Dust in Galaxies at z=10-15: Attenuation and Re-emission Predictions from the FirstLight Simulations

Dust masses and grain properties in z>10 galaxies remain poorly constrained, as JWST UV attenuation and ALMA dust re-emission are rarely modelled consistently. We perform post-processed 3D dust radiative transfer on 208 snapshots of 99 galaxies at z=10-15 from the FirstLight simulations (M*=10 6.5 - 10 9.5,M), applying twelve dust models per snapshot (2496 runs): four dust-to-metal (DTM) prescriptions and three grain size distributions. Metallicity-dependent DTM models reproduce ALMA dust-mass upper limits at z 10 and detections at z about7, as dust-to-stellar mass ratios rise sharply near Z about0.2,Z, typically at M* 10 8.5,M, where grain growth by metal accretion becomes dominant. A constant DTM of 0.4 instead violates more than half of the upper limits. For dust attenuation, SMC-like dust is favored by the observed M UV - β UV relation. The observed UV slopes (β UV-2.3) favor small grains (a λ UV/2π 0.025,μ m) producing steep UV extinction, together with a low abundance of carbonaceous grains or PAHs responsible for the 2175 bump, consistent with silicate-rich dust. Large-grain stellar dust (a 0=0.1,μ m) instead yields nearly grey extinction and UV slopes close to the intrinsic value (β UV,0-2.6). SMC and MW attenuation curves flatten at A V 0.3, showing that relatively flat attenuation does not necessarily imply large grains. For dust re-emission, dust peak temperatures reach 60-80 K, exceeding the 40-50 K commonly assumed in ALMA dust-mass estimates. Predicted IR continuum flux increases with UV luminosity; at current ALMA sensitivities (10-20 μ Jy), detections generally require M UV-21.5. Roman wide-field surveys may identify such rare UV-bright galaxies.