Diffuse Radio Emission in Abell 754

TL;DR

This study presents low-frequency radio observations of galaxy cluster A754, detecting multiple diffuse features, analyzing their spectra, and suggesting their origins as radio galaxy cocoons or shock-related phenomena.

Contribution

First low-frequency imaging of A754 revealing four diffuse features and their spectral analysis, proposing their origins as radio cocoons or shock-related emissions.

Findings

Detected four diffuse radio features at 150 MHz.

Spectral index of the new feature exceeds 2.

The fourth feature's spectrum matches a radio galaxy cocoon.

Abstract

We present a low frequency study of the diffuse radio emission in the galaxy cluster A754. We present new 150 MHz image of the galaxy cluster A754 made with the Giant Metrewave Radio Telescope (GMRT) and discuss the detection of 4 diffuse features. We compare the 150 MHz image with the images at 74, 330 and 1363 MHz; one new diffuse feature is detected. The flux density upperlimits at 330 and 1363 MHz imply a synchrotron spectral index, $\alpha > 2$, ($S\propto \nu^{-\alpha}$) for the new feature. The 'west relic' detected at 74 MHz (Kassim et al 2001) is not detected at 150 MHz and is thus consistent with its non-detection at 1363 MHz (Bacchi et al 2003) and 330 MHz(Kassim et al 2001). Integrated spectra of all the diffuse features are presented. The fourth diffuse feature is located along the proposed merger axis (Zabludoff et al 1995) in A754 and 0.7 Mpc away from the peak of X-ray emission. We have made use of the framework of adiabatic compression model (Ensslin & Gopal-Krishna 2001) to obtain spectra. We show that the spectrum of the fourth diffuse feature is consistent with that of a cocoon of a radio galaxy lurking for about $9\times10^{7}$ yr; no shock compression is required. The other three diffuse emission have spectra steeper than 1.5 and could be cocoons lurking for longer time. We discuss other possibilities such as shocks and turbulent reacceleration being responsible for the diffuse emission in A754.