The extended X-ray emission around HDF130 at z=1.99: an inverse Compton ghost of a giant radio source in the Chandra Deep Field North

TL;DR

This paper reports the discovery of an extended X-ray source in the Chandra Deep Field North, interpreted as an inverse Compton ghost of a now-inactive giant radio galaxy at redshift 1.99, revealing insights into galaxy evolution and electron energy losses.

Contribution

It presents the first detailed analysis of an inverse Compton ghost in deep X-ray observations, linking X-ray emission to past radio activity of a high-redshift galaxy.

Findings

X-ray source has a double-lobed structure consistent with a ghost radio galaxy.

The X-ray spectrum fits a power-law with photon index 2.65.

The source's luminosity and structure suggest it is a relic of a giant radio source no longer powered.

Abstract

One of the six extended X-ray sources found in the Chandra DeepField North is centred on HDF130, which has recently been shown to be a massive galaxy at z=1.99 with a compact radio nucleus. The X-ray source has a roughly double-lobed structure with each lobe about 41 arcsec long, or 345 kpc at the redshift of HDF130. We have analyzed the 2 Ms X-ray image and spectrum of the source and find that it is well fit by a power-law continuum of photon index 2.65 and has a 2--10 keV luminosity of 5.4x10^{43}ergps (if at z=1.99). Any further extended emission within a radius of 60 arcsec has a luminosity less than half this value, which is contrary to what is expected from a cluster of galaxies. The source is best explained as an inverse Compton ghost of a giant radio source, which is no longer being powered, and for which Compton losses have downgraded the energetic electrons, \gamma> 10^4, required for high-frequency radio emission. The lower energy electrons, \gamma~1000, produce X-rays by inverse Compton scattering on the Cosmic Microwave Background. Depending on the magnetic field strength, some low frequency radio emission may remain. Further inverse Compton ghosts may exist in the Chandra deep fields.