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. [ABSTRACT FROM AUTHOR]