{M}_{\odot }$ and $52\pm 11$ K, respectively. In some galaxies dust is abundant (REBELS-14, $M_d^{\prime }\approx 3.4\times 10^7{M}_{\odot }$), or hot (REBELS-18, $T_d^{\prime }\approx 67$ K). The dust distribution is compact ($<0.3$ kpc for 70% of the galaxies). The dust yield per supernova is $0.1\le y_d/M_{\odot }\le 3.3$, with 70% of the galaxies requiring $y_d<0.25M_{\odot }$. Three galaxies (REBELS-12, 14, 39) require $y_d>1M_{\odot }$. With the SFR predicted by the model and a MW extinction curve, REBELS galaxies detected in [CII] nicely follow the local $L_{\mathrm{CII}}-$SFR relation, and are approximately located on the Kennicutt-Schmidt relation. The sample-averaged gas depletion time is of $0.11y_P^{-2}$ Gyr, where $y_P$ is the ratio of the gas-to-stellar distribution radius. For some systems a solution simultaneously matching the observed (${\hat{I}}^2,F_{\mathrm{UV}},F_{158}$) values cannot be found. This occurs when the index $I_m=(F_{158}/F_{\mathrm{UV}})/({\hat{I}}^2-{\hat{I}}_{\mathrm{int}}^2)$, where ${\hat{I}}_{\mathrm{int}}^2$ is the intrinsic UV slope, exceeds $I_m^{\ast }\approx 1120$ for a MW curve. For these objects we argue that the FIR and UV emitting regions are not co-spatial, questioning the use of the IRX-${\hat{I}}^2$ relation.

22 pages, 7 figures, accepted by MNRAS. Comments welcome