The peculiar radio galaxy 4C 35.06: a case for recurrent AGN activity?

Abstract

Using observations obtained with the LOw Fequency ARray (LOFAR), the Westerbork Synthesis Radio Telescope (WSRT) and archival Very Large Array (VLA) data, we have traced the radio emission to large scales in the complex source 4C 35.06 located in the core of the galaxy cluster Abell 407. At higher spatial resolution (∼4 ), the source was known to have two inner radio lobes spanning 31 kpc and a diffuse, low-brightness extension running parallel to them, offset by about 11 kpc (in projection). At 62 MHz, we detect the radio emission of this structure extending out to 210 kpc. At 1.4 GHz and intermediate spatial resolution (∼30 ), the structure appears to have a helical morphology. We have derived the characteristics of the radio spectral index across the source. We show that the source morphology is most likely the result of at least two episodes of AGN activity separated by a dormant period of around 35 Myr. The outermost regions of radio emission have a steep spectral index (α < −1), indicative of old plasma. We connect the spectral index properties of the resolved source structure with the integrated flux density spectral index of 4C 35.06 and suggest an explanation for its unusual integrated flux density spectral shape (a moderately steep power law with no discernible spectral break), possibly providing a proxy for future studies of more distant radio sources through inferring their detailed spectral index properties and activity history from their integrated spectral indices. The AGN is hosted by one of the galaxies located in the cluster core of Abell 407. We propose that it is intermittently active as it moves in the dense environment in the cluster core. In this scenario, the AGN turned on sometime in the past, and has produced the helical pattern of emission, possibly a sign of jet precession/merger during that episode of activity. Using LOFAR, we can trace the relic plasma from that episode of activity out to greater distances from the core than ever before. Using the the WSRT, we detect H I in absorption against the center of the radio source. The absorption profile is relatively broad (FWHM of 288 km s −1 ), similar to what is found in other clusters. The derived column density is N HI ∼ 4 × 10 20 cm −2 for a T spin = 100 K. This detection supports the connection -already suggested for other restarted radio sources -between the presence of cold gas and restarting activity. The cold gas appears to be dominated by a blue-shifted component although the broad H I profile could also include gas with different kinematics. Understanding the duty cycle of the radio emission as well as the triggering mechanism for starting (or restarting) the radio-loud activity can provide important constraints to quantify the impact of AGN feedback on galaxy evolution. The study of these mechanisms at low frequencies using morphological and spectral information promises to bring new important insights in this field.

Radio spectral properties and jet duty cycle in the restarted radio galaxy 3C388

“…To further investigate the spectral properties in different regions of the source we used the colour-colour diagram (α high versus α low , Katz-Stone et al 1993;Katz-Stone & Rudnick 1997;van Weeren et al 2012;Shulevski et al 2015). This plot is useful to inspect and compare the curvature of the radio spectrum in different regions or pixels of the source, independently of the assumption on magnetic field and on the presence of adiabatic compression or expansion.…”

“…More in general, the typical estimated inactive period between two phases of activity varies in the range of a few Myr to a few tens of Myr for restarted radio galaxies of various morphologies, including the well-known double-double radio galaxies (e.g. Schoenmakers et al 2000;Saikia et al 2006;Konar et al 2012;Shulevski et al 2015;Brienza et al 2018). Typically, the duration of this quiescent phase is shorter or at most comparable to the duration of the previous phase of activity, which is usually found to be in the range few tens to few hundreds of Myr.…”

“…Saripalli et al 2012;Jamrozy et al 2007;Shulevski et al 2012;Brienza et al 2018). Other restarted radio galaxies have individually been identified through their peculiar morphologies, for example 3C338 (Burns et al 1983), 4C 35.06 (Shulevski et al 2015), and 3C219 (Clarke et al 1992).…”

“…Major steps forward in the selection and study of restarted radio galaxies have recently been allowed by the advent of the LOw Frequency ARray (LOFAR,van Haarlem et al 2013) through its unprecedented sensitivity and resolution at low frequency, where the remnant lobes are expected to be brighter (see e.g. Shulevski et al 2015;Orrù et al 2015;Brienza et al 2018;Mahatma et al 2019;Jurlin et al 2020;Shabala et al 2020) However, there is one peculiar class of restarted radio galaxies that we may still be missing:. sources showing the footprint of their multi-epoch activity through spatial variations of their spectral index across their radio lobes.…”

Effect of richness on AGN and star formation activities in SDSS galaxy groups

“…As hypothesized by Davies et al (2014), the group environment is responsible for triggering gas inflow, and the environmental influence may last more than 1 Gyr. However, the resulting gas accretion on SMBHs occurs at some point during this phase, and the AGN phenomenon is on and off within a period of ∼ 100 Myr (Shulevski et al 2015). A random subset of active galaxies is observed in one snapshot, which might be different from another snapshot.…”