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

TL;DR

This study analyzes a young, high-redshift galaxy's stellar metallicity and star formation history using deep spectroscopic data, revealing low metallicity and implications for early galaxy evolution.

Contribution

It provides a detailed case study of a z=4.77 galaxy, combining spectral modeling and observations to infer metallicity, star formation, and ionizing photon escape, highlighting challenges in such analyses.

Findings

Galaxy has low metallicity ([Z/H]=-2.15 to -1.15)

High Hα luminosity suggests high ionizing photon escape

Lyα escape linked to low metallicity

Abstract

Directly characterising the first generations of stars in distant galaxies is a key quest of observational cosmology. We present a case study of ID53 at z=4.77, the UV-brightest (but L*) star-forming galaxy at z>3 in the MUSE eXtremely Deep Field with a mass of $10^9$ M$_{\odot}$. In addition to very strong Lyman-$\alpha$ (Ly$\alpha$) emission, we clearly detect the (stellar) continuum and an 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 Hubble Space Telescope 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 an attenuation law. Models with a young age and low metallicity (log10(age/yr)=6.5-7.6 and [Z/H]=-2.15 to -1.15) are preferred. The intrinsic H$\alpha$ luminosity of the best-fit models is an order of magnitude higher than the H$\alpha$ luminosity inferred from Spitzer/IRAC data, which either suggests a high escape fraction of ionising photons, a high relative attenuation of nebular to stellar dust, or a complex SFH. The metallicity appears lower than the metallicity in more massive galaxies at z=3-5, consistent with the scenario according to which younger galaxies have lower metallicities. This chemical immaturity likely facilitates Ly$\alpha$ escape, explaining why the Ly$\alpha$ equivalent width is anti-correlated 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.