Abstract
Active galactic nuclei (AGN) at the centres of galaxies can cycle between periods of activity and of quiescence. Characterising the duty-cycle of AGN is crucial for understanding their impact on the evolution of the host galaxy. For radio AGN, their evolutionary stage can be identified from a combination of morphological and spectral properties. We summarise the results we have obtained in the last few years by studying radio galaxies in various crucial phases of their lives, such as remnant and restarted sources. We used morphological information derived from LOw Frequency ARray (LOFAR) images at 150 MHz, combined with resolved spectral indices maps, obtained using recently released images at 1400 MHz from the APERture Tile In Focus (Apertif) phased-array feed system installed on the Westerbork Synthesis Radio Telescope. Our study, limited so far to the Lockman Hole region, has identified radio galaxies in the dying and restarted phases. We found large varieties in their properties, relevant for understanding their evolutionary stage. We started by quantifying their occurrences, the duration of the ‘on’ (active) and ‘off’ (dying) phase, and we compared the results with models of the evolution of radio galaxies. In addition to these extreme phases, the resolved spectral index images can also reveal interesting secrets about the evolution of apparently normal radio galaxies. The spectral information can be connected with, and used to improve, the Fanaroff–Riley classification, and we present one example of this, illustrating what the combination of the LOFAR and Apertif surveys now allow us to do routinely.
Highlights
Since the first studies were conducted on active galactic nuclei (AGN), it has been suggested that the release of energy from super-massive black holes (SMBH) goes in cycles, with alternating periods of activity and periods of quiescence, see, e.g., [1,2,3,4] and references therein
We summarised some of the recent results on the study of the life cycle of radio galaxies, obtained by focusing on the elusive phases of remnant and restarted radio sources
The results presented here were obtained by using LOw Frequency ARray (LOFAR) images at 150 MHz—for the selection based on the morphology—in combination with APERture Tile In Focus (Apertif) images at 1400 MHz to derive spatially resolved, spectral information at low frequencies
Summary
Since the first studies were conducted on active galactic nuclei (AGN), it has been suggested that the release of energy from super-massive black holes (SMBH) goes in cycles, with alternating periods of activity and periods of quiescence, see, e.g., [1,2,3,4] and references therein. To achieve the goals of obtaining larger (and less biased) samples of remnant and restarted radio galaxies, and a more systematic and quantitative assessment of the life cycle, two elements are important: (1) being able to trace the morphology and structures on different scales (i.e., the presence of extended, low surface brightness amorphous features, while at the same time, having high spatial resolution to identify the presence of compact structures); and (2) obtaining the (resolved) spectral information down to the lowest frequencies. If the steepening is large, with spectral indices steeper than α = 1.2 (i.e., ultra-steep spectrum, USS), it is the signature of plasma devoid of any fresh particle injection [21] This provides a key signature for tracing remnant radio galaxies: inactive or dying radio AGN where the central engine has stopped or its activity is substantially dimmed. For example, non-uniform magnetic fields, can complicate the interpretation, observing extreme spectral indices can provide unambiguous evidence of both stopped and restarted activity
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