The ALMA REBELS Survey: cosmic dust temperature evolution out to z 7

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) × 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) × 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.