2025MNRAS.544.1502B

We present an Atacama Large Millimeter/submillimeter Array (ALMA) Band 9 continuum detection ($3.3 \sigma$) of MACS0416_Y1 that confirms the suspected warm dust (91$^{+62}_{-35}$ K) of this Lyman-Break Galaxy (LBG) at $z = 8.3$ with $\log _{10} M_{\ast }/$M$_{\odot } = 9.0 \pm 0.1$. A modified blackbody fit to the ALMA Bands 3 through 9 data of MACS0416_Y1 finds an intrinsic infrared luminosity of 1.0$^{+1.8}_{-0.6} \times {} 10^{12}\ \mathrm{L_{\odot }}$, placing this UV-selected LBG in the regime of Ultra Luminous Infrared Galaxies. Its luminous but modest dust reservoir (1.4$^{+1.3}_{-0.5} \times {} 10^{6}\ \mathrm{{\rm M}_{\odot }}$) is cospatial to regions with a UV-continuum slope $\beta _{\rm UV} \approx -1.5$ as seen by James Webb Space Telescope (JWST) imaging. Although this implies some dust obscuration, the JWST photometry implies less obscured star formation than seen in the complete characterization by ALMA, implying some spatial separation of dust and stars on scales below 200 pc, i.e. smaller than those probed by JWST and ALMA. This source is an extreme example of dust-obscured star formation contributing strongly to the cosmic build-up of stellar mass, which can only be revealed through direct and comprehensive observations in the (sub)mm regime.