A new search for distant radio galaxies in the Southern hemisphere -- II. 2.2 micron imaging

TL;DR

This study presents K-band imaging of a large sample of ultra-steep-spectrum radio sources to identify high-redshift radio galaxies, revealing their positional relationships with radio features and morphological characteristics.

Contribution

It provides the largest K-band counterpart sample for USS-selected radio galaxies and analyzes their spatial and morphological properties at high redshift.

Findings

93% detection rate of K-band counterparts

Most identifications lie near the radio lobes' midpoint

Majority show no alignment between near-infrared and radio axes

Abstract

We have compiled a sample of 234 ultra-steep-spectrum (USS) selected radio sources in order to find high-redshift radio galaxies. The sample covers the declination range -40deg < DEC < -30deg in the overlap region between the 1400-MHz NRAO VLA Sky Survey, 408-MHz Revised Molonglo Reference Catalogue and the 843-MHz Sydney University Molonglo Sky Survey (the MRCR-SUMSS sample). This is the second in a series of papers on the MRCR-SUMSS sample, and here we present the K-band (2.2 micron) imaging of 173 of the sources primarily from the Magellan and the Anglo-Australian Telescopes. We detect a counterpart to the radio source in 93% of the new K-band images which, along with previously published data, makes this the largest published sample of K-band counterparts to USS-selected radio galaxies. The location of the K-band identification has been compared to the features of the radio emission for the double sources. We find that the identification is most likely to lie near the midpoint of the radio lobes rather than closer to the brighter lobe, making the centroid a less likely place to find the optical counterpart. 79% of the identifications are less than 1 arcsec from the radio lobe axis. These results differ from studies of low-redshift radio samples where the environments are typically not nearly so dense and disturbed as those at high redshift. In contrast to some literature samples, we find that the majority of our sample shows no alignment between the near-infrared and radio axes. Several different morphologies of aligned structures are found and those that are aligned within 10 degrees are consistent with jet-induced star formation. ...abridged...