Age-dating Early Quiescent Galaxies: High Star-Formation Efficiency, but Consistent with Direct, Higher-Redshift Observations

We present a detailed analysis of JWST/NIRSpec and NIRCam observations of ZF-UDS-7329, a massive, quiescent galaxy at redshift z=3.2, which has been put forward to challenge cosmology and galaxy formation physics. Our study extends previous works by focusing on the impact of different star formation history (SFH) priors, stellar libraries, metallicity, and initial mass function assumptions. Our results show that ZF-UDS-7329, with a formed stellar mass of M_⋆≈ 10^11.4 M_⊙ and a specific star formation rate of sSFR≈ 0.03 Gyr^-1, formed efficiently in the first billion years of the Universe. In agreement with previous work, we find that the spectrum is consistent with mass-weighted stellar ages of 1.3-1.8 Gyr, depending on the SFH prior used. A physically motivated rising SFH prior makes the formation history of ZF-UDS-7329 compatible with stellar mass and star-formation rate estimates of high-redshift (z>6) galaxies. Using NIRCam imaging, we identify a colour gradient indicative of an old, quiescent bulge and a younger disc component, as expected from a complex formation history. The inferred SFH is consistent a high stellar fraction of f_⋆=M_⋆/(f_b · M_ h) ≈ 100% at z=7-12, implying an extremely high integrated star-formation efficiency. However, when considering cosmic variance and possible mergers as expected in over-dense environments - as traced by ZF-UDS-7329 - the stellar fractions could be reduced to f_⋆≈ 50%, which is more consistent with galaxy formation models and the stellar-to-halo mass relation at lower redshifts. We conclude that ZF-UDS-7329 forms extremely efficient in the early universe, but does not necessitate unseen galaxies at higher redshifts since the inferred SFR of ancestors are consistent with those seen in z>6 galaxies.