The ALMA REBELS survey: the dust content of z ∼ 7 Lyman break galaxies

Abstract

We include a fully coupled treatment of metal and dust enrichment into the Delphi semi-analytic model of galaxy formation to explain the dust content of 13 Lyman Break Galaxies (LBGs) detected by the Atacama Large millimetre Array (ALMA) REBELS Large Program at z ≃ 7. We find that the galaxy dust mass, Md, is regulated by the combination of SNII dust production, astration, shock destruction, and ejection in outflows; grain growth (with a standard timescale τ0 = 30 Myr) plays a negligible role. The model predicts a dust-to-stellar mass ratio of $\sim 0.07-0.1{{\ \rm per\ cent}}$ and a UV-to-total star formation rate relation such that log(ψUV) = −0.05 [log(ψ)]2 + 0.86 log(ψ) − 0.05 (implying that 55-80 per cent of the star formation is obscured) for REBELS galaxies with stellar mass $M_* = 10^{9-10} \rm M_\odot$. This relation reconciles the intrinsic UV luminosity of LBGs with their observed luminosity function at z = 7. However, 2 out of the 13 systems show dust-to-stellar mass ratios ($\sim 0.94-1.1{{\ \rm per\ cent}}$) that are up to 18 × larger than expected from the fiducial relation. Due to the physical coupling between dust and metal enrichment, even decreasing τ0 to very low values (0.3 Myr) only increases the dust-to-stellar mass ratio by a factor ∼2. Given that grain growth is not a viable explanation for such high observed ratios of the dust-to-stellar mass, we propose alternative solutions.

DUSTiER (DUST in the Epoch of Reionization): dusty galaxies in cosmological radiation-hydrodynamical simulations of the Epoch of Reionization with RAMSES-CUDATON

“…this can then be explained by lower metallicities in DUSTiER galaxies (≈ 1.5× lower for the highest stellar masses). Our results are similar to those of Dayal et al (2022), who report lower dust masses at fixed stellar mass, thus implying higher metallicities in DUSTiER. Note that the fiducial model of Popping et al (2017) and the model of Vijayan et al (2019) both predict a jump in the average DTM as a function of stellar mass, but at higher stellar masses, with a slighter difference before and after the jumps.…”

“…We find DUSTiER sits at an intermediate position in terms of the degree with which extinction affects the UVLF: whereas the impact on the UVLF occurs for M AB1600 <-19 in DUSTiER, in the simulations of Wu et al (2020) it occurs as soon as M AB1600 ∼ -17.5, and in the simulations of Vijayan et al (2020) it occurs as late as -21. Dayal et al (2022) find a result close to that of Vijayan et al (2020), but with considerably fewer M AB1600 < −21 galaxies. The fact that Vijayan et al (2020) observe that the UVLF is only extincted in brighter galaxies than in DUSTiER is intriguing since Fig.…”

“…For the highest stellar mass haloes (M ★ > 10 8 M ) our median dust masses are a good match to the locus of observational points, as well as to the "high-cond" model of Popping et al (2017) at z = 6 towards the end of Reionization. For the highest stellar mass galaxies, our predictions are also in quite good agreement with their "fiducial" model, but overshoot the results of Vijayan et al (2019) and Dayal et al (2022) by almost a factor of 10. There appears to be two regimes of dust accumulation, with a sudden increase of a factor ∼ 10 3 in dust mass taking place around a stellar mass of 10 5 M to 10 6 M (depending on the redshift).…”

“…As with the other observables we have investigated, there is very little redshift evolution or scatter around the median. Whereas the median dust masses and DTM values agreed well with the predictions of the 'high-cond' model of Popping et al (2017) and Dayal et al (2022), here we under-predict the median DTG when compared to the 'high-cond ' Popping et al (2017) model, and end up with a slightly better agreement with the DTG from their 'fiducial' model. This discrepancy with respect to the Popping et al (2017) findings likely arises from the definition of DTG, and modelling of the hot and cold phases of the ISM.…”

“…One possible explanation we set ourselves to investigate and quantify in this paper is dust, which is not accounted for in CoDa I nor CoDa II . Indeed, it is increasingly clear that massive star forming galaxies in the high redshift Universe already contain large dust masses (significant fractions of their stellar mass, as in Schaerer et al (2015); Béthermin et al (2015); Laporte et al (2017); Burgarella et al (2020); Dayal et al (2022)). Dust is coupled to gas and ionising photons in several ways that can interest us.…”