SERENADE II: An ALMA Multi-Band Dust-Continuum Analysis of 28 Galaxies at $5<z<8$ and the Physical Origin of the Dust Temperature Evolution

Abstract

We present an analysis of ALMA multi-band dust-continuum observations for 28 spectroscopically-confirmed bright Lyman-break galaxies at $5<z<8$. Our sample consists of 11 galaxies at $z\sim 6$ newly observed in our ALMA program, which substantially increases the number of $5<z<8$ galaxies with both rest-frame 88 and 158 $\mu \mathrm{m}$ continuum observations, allowing us to simultaneously measure the IR luminosity and dust temperature for a statistical sample of $z\gtrsim 5$ galaxies for the first time. We derive the relationship between the UV slope (${\beta }_{\mathrm{UV}}$) and infrared excess (IRX) for the $z\sim 6$ galaxies, and find a shallower IRX-${\beta }_{\mathrm{UV}}$ relation compared to the previous results at $z\sim 2$--4. Based on the IRX-${\beta }_{\mathrm{UV}}$ relation consistent with our results and the ${\beta }_{\mathrm{UV}}$-$M_{\mathrm{UV}}$ relation including fainter galaxies in the literature, we find a limited contribution of the dust-obscured star formation to the total SFR density, $\sim 30\%$ at $z\sim 6$. Our measurements of the dust temperature at $z\sim 6-7$, $T_{\mathrm{dust}}=40.9_{-9.1}^{+10.0}\mathrm{K}$ on average, supports a gentle increase of $T_{\mathrm{dust}}$ from $z=0$ to $z\sim 6$--7. Using an analytic model with parameters consistent with recent {\it{JWST}} results, we discuss that the observed redshift evolution of the dust temperature can be reproduced by an $\sim 0.6\mathrm{dex}$ increase in the gas depletion timescale and $\sim 0.4\mathrm{dex}$ decrease of the metallicity. The variety of $T_{\mathrm{dust}}$ observed at high redshifts can also be naturally explained by scatters around the star-formation main sequence and average mass-metallicity relation, including an extremely high dust temperature of $T_{\mathrm{dust}}>80\mathrm{K}$ observed in a galaxy at $z=8.3$.

Submitted to ApJ