Professor Andrew Bunker

Abstract:

Distant quasars (QSOs) in galaxy overdensities are considered key actors in the evolution of the early Universe. In this work, we performed an analysis of the kinematic and physical properties of the BR1202-0725 system at z ∼ 4.7, one of the most overdense fields known in the early Universe, consisting of a QSO, a sub-millimetre galaxy (SMG), and three Lyman- α emitters. We used data from the JWST/NIRSpec integral field unit to analyse the rest-frame optical emission of each source in the system. Based on the continuum and Balmer line emission, we estimated a bolometric luminosity of log( L bol /[erg s −1 ]) = 47.2 ± 0.4 and a black hole mass of log( M BH / M ⊙ ) = 10.1 ± 0.5 for the QSO, which are consistent with previous measurements obtained with ground-based observations. The NIRSpec spectra of the SMG revealed, instead, unexpected [O  III ] and H α +[N  II ] profiles. The overall [O  III ] line profile is blueshifted by more than 700 km s −1 relative to the systemic velocity of the galaxy. Additionally, both the [O  III ] and H α +[N  II ] lines show prominent broad (∼1300 km s −1 ), blueshifted wings associated with outflowing ionized gas. The analysis of NIRSpec and X-ray observations indicates that the SMG likely hosts an accreting supermassive black hole, which is supported by the following results: (i) the excitation diagnostic diagram is consistent with ionization from an active galactic nucleus (AGN); (ii) the X-ray luminosity is higher than 10 44 erg s −1 ; and (iii) it hosts a fast outflow ( v out ∼ 5000 km s −1 ), comparable to the ones observed in luminous QSOs. Therefore, the QSO-SMG pair of BR1202-0725 represents one of the highest-redshift double AGNs found to date, with a projected separation of 24 kpc. Finally, we investigated the environment of this system and found four new galaxies, detected in both [O  III ] and H α emission, at the same redshift of the QSO and within a projected distance of 5 kpc from it. This overdense system includes at least ten galaxies in a field of view of only 980 kpc 2 .

Abstract:

We investigate the unusual emission-line luminosity ratios observed in the JWST Deep Extragalactic Survey (JADES) NIRSpec spectroscopy of GN-z11, which reveal exceptionally strong emission lines and a significant detection of the rarely observed N iii] , multiplet. These features suggest an elevated N/O abundance, challenging existing models of stellar populations and nebular emission. To assess whether Wolf–Rayet (WR) stars can account for the observed line ratios, we construct a suite of stellar and nebular models incorporating high-resolution stellar spectral libraries, enabling a more accurate treatment of WR evolution and its influence on the ionizing radiation field. We find that the inclusion of WR stars is essential for reproducing the observed position of GN-z11 in the C iii]/He ii versus C iii]/C iv diagnostic plane, resolving discrepancies from previous studies. The model-derived metallicity (0.07 Z/Z 0.15), ionization parameter (−2), and stellar ages are consistent with the literature estimates. However, our models underpredict the N iii/O iii] ratio, suggesting that WR stars alone cannot fully explain the nitrogen enrichment. This suggests that additional mechanisms, such as rapid chemical enrichment in a young, metal-poor environment, may be necessary to explain the nitrogen excess. While our models successfully reproduce most observed line ratios, further refinements to the models are needed to fully characterize the stellar populations and the enrichment processes of high-redshift galaxies like GN-z11.