Professor Andrew Bunker

Abstract:

Context . Collisional ring galaxies are a rare class of interacting systems, comprising only ∼0.01% of galaxies in the local Universe. Their formation is typically attributed to a head-on collision of a massive galaxy with a compact satellite (intruder), triggering density waves propagating outwards that produce the characteristic ring morphology. Here, we report the discovery and detailed analysis of GS18660, the most distant ring galaxy known to date, at z = 3.076, based on JWST data obtained within of the GA-NIFS programme. Aims . This work aims to characterise the physical and dynamical properties of GS18660 and shed light on the formation of its ring. Specifically, we analysed the ionised gas properties, stellar populations, and gas kinematics of the system, and used the observed geometry to constrain the timescale of the collision. Methods . Our analysis is based on NIRSpec integral field spectrograph (IFS) data, including low-resolution ( R ∼ 100) spectroscopy covering 0.6–5.3 μ m (∼0.2–1.3 μ m rest-frame), and high-resolution ( R ∼ 2700) spectroscopy covering 1.66–3.17 μ m (0.4–0.8 μ m rest-frame). Multi-wavelength techniques (e.g. emission line diagnostics, full spectral fitting, and gas dynamics) were applied to derive nebular gas conditions and stellar population properties. Results . Gas kinematic analysis reveals that GS18660 exhibits a rotating disc component with an additional radial expansion velocity of ∼200 km s −1 , consistent with a propagating collisional wave. Nebular line diagnostics indicate an intense star formation (star formation rate SFR ∼100 M ⊙ yr −1 ) along the ring and in the nucleus. Stellar population analysis shows that the most recent star formation episode, occurring within the last ≈50 Myr, predominantly took place in the ring. We also identify a close companion, the intruder galaxy responsible for the collision, moving away with a relative velocity of ∼425 km s −1 . Conclusions . The evidence favours a collisional origin for the ring in GS18660, though the presence of a recently formed bar (and hence a resonance ring) cannot be completely excluded. This discovery provides a unique window to the physics of galaxy interactions and the rapid assembly of stellar structures in the early Universe.

Abstract:

We systematically search for galaxy overdensities using 20 independent fields with a minimum of six filters (F090W, F115W, F150W, F277W, F356W, and F444W) from BEACON, the JWST Cycle 2 NIRCam pure-parallel imaging survey. We apply an adaptive kernel-density estimation method that incorporates the full photometric redshift probability distribution function of each galaxy to map galaxy overdensities, and identify 207 significant (>4σ) overdensities at 1.5 < z < 5. We measure the quenched galaxy fraction, the average specific star formation rate (sSFR), the total dark matter mass, and the local galaxy density of each system. By investigating the correlation among these observables, we find that galaxy quenching proceeds in two paths. (i) Overdensities within more massive halos exhibit higher quenched fractions and lower averaged sSFRs. This trend weakens at z ≳ 2, consistent with cold gas streams penetrating shock-heated massive halos and sustaining star formation activity at early times. (ii) We also find a dependence of the same parameters on local densities at z < 2, where the quenched fraction increases and the sSFR decreases toward higher densities. The environmental trend in sSFR weakens at z ∼ 2–3 and shows tentative evidence for a reversal at z > 3, potentially due to a larger cold gas supply in earlier times. Our study reveals a complex interplay between individual galaxies and large-scale environmental properties, marking the onset of environmental effects on galaxy quenching in massive halos at cosmic noon.

Abstract:

This Letter presents JWST MIRI/Medium-Resolution Spectrometer (MRS) observations of the central r ≈ 40–240 pc regions of five active galactic nuclei (AGN) spanning a wide range of luminosities (log Lbol/erg s−1 ≈ 39.8–43.8). Combining multiphase diagnostics from polycyclic aromatic hydrocarbons (PAHs), molecular hydrogen (H2), and ionized gas at spatial scales of ∼4–24 pc, this study presents a spatially resolved investigation into the effects of the two distinct AGN feedback modes—radiative and kinetic—on the surrounding medium. The results indicate that these two feedback modes, associated with AGN irradiation and shock processing, respectively, collectively drive the relative suppression of PAH emission in the nuclear regions of the targets studied here. Moreover, the coexistence of these two AGN feedback modes, especially the shock processing associated with either jets or outflows, in the central regions of AGN naturally explains both the bimodal distribution of PAH band ratios observed in the targets studied here and the seemingly disparate results reported in the literature. Although based on a limited sample, these findings provide new insights into calibrating star formation rates from PAH emission in AGN and, more importantly, lay the groundwork for a practical framework to diagnose and quantify AGN feedback in the JWST era.

Abstract:

Finding the first (Population III or Pop III) stars is one of the fundamental quests of astronomy, aiming to deliver the missing link in how stars form at early cosmic times. Yet their initial mass function, formation sites and feedback remain highly uncertain, as well as the timing and topology of the transition to metal-enriched star formation. The observability of their peculiar spectral features is also debated, due to their short lifetime and faintness. This review summarizes current theoretical expectations for Pop III star formation, and the main observational strategies that have been adopted to constrain their properties across cosmic time, including near-field cosmology studies, direct searches for extremely metal-poor star-forming complexes and/or hard-ionizing spectral signatures at high and intermediate redshifts, and prospects for identifying Pop III activity up to Cosmic Dawn. The combination of JWST spectroscopy, time-domain searches, lensing surveys, stellar archaeology, absorption-line studies, as well as improved simulations, is yielding a growing number of observational candidates and narrowing the allowed parameter space for the first stars, setting the stage for a ‘’golden era’’ of Pop III searches.