Powerful yet lonely: Is 3C 297 a high-redshift fossil group?

TL;DR

This study investigates the high-redshift radio galaxy 3C 297, revealing cluster-like X-ray features but no nearby galaxies, suggesting it may be a fossil group with a merged stellar mass and an X-ray halo.

Contribution

The paper provides new X-ray and optical observations of 3C 297, proposing it as a potential high-redshift fossil group despite its isolated optical environment.

Findings

Detection of hot gas halo in X-ray observations.

Absence of companion galaxies at similar redshift.

Host galaxy shows characteristics of a fossil group.

Abstract

The environment of the high-redshift (z=1.408), powerful radio-loud galaxy 3C 297 has several distinctive features of a galaxy cluster. Among them, a characteristic halo of hot gas revealed by Chandra X-ray observations. In addition, a radio map obtained with the Very Large Array (VLA) shows a bright hotspot in the northwestern direction, created by the interaction of the AGN jet arising from 3C 297 with its environment. In the X-ray images, emission cospatial with the northwestern radio lobe is detected, and peaks at the position of the radio hotspot. The extended, complex X-ray emission observed with our new Chandra data is largely unrelated to its radio structure. Despite having attributes of a galaxy cluster, no companion galaxies have been identified from 39 new spectra of neighboring targets of 3C 297 obtained with the Gemini Multi-Object Spectrograph. None of the 19 galaxies for which a redshift was determined lies at the same distance as 3C 297. The optical spectral analysis of the new Gemini spectrum of 3C 297 reveals an isolated Type-II radio-loud AGN. We also detected line broadening in [O II](3728) with a FWHM about 1700 km/s and possible line shifts of up to 500-600 km/s. We postulate that the host galaxy of 3C 297 is a fossil group, in which most of the stellar mass has merged into a single object, leaving behind an X-ray halo.