Spectral ageing in the lobes of FR-II radio galaxies: New methods of analysis for broadband radio data

TL;DR

This paper introduces the BRATS software for detailed broadband spectral analysis of radio galaxies, compares spectral ageing models, and reveals new insights into source physics and hot spot dynamics using high-resolution data.

Contribution

It develops the BRATS software and applies it to JVLA data, comparing spectral ageing models and providing new high-resolution spectral maps and physical insights.

Findings

Tribble model fits observations well and is physically realistic

High injection indices are observed, higher than previously assumed

Discrepancy in hot spot advance speeds persists at high spectral resolution

Abstract

The broad-bandwidth capabilities of next generation telescopes such as the JVLA mean that the spectrum of any given source varies significantly within the bandwidth of any given observation. Detailed spectral analysis taking this variation into account is set to become standard practice when dealing with any new broadband radio observations; it is therefore vital that methods are developed to handle this new type of data. In this paper, we present the Broadband Radio Astronomy ToolS (BRATS) software package and, use it to carry out detailed analysis of JVLA observations of three powerful radio galaxies. We compare two of the most widely used models of spectral ageing, the Kardashev-Pacholczyk and Jaffe-Perola models and also results of the more complex, but potentially more realistic, Tribble model. We find that the Tribble model provides both a good fit to observations as well as providing a physically realistic description of the source. We present the first high-resolution spectral maps of our sources and find that the best-fitting injection indices across all models take higher values than has previously been assumed. We present characteristic hot spot advance speeds and make comparison to those derived from dynamical ages, confirming the previously known discrepancy in speed remains present when determined at high spectral resolutions. We show that some previously common assumptions made in determining spectral ages with narrow-band radio telescopes may not always hold and strongly suggest these are accounted for in future investigations.