HST images and properties of the most distant radio galaxies

TL;DR

This study uses HST imaging to analyze the morphology and properties of 11 high-redshift radio galaxies, revealing diverse structures, alignment effects, and evolutionary links to present-day galaxies.

Contribution

First detailed HST morphological analysis of high-redshift radio galaxies showing diverse structures and implications for galaxy evolution.

Findings

Most galaxies have irregular, clumpy structures suggestive of merging.

UV continuum emission is elongated and aligned with radio axes, but varies case by case.

High-redshift radio galaxies may evolve into brightest cluster galaxies.

Abstract

We present Hubble Space Telescope images of 11 high redshift radio galaxies (between $z=2.3$ and $z=3.6$). The galaxies were observed with the WFPC2 camera in a broad band filter (F606W or F707W, roughly equivalent to V or R-band), for 2 orbits each. We find that on the scale of the HST observations there is a wide variety of morphological structures of the hosting galaxies: most objects have a clumpy, irregular appearance, consisting of a bright nucleus and a number of smaller components, suggestive of merging systems. Some observed structures could be due (at least partly) to the presence of dust distributed through the galaxies. The UV continuum emission is generally elongated and aligned with the axis of the radio sources, however the characteristics of the ``alignment effect'' differ from case to case, suggesting that the phenomenon cannot be explained by a single physical mechanism. We compare the properties of our radio galaxies with those of the UV dropout galaxies and conclude that (i) the most massive radio galaxies may well evolve from an aggregate of UV dropout galaxies and (ii) high redshift radio galaxies probably evolve into present day brightest cluster galaxies.