Quantifying the Effects of Known Unknowns on Inferred High-redshift Galaxy Properties: Burstiness, IMF, and Nebular Physics

Abstract

The era of the James Webb Space Telescope ushers stellar population models into uncharted territories, particularly at the high-redshift frontier. In a companion paper, we apply the Prospector Bayesian framework to jointly infer galaxy redshifts and stellar population properties from broadband photometry as part of the UNCOVER survey. Here we present a comprehensive error budget in spectral energy distribution (SED) modeling. Using a sample selected to have photometric redshifts higher than 9, we quantify the systematic shifts stemming from various model choices in inferred stellar mass, star formation rate (SFR), and age. These choices encompass different timescales for changes in the star formation history (SFH), nonuniversal stellar initial mass functions (IMF), and the inclusion of variable nebular abundances, gas density, and ionizing photon budget. We find that the IMF exerts the strongest influence on the inferred properties: the systematic uncertainties can be as much as 1 dex, 2–5 times larger than the formal reported uncertainties in mass and SFR, and importantly, exceed the scatter seen when using different SED fitting codes. Although the assumptions on the lower end of the IMF induce degeneracy, our findings suggest that a common practice in the literature of assessing uncertainties in SED-fitting processes by comparing multiple codes is substantively underestimating the true systematic uncertainty. Highly stochastic SFHs change the inferred SFH by much larger than the formal uncertainties, and introduce ∼0.8 dex systematics in SFR averaged over a short timescale and ∼0.3 dex systematics in average age. Finally, employing a flexible nebular emission model causes ∼0.2 dex systematic increase in mass and SFR, comparable to the formal uncertainty. This paper constitutes an initial step toward a complete uncertainty estimate in SED modeling.

The Heavy Metal Survey: Star Formation Constraints and Dynamical Masses of 21 Massive Quiescent Galaxies at z = 1.3–2.3

“…Assuming a Salpeter (1955) IMF would increase the stellar masses by 0.2 dex (Figure 11) such that they exceed M dyn for the majority of the galaxies. Thus, the IMF assumption causes the largest (systematic) uncertainty in our stellar mass estimates (see also Wang et al 2024). Combining both stellar mass effects and the color gradient correction (assumption set 8 in Figure 11), would lead to stellar masses vastly exceeding the dynamical masses for nearly all galaxies.…”

Two Distinct Classes of Quiescent Galaxies at Cosmic Noon Revealed by JWST PRIMER and UNCOVER

“…  < M M log 9 galaxies predict that very bursty SFHs are needed to explain observations (e.g., Anglés-Alcázar et al 2017;Faucher-Giguère 2018;Ma et al 2018;Sun et al 2023;Dome et al 2024), but similar studies at lower redshifts do not exist. Moreover, stellar population modeling from broadband photometry alone is largely insufficient to resolve burst durations on timescales shorter than 100 Myr (e.g., Suess et al 2022b;Wang et al 2024a), whereas one would need <40 Myr timescales in order to separate candidate mini-quenched galaxies from recently quenched galaxies (e.g., Dome et al 2024). Spectroscopy of a sample of these recently quenched galaxies would offer more accurate SFH reconstruction on shorter timescales and allow us to identify observables that isolate mini-quenched galaxies from other low-mass galaxies.…”

RUBIES: Evolved Stellar Populations with Extended Formation Histories at z ∼ 7–8 in Candidate Massive Galaxies Identified with JWST/NIRSpec

“…For completeness, we also include the second set of lower number density, based on the CEERS +UDS volume, as open gray symbols in Figure 7(b). Additional systematic uncertainties in SED fitting, e.g., the initial mass function, which can likewise decrease the tension with the standard model, are presented in Wang et al (2024c).…”

“…Paradoxically, the cores of local massive galaxies appear to have bottom-heavy stellar initial mass functions (e.g., Conroy & van Dokkum 2012), with some evidence that this persists or even strengthens at z ∼ 2 (van Dokkum et al 2024; though see Mercier et al 2023, for an alternate take). This would increase the inferred stellar masses (without changing the observed SED in any way; Wang et al 2024c) by a factor of a few, further increasing the tension with the cosmic baryon fraction.…”