Kavli Affiliate: Roberto Maiolino
| First 5 Authors: Crispin Turner, Sandro Tacchella, Francesco D’Eugenio, Stefano Carniani, Mirko Curti
| Summary:
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
$M_{star} approx 10^{11.4}~M_{odot}$ and a specific star formation rate of
$mathrm{sSFR} approx 0.03$ 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 $f_{star}=M_{star}/(f_b
cdot M_{rm h}) approx 100%$ 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 $f_{star} approx
50%$, 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.
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