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

Abstract

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 ($M_*=10^{6.5}$-$10^{9.5}\,M_\odot$), 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 $z\gtrsim10$ and detections at $z\sim7$, as dust-to-stellar mass ratios rise sharply near $Z\sim0.2\,Z_\odot$, typically at $M_*\gtrsim10^{8.5}\,M_\odot$, 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 $M_{\rm UV}$-$β_{\rm UV}$ relation. The observed UV slopes ($β_{\rm UV}\simeq-2.3$) favor small grains ($a\lesssimλ_{\rm UV}/2π\simeq0.025\,μ{\rm m}$) producing steep UV extinction, together with a low abundance of carbonaceous grains or PAHs responsible for the 2175 $\mathrm{\mathring{A}}$ bump, consistent with silicate-rich dust. Large-grain stellar dust ($a_0=0.1\,μ{\rm m}$) instead yields nearly grey extinction and UV slopes close to the intrinsic value ($β_{\rm UV,0}\simeq-2.6$). SMC and MW attenuation curves flatten at $A_{\rm V}\gtrsim0.3$, 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 $M_{\rm UV}\lesssim-21.5$. Roman wide-field surveys may identify such rare UV-bright galaxies.