Serendipitous discovery of a dying Giant Radio Galaxy associated with NGC 1534, using the Murchison Widefield Array

TL;DR

This paper reports the serendipitous discovery of a nearby, dying giant radio galaxy associated with NGC 1534 using the Murchison Widefield Array, revealing a rare class of low surface brightness, steep-spectrum radio galaxies.

Contribution

It presents the first detailed radio observations of NGC 1534, identifying it as a rare, low surface brightness, dying radio galaxy with extreme spectral properties and estimating its space density.

Findings

NGC 1534 is the lowest surface brightness radio galaxy known.

The radio lobes have an extremely steep spectral index of -2.1.

Estimated space density of such dying radio galaxies is 7×10⁻⁷ Mpc⁻³.

Abstract

Recent observations with the Murchison Widefield Array at 185~MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby ($z = 0.0178$) radio galaxy associated with NGC\,1534. The diffuse emission presented here is the first indication that NGC\,1534 is one of a rare class of objects (along with NGC\,5128 and NGC\,612) in which a galaxy with a prominent dust lane hosts radio emission on scales of $\sim$700\,kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with disks. NGC1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2\,mJy\,arcmin$^{-2}$. The radio lobes have one of the steepest spectral indices yet observed: $\alpha=-2.1\pm0.1$, and the core to lobe luminosity ratio is $<0.1$\%. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as $7\times10^{-7}$\,Mpc$^{-3}$ and argue that normal AGN cannot spend more than 6\% of their lifetime in this phase if they all go through the same cycle.