Probing cosmic dawn: modelling the assembly history, SEDs, and dust content of selected z ∼ 9 galaxies

Abstract

The presence of spectroscopically confirmed Balmer breaks in galaxy spectral energy distributions (SEDs) at z > 9 provides one of the best probes of the assembly history of the first generations of stars in our Universe. Recent observations of the gravitationally lensed source, MACS 1149 JD1 (JD1), indicate that significant amounts of star formation likely occurred at redshifts as high as z 15. The inferred stellar mass, dust mass, and assembly history of JD1, or any other galaxy at these redshifts that exhibits a strong Balmer break, can provide a strong test of our best theoretical models from high-resolution cosmological simulations. In this work, we present the results from a cosmological radiation-hydrodynamics simulation of the region surrounding a massive Lyman-break galaxy. For two of our most massive systems, we show that dust preferentially resides in the vicinity of the young stars thereby increasing the strength of the measured Balmer break such that the simulated SEDs are consistent with the photometry of JD1 and two other z > 9 systems (GN-z10-3 and GN-z9-1) that have proposed Balmer breaks at high redshift. We find strong variations in the shape and luminosity of the SEDs of galaxies with nearly identical stellar and halo masses, indicating the importance of morphology, assembly history, and dust distribution in making inferences on the properties of individual galaxies at high redshifts.Our results stress the importance that dust may play in modulating the observable properties of galaxies, even at the extreme redshifts of z > 9.

The absence of [C ii] 158 $\mu$m emission in spectroscopically confirmed galaxies at z > 8

“…The [O III] / [C II] emission line ratio for high redshift galaxies. Our work on MACS1149 JD1 and A2744 YD4 together with the z = 7.2 LAE (Inoue et al 2016) indicate ratios well above those seen in local metal poor dwarfs(Madden et al 2013, grey circles) as well as numerical simulations capable of predicting both lines(Katz et al 2019, black open symbols). The maximum effect of CMB attenuation is indicated by dashed lines below the current limits (see text for details).…”

“…Likewise when studying the [Oiii]88µm/ [Cii]158µm line ratio, we find anomalously high values. We examine this line ratio with a recent hydrodynamical simulation of the ISM in early galaxies (Katz et al 2019) and suggest that a low gas-phase metallicity may not be the sole explanation for this [C II] deficit. Other hypotheses include a high ionisation parameter consistent with trends seen in UV spectroscopy of similar z > 7 sources or the suppression of neutral gas and hence [Cii]158µm emission via energetic feedback from intermittent star formation.…”

Balmer breaks in simulated galaxies at z>6

“…Furthermore, Katz et al (2019) point out that strong Balmer breaks are rare in their simulations if one considers dust-free galaxies, which is consistent with our findings. In difference to the Katz et al (2019) study, we find that galaxies with Balmer breaks as strong as the one observed in JD1 are rare in the simulations at z ∼ 9, even when considering differential obscuration. This means that the main difference in our results stem from the different treatment of dust effects.…”

“…Thus, the effect of differential obscuration may become significantly smaller in cases where dust scattering is considered. In addition to finding an analogue to JD1, Katz et al (2019) show that they find 3 objects that exhibit strong Balmer breaks in their simulation volume. It is, however not entirely clear how likely we are to find such galaxies in their simulations when considering a larger sample of objects (especially when considering possible viewing-angle effects).…”

“…With a detailed analysis combining observations over a wide wavelength range, Hashimoto et al (2018) found that the spectral energy distribution (SED) of JD1 is consistent with the object having a star formation history (SFH) with two episodes of high star formation and an intermediate phase (with a duration of ∼ 100 to 200 Myr) of low/no star formation activity. An alternative interpretation is that the Balmer break of JD1 is primarily due to selective dust attenuation, in the sense that the light from young stars is far more heavily extinguished than the light from old stars (Katz et al 2019). There are other examples of galaxies that seem to exhibit evolved features such as strong Balmer breaks at lower redshifts (z 5 Mawatari et al 2016).…”