A small radio galaxy at z=4.026

TL;DR

This paper reports the discovery of a small radio galaxy at redshift 4.026, revealing complex radio structures and challenging simple beaming models for high-redshift AGN.

Contribution

It presents detailed radio observations of a high-redshift radio galaxy, highlighting its complex structure and implications for understanding AGN evolution.

Findings

Revealed a rich radio structure at z=4.026

Detected a bright hotspot indicating dense interstellar medium

Challenged simple beaming models for high-redshift AGN

Abstract

Less than $200$ radio-loud active galactic nuclei (AGN) are known above redshift $4$. Around $40$ of them have been observed at milliarcsecond (mas) scale resolution with very long baseline interferometry (VLBI) technique. Some of them are unresolved, compact, relativistically beamed objects, blazars with jets pointing at small angles to the observer's line of sight. But there are also objects with no sign of relativistic beaming possibly having larger jet inclination angles. In a couple of cases, X-ray observations indicate the presence of relativistic beaming in contrary to the VLBI measurements made with the European VLBI Network (EVN). J1420$+$1205 is a prominent example, where our $30-100$ mas-scale enhanced Multi Element Remotely Linked Interferometer Network (e-MERLIN) radio observations revealed a rich structure reminiscent of a small radio galaxy. It shows a bright hotspot which might be related to the denser interstellar medium around a young galaxy at an early cosmological epoch.