Professor Andrew Bunker

Abstract:

We present observations of the Lyman-break galaxy HZ10 with the JWST/NIRSpec integral field unit in high and low spectral resolution (G395H, spectral resolving power and PRISM, , respectively), as part of the GA-NIFS programme. By spatially resolving the source (spatial resolution or kpc), we find three spatially and spectrally distinct regions of line emission along with one region of strong continuum emission, all within a projected distance of kpc. The R2700 data features strong detections in H, [O iii] , [N ii] , H, and [S ii] . The R100 data additionally contain a strong detection of the Ly break, rest-frame UV and optical continuum, and [O ii] . None of the detected lines present strong evidence for active galactic nucleus excitation from line diagnostic diagrams, and no high-ionization lines are detected. Using the detected lines, we constrain the electron density and metallicity ( solar) in each component. Spaxel-by-spaxel fits reveal a strong east-west velocity gradient and significant line asymmetries (possibly indicating tidal features or outflows). The western component features a very red UV slope () and significant H emission, suggesting an evolved population and active star formation. A comparison to high-resolution ( or kpc) [] m imaging obtained with the Atacama Large Millimetre/submillimetre Array (ALMA) reveals areas of dust obscuration. Altogether, these data suggest that HZ10 represents an ongoing merger, with a complex distribution of stars, gas, and dust Gyr after the big bang

Abstract:

The unexpectedly high nitrogen-to-oxygen (N/O) ratios observed in high-redshift (z) galaxies have challenged our understanding of early star formation. Notably, many of these nitrogen-rich galaxies show signatures of active galactic nuclei (AGNs), suggesting a possible connection between black hole formation and nitrogen enrichment. To explore this connection, we analyse stacked spectra of broad-line and narrow-line AGNs using deep Near Infrared Spectrograph data from the JWST Advanced Deep Extragalactic Survey. We identify a significant N iii] quintuplet and a high electron density ( cm) only in the broad-line AGN stack, indicating nitrogen-rich (, ) and dense gas similar to the high-z nitrogen-rich galaxies. Our findings suggest that dense nuclear star formation may trap nitrogen-rich gas in proto-globular clusters, in line with the high N/O observed in local globular clusters; associated runaway stellar collisions could produce intermediate-mass black hole seeds, as predicted by some models and simulations, whose accretion results into AGN signatures. These findings support scenarios connecting the early black hole seeding and growth to merging processes within and between proto-globular clusters in primeval galaxies.

Abstract:

The topology of reionization and the environments where galaxies efficiently produce ionizing photons are key open questions. For the first time, we investigate the trend between ionizing photon production efficiency, ξion, and galaxy overdensity, log(1+δ) . We analyze the ionizing properties of 79 galaxies between 1.0 < z < 5.2 using JWST NIRSpec medium-resolution spectra from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey (SMILES) program. Among these, 67 galaxies have Hα coverage, spanning 1.0 < z < 3.1. The galaxy overdensity, log(1+δ) , is measured using the JADES photometric catalog, which covers the SMILES footprint. For the subset with Hα coverage, we find that logξion is positively correlated with log(1+δ) , with a slope of 0.94−0.46+0.46 . Additionally, the mean ξion for galaxies in overdense regions ( log(1+δ)>0.1 ) is 2.43 times that of galaxies in lower density regions ( log(1+δ)<0.1 ). This strong trend is found to be independent of redshift evolution. Furthermore, our results confirm the robust correlations between ξion and the rest-frame equivalent widths of the [O iii] or Hα emission lines. Our results suggest that galaxies in high-density regions are efficient producers of ionizing photons.

Abstract:

We present a comprehensive analysis of galaxy close-pair fractions and major merger rates to evaluate the importance of mergers in the hierarchical growth of galaxies over cosmic time. This study focuses on the previously poorly understood redshift range of using JADES observations. Our mass-complete sample includes primary galaxies with stellar masses of , having major companions (mass ratio ) selected by pkpc projected separation and redshift proximity criteria. Pair fractions are measured using a statistically robust method incorporating photometric redshift posteriors and available spectroscopic data. The pair fraction evolves with redshift and shows dependence on the stellar mass: at there is an increase up to , followed by a turnover, while at higher stellar masses there is a flattening and weak decline with increasing redshift. Similarly, the derived galaxy major merger rate increases and flattens beyond to per galaxy, showing a weak scaling with stellar mass, driven by the evolution of the galaxy stellar mass function. A comparison between the cumulative mass accretion from major mergers and the mass assembled through star formation indicates that major mergers contribute approximately to the total mass growth over the studied redshift range, which is in agreement with the ex situ mass fraction estimated from our simple numerical model. These results highlight that major mergers contribute little to the direct stellar mass growth compared to in situ star formation but could still play an indirect role by driving star formation itself.