Deciphering stellar metallicities in the early Universe: Case study of a young galaxy at z = 4.77 in the MUSE eXtremely Deep Field

Abstract

Directly characterising the first generations of stars in distant galaxies is a key quest of observational cosmology. Here, we present a case study of ID53 at z = 4.77, the UV-brightest (yet L ) star-forming galaxy at z > 3 in the MUSE eXtremely Deep Field with a mass of ≈ 10 9 M . Besides very strong Lyman-α (Lyα) emission, we clearly detect the (stellar) continuum and a NV P-Cygni feature, interstellar absorption, fine-structure emission and nebular CIV emission-lines in the 140 hr spectrum. Continuum emission from two spatially resolved components in the HST data are blended in the MUSE data, but we show that the nebular CIV emission originates from a subcomponent of the galaxy. The UV spectrum can be fit with recent BPASS stellar population models combined with single burst or continuous star formation histories (SFHs), a standard initial mass function and attenuation law. Models with a young age and low metallicity (log 10 (age/yr)=6.5-7.6 and [Z/H]=-2.15 to -1.15) are preferred, but the details depend on the assumed SFH. The intrinsic Hα luminosity of the best-fit models is an order of magnitude higher than the Hα luminosity inferred from Spitzer/IRAC data, which either suggests a high escape fraction of ionising photons, a high relative nebular to stellar dust attenuation or a complex star formation history. The metallicity appears lower than the metallicity in more massive galaxies at z = 3 − 5, consistent with the picture in which younger galaxies have lower metallicities. Such chemical immaturity likely facilitates Lyα escape, explaining why the Lyα equivalent width anti-correlates with stellar metallicity. Finally, we stress that uncertainties in SFHs impose a challenge for future inferences of the stellar metallicity of young galaxies. This highlights the need for joint (spatially resolved) analyses of stellar spectra and photo-ionisation models.

Live Fast, Die $α$-Enhanced: The Mass-Metallicity-$α$ Relation of the Milky Way's Disrupted Dwarf Galaxies

“…The MZR is offset to lower metallicities, but runs almost parallel to the z = 0 MZR. Further, these studies infer α-enhancement at z ≈ 2 − 3 on the order of [α/Fe]≈ 0.4, i.e., (α/Fe)≈ 2.5×(α/Fe) , based on e.g., 1 A handful of constraints at higher redshifts (z ≈ 4 − 8) exist on individual objects (e.g., Shapley et al 2017;Matthee et al 2021) or via calibrations where the current systematic uncertainties are likely on the order of the expected evolution in the abundance trends (e.g., Faisst et al 2016;Jones et al 2020). This is set to change soon thanks to JWST (Shapley et al 2021;Strom et al 2021;Curti et al 2021).…”