Professor Andrew Bunker

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.