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

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 [CII] 158-mu m line and underlying dust continuum measurements, we derive T-d in the continuum and [CII] detected z approximate to 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 10(7) M-circle dot. 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(d)(ep) = M-g/SFR, induced by the higher cosmological accretion rate at early times; this hypothesis yields T-d proportional to (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-d proportional to N-H(1/6) (T-d proportional to Z(-1/6)). REBELS galaxies are on average relatively transparent, with effective gas column densities around N-H similar or equal to (0.03-1) x 10(21) 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.