An empirical study of dust properties at the earliest epochs

Abstract

We present an empirical analysis of the properties of dust-continuum emission in a sample of 17 galaxies in the early Universe ($4 < z < 8$) with well-sampled far-infrared (FIR) spectral energy distributions (SEDs) compiled from the literature. We place our results into context by self-consistently comparing to samples of nearby star-forming galaxies, luminous infrared galaxies (LIRGs), and quasars. With the exception of two sources, we find no significant evolution in the dust emissivity index across cosmic time, measuring a consistent value of $\beta_\text{IR} = 1.8 \pm 0.3$ at $z > 4$, suggesting the effective dust properties do not change dramatically for most galaxies. Despite having comparable stellar masses, we find the high-redshift galaxies to be similar to, or even more extreme than, LIRGs in the HERUS sample in terms of dust temperature ($T_\text{dust} > 40 \, \mathrm{K}$) and IR luminosity ($L_\text{IR} > 10^{11} \, \mathrm{L_\odot}$). We find the dust temperature evolves mildly towards high redshift, though the LIRGs and quasars exhibit elevated temperatures indicating a more efficient and/or additional heating mechanism. Where available, we compare stellar-mass estimates to our inferred dust masses, whose degeneracy with dust temperature can only be mitigated with a well-constrained SED. In merely half of the cases the dust yield may be explained by supernovae alone, with four sources ($44\%$) significantly exceeding a highly optimistic yield where $M_\text{dust} \approx 0.01 M_*$. We discuss possible explanations for this apparent inconsistency and potential observational biases in the measurements of the dust properties of high-redshift galaxies, including in the current IR-bright sample.