2022MNRAS.513.3122S

ALMA observations have revealed the presence of dust in the first generations of galaxies in the Universe. However, the dust temperature T_d remains mostly unconstrained due to the few available FIR continuum data at redshift z > 5. This introduces large uncertainties in several properties of high-z galaxies, namely their dust masses, infrared luminosities, and obscured fraction of star formation. Using a new method based on simultaneous [C \scriptstyle \rm II] 158-µm line and underlying dust continuum measurements, we derive T_d in the continuum and [C \scriptstyle \rm II] detected z ≃ 7 galaxies in the ALMA Large Project REBELS sample. We find 39 < T_d < 58 K, and dust masses in the narrow range M_d = (0.9-3.6) x 10⁷ M_☉. These results allow us to extend for the first time the reported T_d(z) relation into the Epoch of Reionization. We produce a new physical model that explains the increasing T_d(z) trend with the decrease of gas depletion time, t_dep = M_g/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields T_d ∝ (1 + z)^0.4. The model also explains the observed T_d scatter at a fixed redshift. We find that dust is warmer in obscured sources, as a larger obscuration results in more efficient dust heating. For UV-transparent (obscured) galaxies, T_d only depends on the gas column density (metallicity), T_{\rm d} \propto N_{\rm H}^{1/6} (T_d ∝ Z^–1/6). REBELS galaxies are on average relatively transparent, with effective gas column densities around N_H ≃ (0.03-1) x 10²¹ cm^–2. We predict that other high-z galaxies (e.g. MACS0416-Y1, A2744-YD4), with estimated T_d ≫ 60 K, are significantly obscured, low-metallicity systems. In fact, T_d is higher in metal-poor systems due to their smaller dust content, which for fixed L_IR results in warmer temperatures.