Abstract
AbstractActive galactic nuclei show episodic activity, which can be evident in galaxies that exhibit restarted radio jets. These restarted jets can interact with their environment, leaving signatures in the radio spectral energy distribution. Tracing these signatures requires resolved spectral index measurements over a broad frequency range. We present such a study for the radio galaxy 3C 293. Using the International Low Frequency Array Telescope (ILT), we probe spatial scales as fine as ∼0.2″ at 144 MHz and to constrain the spectrum, we combine these data with Multi‐Element Radio Linked Interferometer Network and Very Large Array archival data at frequencies up to 8.4 GHz. In the inner lobes (∼2 kpc), we detect the presence of a compact steep spectrum (CSS) source with a spectral turnover at ∼225 MHz, caused by free‐free absorption from the rich surrounding medium. We find evidence of interaction of the lobes with the rich interstellar medium of the host galaxy. The outer lobes (extending up to ∼100 kpc) have a remarkably uniform spatial distribution of spectral index and only mild spectral curvature (of ∼0.2). Overall, it appears that 3C 293 has gone through multiple (two to three) epochs of activity. This study adds 3C 293 to the new sub‐group of restarted galaxies with short interruption time periods (≲1 Myr). This is the first time a spatially resolved study has been performed that simultaneously studies a young CSS source as well as the older outer lobes at such low frequencies.
Highlights
Over the past decades, active galactic nuclei (AGNs) have been demonstrated to show episodic activity and have been identified in different phases of their life cycle
This paper is structured in the following manner: In Sect. 1, we give an introduction to the source, in Sect. 2, we describe the data and the data reduction procedures, and the procedure to make spectral index maps; in Sect. 3 we present the large and small-scale source morphology, the large and smallscale spectral index, large-scale spectral age analysis, and the absorption models for the inner lobes in the centre
Outer lobes The large structure of 3C 293 is shown in Figure 2 at 144 MHz. seen before at higher frequencies (Bridle et al 1981; Joshi et al The large-scale morphology is in agreement with what has been 2011)
Summary
Active galactic nuclei (AGNs) have been demonstrated to show episodic activity and have been identified in different phases of their life cycle. The life cycle of a radio galaxy is understood to start from a phase of morphologically compact radio emission with an absorbed or steep spectrum. Such sources are called compact steep spectrum (CSS) and gigahertz peaked spectrum (GPS) sources (O’Dea 1998; Fanti 2009; O’Dea & Saikia 2021). These sources show morphological similarities to large-scale radio sources, but with sizes of just a few kiloparsecs (similar to galactic scales), and they are thought to develop into large-scale radio galaxies. Intermittent AGN activity may prevent some of these types of sources from growing to large-scale radio galaxies
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