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arXiv · 2608.22835

ASKAP discovery of a pair of large radio bubbles: on the origin of odd radio circles

Renzhi Su·Minfeng Gu·Hengxiao Guo·Neelesh Amrutha·Liang Cheng·Fangzheng Shi·Wenwen Zuo·Ruiyu Zhang·Guobin Mou·Fulai Guo·Wenke Ren·Xuecheng Zheng
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Abstract

We report the serendipitous discovery of a large, low-surface-brightness radio bubble in 944 MHz continuum data from the ASKAP Evolutionary Map of the Universe (EMU) survey. The structure, centred on the elliptical galaxy LEDA 217397 at a redshift of $z=0.040$, spans $\sim$8.4 arcmin, corresponding to a projected diameter of $\sim$399 kpc, and consists of two partly overlapping shells both with radii of $\sim$114 kpc. The integrated flux density of the bubble is $56.8\pm2.9$ mJy at 944 MHz, implying a rest-frame 1.4 GHz luminosity of $\sim$ $1.4\times10^{23}$ W Hz$^{-1}$. Combining the EMU measurement with MWA GLEAM-X data at 88--185 MHz, we derive a steep integrated spectral index of $\alpha=-1.04\pm0.04$, and a two-frequency spectral-index map suggesting a possible exterior flattening. Spectral Energy Distribution (SED) fitting indicates a massive ($\log M_{\ast}/M\odot = 10.97\pm0.09$), quiescent (SFR=$0.025\pm0.083\,M\odot$ yr$^{-1}$) early-type host with no mid-infrared AGN signature and no overdense environment. We compare the bubble with odd radio circles (ORCs) and large radio shells, and discuss three scenarios for its origin: a starburst-driven wind, a merger-driven shock, and AGN jet-inflated bubbles. The starburst wind is disfavoured on energetic grounds ($\gtrsim$$10^{59}$ erg required versus $\sim$$10^{8}$ yr electron lifetimes), and neither a halo-scale merger shock nor a spherical nuclear blast wave can explain the unusually regular, double-shell geometry; a bipolar nuclear outburst -- a relic AGN jet episode, possibly triggered by a supermassive-black-hole merger -- provides the most natural explanation, with later shocks possibly re-energising the plasma. Deeper broad-band radio, polarimetric, spectroscopic and X-ray observations are needed to confirm its nature.

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Renzhi Su, Minfeng Gu, Hengxiao Guo, Neelesh Amrutha, Liang Cheng, Fangzheng Shi, Wenwen Zuo, Ruiyu Zhang, Guobin Mou, Fulai Guo, Wenke Ren, Xuecheng Zheng. 2026-08-24. ASKAP discovery of a pair of large radio bubbles: on the origin of odd radio circles. https://arxiv.org/abs/2608.22835

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