Jet reorientation in active galactic nuclei: two winged radio galaxies

Abstract

Winged, or X‐shaped, radio sources form a small class of morphologically peculiar extragalactic sources. We present multifrequency radio observations of two such sources. We derive maximum ages since any re‐injection of fresh particles of 34 and 17 Myr for the wings of 3C 223.1 and 3C 403 respectively, based on the lack of synchrotron and inverse Compton losses. On morphological grounds we favour an explanation in terms of a fast realignment of the jet axis which occurred within a few Myr. There is no evidence for merger activity, and the host galaxies are found to reside in no more than poor cluster environments. A number of puzzling questions remain about those sources: in particular, although the black hole could realign on sufficiently short time‐scales, the origin of the realignment is unknown.

On the origin of X-shaped radio galaxies

“…Although a careful analysis of photometric and spectroscopic evidence led Dennett- Thorpe et al (2002) to conclude there is no evidence in the galaxy or its environs for a recent merger activity in either 3C 223.1 or 3C 403 (see, also, Landt et al 2010, for a similar assertion about a larger sample of XRGs), they point out that the backflow models are not consistent with the relative sizes and spectra of the wings in these sources. Hence Dennett-Thorpe et al suggest that the required rapid change in the jet axis within several Myr is either caused by a delayed SMBH merger following ingestion of a small galaxy by the radio galaxy, which left no obvious signature in the latter's stellar population, or, alternatively, the axis flip may have occurred due to accretion disk instabilities (e.g., Natarajan & Pringle 1998).…”

“…In this way, the strong tendency for the wings to align with the optical minor axis of the host elliptical can be readily understood. Although appealing for its simplicity, as noted by several authors (e.g., Dennett-Thorpe et al 2002), this hydrodynamic backflow model is severely challenged by the key observation that in several XRGs the wings are found to be distinctly more extended than the primary lobes, even though the latter are supposed to be advancing supersonically with respect to the external medium (whose ram pressure causes a bright hot spot marking the leading edge of the lobe), in contrast to the subsonically growing wings. Examples of such XRGs can be found, e.g., in Leahy & Parma (1992); SS09 and Sect.…”

“…A particularly detailed model argues that the central SMBH is realigned with the binary orbital plane during a process that involves a strong interaction between the binary and the accretion disk that twists and warps the disk which quickly realigns the spin axis of the SMBH ejecting the jets (Liu 2004). Alternatively, the jet realignment can be a more violent process involving the coalescence of the jetted SMBH with another SMBH in the throes of a galaxy merger (Rottmann 2001;Zier & Biermann 2001;Merritt & Ekers 2002;Dennett-Thorpe et al 2002;GBW03). This latter mechanism is particularly interesting since, it also results in a copious emission of gravitational waves, which for the lower mass range of SMBHs, lies in the Laser Interferometer Space Antenna (LISA) frequency band.…”

Wise J233237.05–505643.5: A Double-Peaked, Broad-Lined Active Galactic Nucleus With a Spiral-Shaped Radio Morphology

“…Usually one pair of more extended lobes with low surface brightness is oriented at a large angle with respect to a pair of less extended, high surface brightness lobes. Their radio morphology has been suggested as a sign of rapidly changing SMBH spin, possibly tracing a SMBH merging event (Dennett-Thorpe et al 2002;Merritt & Ekers 2002;Gopal-Krishna et al 2003). Other proposed explanations for XRGs include back-flowing material from jets deflected off the ISM, outflow from overpressured cocoons, or interactions of jets with stellar and intergalactic shells (Leahy & Williams 1984;Worrall et al 1995;Capetti et al 2002;Hodges-Kluck et al 2010).…”

Starbursts and black hole masses in X-shaped radio galaxies: Signatures of a merger event?

“…A recent merger of two supermassive black holes (SMBHs), where the wings constitute the relic emission of the past radio jets, is one of several scenarios proposed to explain this peculiar radio morphology (e.g., Rottmann 2001;Merritt & Ekers 2002;Komossa 2006;Gergely & Biermann 2009;Hodges-Kluck et al 2010b;Mezcua et al 2011). Alternative models employ jet-axis reorientation (e.g., Dennett-Thorpe et al 2002), two unresolved active galactic nuclei (AGN; Lal & Rao 2007), backflow of material from the main lobes into the wings (e.g., Leahy & Williams 1984;Capetti et al 2002;Hodges-Kluck et al 2010a;Landt et al 2010;Hodges-Kluck & Reynolds 2011), or non-ballistic precession assuming a pre-merger state of the two black holes (BHs; Gong 2008;Gong et al 2011). The existence of very few X-shaped sources with FRI edge-darkened radio lobes (Saripalli et al 2008;Saripalli & Subrahmanyan 2009) was initially used as a strong argument against the backflow scenario.…”