Observing galaxy mergers at the epoch of reionization

TL;DR

This study investigates galaxy mergers at redshifts around 7-8 using Hubble data, finding some galaxy pairs and discussing implications for cosmic variance and redshift accuracy.

Contribution

It provides the first observational evidence of galaxy pairs at these high redshifts and analyzes potential mechanisms behind their appearance.

Findings

6 out of 22 galaxies at z~8 have companions within 1'

Projection coincidences and lensing unlikely explain all pairs

Potential contamination from lower redshift sources

Abstract

The galaxies with photometric redshifts observed in a close angular proximity might be either projection coincidences, strongly lensed images of the same galaxy, or separate galaxies that are in a stage of merging. We search for the groups of galaxies in the Hubble Ultra Deep Field (HUDF09) in $z\sim7$ and $z\sim8$ drop-out samples. We find no close pairs among 50 galaxies in the $z\sim7$ sample, while in the $z\sim8$ sample we find that 6 out of 22 galaxies have a companion within $\sim1'$ (3 pairs). Adopting a numerical simulation and performing forward modeling we show that even though mergers are unlikely to have such a high fraction, the projection coincidences and the strong lensing are even less likely mechanisms to account for all of three pairs. Alternatively, there is a possibility of the contamination in the drop-out catalog from lower redshifts, which potentially can account for all of the groups. Finally, we make projection on the sensitivity to mergers of the James Webb Space Telescope, and discuss the possible applications of the high-redshift merging galaxies for decreasing cosmic variance effect on the luminosity function and for improving the accuracy of photometric redshifts in general.