On the origin of X-shaped radio-sources: New insights from the properties of their host galaxies

Abstract

Abstract. A significant fraction of extended radio sources presents a peculiar X-shaped radio morphology: in addition to the classical double lobed structure, radio emission is also observed along a second axis of symmetry in the form of diffuse wings or tails. We re-examine the origin of these extensions relating the radio morphology to the properties of their host galaxies. The orientation of the wings shows a striking connection with the structure of the host galaxy as they are preferentially aligned with its minor axis. Furthermore, wings are only observed in galaxies of high projected ellipticity. Hydrodynamical simulations of the radio-source evolution show that X-shaped radio-sources naturally form in this geometrical situation: as a jet propagates in a non-spherical gas distribution, the cocoon surrounding the radio-jets expands laterally at a high rate producing wings of radio emission, in a way that is reminiscent of the twin-exhaust model for radio-sources.

Origin of X-shaped radio-sources: further insights from the properties of their host galaxies

“…The same result has been obtained by Capetti et al (2002) from the study of a sample of nine XRSs selected from the literature. Thanks to the larger size of the sample considered here, the statistical significance is strongly improved.…”

“…Since the XRSs samples in these two studies do not have objects in common, the overall probability that XRS wings are randomly oriented with respect to their hosts is even lower than the ∼10 −3 probability quoted here. Capetti et al (2002) also find that wings form in galaxies with a larger ellipticity with respect to a reference sample of non-winged FR II radiogalaxies. We cannot confirm the overall validity of this finding since the 22 XRSs for which we can measure the optical axis are at a much larger distance than those studied by Capetti et al, with a median redshift of the two samples being 0.31 and 0.085, respectively.…”

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

“…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). These models have been tested in X-ray and optical spectroscopic morphological studies (e.g., Saripalli & Subrahmanyan 2009;Cheung et al 2009;Landt et al 2010;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.…”