The REBELS ALMA 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 $z\approx 7$ galaxies in the ALMA Large Project REBELS sample.
We find $39\ \mathrm{K} < T_d < 58\ \mathrm{K}$, and dust masses in the narrow
range $M_d = (0.9-3.6)\times 10^7 M_{\odot}$. 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,
$t_{dep}=M_g/\mathrm{SFR}$, induced by the higher cosmological accretion rate
at early times; this hypothesis yields $T_d \propto (1+z)^{0.4}$. 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), $T_d \propto N_H^{1/6}$ ($T_d
\propto Z^{-1/6}$). REBELS galaxies are on average relatively transparent, with
effective gas column densities around $N_H \simeq (0.03-1)\times 10^{21}
\mathrm{cm}^{-2}$. We predict that other high-$z$ galaxies (e.g. MACS0416-Y1,
A2744-YD4), with estimated $T_d \gg 60$ 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.
