Professor Andrew Bunker

Abstract:

We report the discovery of a galaxy protocluster candidate (dubbed MACS0416-OD-z8p5) at a spectroscopic redshift of z ∼ 8.47, dating back to ∼550 Myr after the Big Bang. The observations are part of the JWST Cycle 3 Treasury Program Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (or SAPPHIRES) using NIRCam grism observations. Using wide-field slitless spectroscopy (WFSS) obtained in the MACS0416 parallel field, we robustly confirm nine galaxies at zspec ∼ 8.47 via emission-line detections of [O iii] 5008 Å (with >5σ) and tentatively confirm one additional galaxy (at ∼3σ). This discovery represents the highest-redshift spectroscopically confirmed galaxy overdensity known to date and is ∼6–8 times more dense than the average number density of galaxies at the same redshift. Furthermore, a galaxy hosting a low-mass active galactic nucleus (a “little red dot”) is found as a member, suggesting that black hole growth may already be underway in dense environments at early cosmic times. This finding of a massive dark matter halo hosting a galaxy overdensity at z ∼ 8.5 is surprising given that our survey covered only a small, random field ( 16.5arcmin2 ) as part of a pure-parallel observation. Comparison with cosmological simulations shows that the likelihood of finding such a large-scale structure is <5% under the current galaxy formation scenario and within the observed survey volume. Our results demonstrate the power of WFSS observations for building complete samples of line-emitting galaxies and suggest an important role for overdensities in enhancing galaxy formation by funneling large-scale gas supplies into small cosmological volumes.

Abstract:

Abstract Galaxy mergers and interactions are often invoked to explain enhanced star formation, black hole growth, and mass build-up of galaxies at later cosmic times, but their effect is poorly understood at high redshift (z > 2). We use JADES data to analyse a mass-complete sample of 1968 galaxies at z = 3 − 9 with log(M⋆/M⊙) = [8, 10], identifying major merger pairs (projected separation of 5 − 100 pkpc, mass ratio ≥1/4) using a probabilistic method. To look for signatures of enhancement in multiple physical properties, we carefully build a control sample of non-pairs that are simultaneously matched in redshift, stellar mass, isolation, and environment to the pair sample. We find a moderate enhancement in specific star-formation rate (sSFR) of a factor of 1.10 ± 0.05 at separations ≲ 20 kpc in pairs compared to their matched controls. We find that the interaction-related sSFR enhancement is most apparent at longer SFR averaging timescales (50–100 Myr), whereas the enhancement is not detected on 5—10 Myr timescales, where bursty star formation might be driven by internal processes. By averaging star formation histories, we find two distinct populations: one with monotonically rising SFRs and another with an earlier peak in SFR, potentially corresponding to different interaction stages. Finally, we find no significant excess of AGN in pairs, consistent with galaxy interactions not strongly triggering black hole activity at the separations probed (>5 kpc). Similarly, we also do not detect an excess in the fraction of Lyman-α emitters in pairs, suggesting that at the probed separations, galaxy interactions are not efficient at enhancing Lyman-α photon production and escape, which may only become important at the smallest physical scales.

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.