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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. 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 ≈109 M. In addition to very strong Lyman-α (Lyα) emission, we clearly detect the (stellar) continuum and an N V P Cygni feature, interstellar absorption, fine-structure emission and nebular C IV emission lines in the 140 h 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 C IV 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, 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 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 Lyα escape, explaining why the Lyα 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.

Based on observations obtained with the Very Large Telescope under the large program 1101.A-0127.

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https://hal-insu.archives-ouvertes.fr/insu-03711525
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Submitted on : Saturday, July 2, 2022 - 7:26:11 AM
Last modification on : Saturday, September 24, 2022 - 12:02:05 PM
Long-term archiving on: : Monday, October 3, 2022 - 6:24:05 PM

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Jorryt Matthee, Anna Feltre, Michael Maseda, Themiya Nanayakkara, Leindert Boogaard, et al.. Deciphering stellar metallicities in the early Universe: case study of a young galaxy at z = 4.77 in the MUSE eXtremely Deep Field. Astronomy and Astrophysics - A&A, 2022, 660, ⟨10.1051/0004-6361/202142187⟩. ⟨insu-03711525⟩

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