The Chandra View of Nearby X-shaped Radio Galaxies

📝 Abstract

We present new and archival Chandra X-ray Observatory observations of X-shaped radio galaxies within z < 0.1 alongside a comparison sample of normal double-lobed FR I and II radio galaxies. By fitting elliptical distributions to the observed diffuse hot X-ray emitting atmospheres, we find that the ellipticity and the position angle of the hot gas follows that of the stellar light distribution for radio galaxy hosts in general. Moreover, compared to the control sample, we find a strong tendency for X-shaped morphology to be associated with wings directed along the minor axis of the hot gas distribution. Taken at face value, this result favors the hydrodynamic backflow models for the formation of X-shaped radio galaxies which naturally explain the geometry; the merger-induced rapid reorientation models make no obvious prediction about orientation.

💡 Analysis

We present new and archival Chandra X-ray Observatory observations of X-shaped radio galaxies within z < 0.1 alongside a comparison sample of normal double-lobed FR I and II radio galaxies. By fitting elliptical distributions to the observed diffuse hot X-ray emitting atmospheres, we find that the ellipticity and the position angle of the hot gas follows that of the stellar light distribution for radio galaxy hosts in general. Moreover, compared to the control sample, we find a strong tendency for X-shaped morphology to be associated with wings directed along the minor axis of the hot gas distribution. Taken at face value, this result favors the hydrodynamic backflow models for the formation of X-shaped radio galaxies which naturally explain the geometry; the merger-induced rapid reorientation models make no obvious prediction about orientation.

📄 Content

arXiv:0912.3001v1 [astro-ph.HE] 15 Dec 2009 The Chandra View of Nearby X-shaped Radio Galaxies Edmund J. Hodges-Kluck1, Christopher S. Reynolds1, Chi C. Cheung2,3 & M. Coleman Miller1 ABSTRACT We present new and archival Chandra X-ray Observatory observations of X-shaped radio galaxies within z ∼0.1 alongside a comparison sample of normal double-lobed FR I and II radio galaxies. By fitting elliptical distributions to the observed diffuse hot X-ray emitting atmospheres (either the interstellar or intragroup medium), we find that the ellipticity and the position angle of the hot gas follows that of the stellar light distribution for radio galaxy hosts in general. Moreover, compared to the control sample, we find a strong tendency for X-shaped morphology to be associated with wings directed along the minor axis of the hot gas distribution. Taken at face value, this result favors the hydrodynamic backflow models for the formation of X-shaped radio galaxies which naturally explain the geometry; the merger-induced rapid reorientation models make no obvious prediction about orientation. Subject headings: galaxies: active, galaxies: intergalactic medium 1. Introduction “Winged” and X-shaped radio galaxies (XRGs) are centro-symmetric subclasses of Fanaroff- Riley (FR) type I and II radio galaxies (Fanaroff& Riley 1974) which exhibit a second, fainter pair of wings lacking terminal hot spots in addition to the symmetric double lobe structure seen in ordinary FR II galaxies (Leahy & Williams 1984). These galaxies comprise up to 10% of radio galaxies, although the galaxies in which the length of the wings exceeds 80% of that of the primary lobes (“classical” XRGs) are a very small subset of radio galaxies (Leahy & Parma 1992; Cheung 2007). The primary lobes of these galaxies, as in other radio galaxies, are produced by a jet emanating from an active galactic nucleus (AGN) which becomes decollimated as it propagates into and interacts with the surrounding medium. In FR II radio galaxies, the terminal shock where the jet rams into its environment is characterized by a radio hot spot (also often seen in the X-ray band, e.g. Hardcastle et al. 2004) which produces the so-called “edge-brightened” morphology. The absence of these hot spots in the fainter wings of XRGs implies that they do not harbor an active 1Department of Astronomy, University of Maryland, College Park, MD 20742-2421 2NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771 3Current address: National Research Council Research Associate, Space Science Division, Naval Research Labo- ratory, Washington, DC 20375 – 2 – jet, although Lal & Rao (2007) argue in favor of a dual-AGN origin in which the pairs of lobes emanate from separate, unresolved AGN. The X-shaped morphology is of interest because two remarkably disparate classes of models have been invoked to explain it. The first class is predicated on the reorientation of the jets either by realignment of the supermassive black hole (SMBH) spin or the accretion disk, whereas the second purports to explain the distorted morphology as the result of hydrodynamic interaction between the radio lobe and its surrounding gaseous environment on kiloparsec scales. In the first case, the most common explanation for the X-shaped morphology is that the SMBH has its spin axis realigned, either via merger or precession. The possibility that XRG morphology is produced by a SMBH merger is of considerable interest as a potential estimate of merger rates and hence source rates for the Laser Interferometer Space Antenna; because fossil lobes will quickly decay due to adiabatic expansion, an XRG would be a sign of a recent merger. In this scenario (e.g. Rottmann 2001; Zier & Biermann 2001; Merritt & Ekers 2002), a SMBH binary formed by galactic merger and dynamical friction hardens for an unknown length of time via three-body interactions until gravitational radiation becomes effective at radiating orbital energy. At this point, the binary quickly coalesces, and the radio jets realign along the direction of the angular momentum of the final merged black hole. The jet begins propagating in the new direction, leaving a decaying pair of radio lobes along the old spin axis. A significant objection to the merger model (Bogdanovi´c et al. 2007) is that the linear momentum “kick” imparted to the coalesced SMBH can exceed several times 103 km s−1 if the spins of the black holes are random at the time of merger, sufficient to escape the galaxy. Coaligned spins reduce the magnitude of the kick, but would not result in an X-shaped source. Although not a problem for rapid reorientation models which rely on precession or other realigning mechanisms (Ekers et al. 1978; Rees 1978; Klein et al. 1995; Dennett-Thorpe et al. 2002), an additional objection to the “fossil” lobe scenario is that the secondary lobe lengths in some XRGs are inconsistent with the fast lobe decay expected when a jet changes alignment (Saripalli & Subrahmanyan 2009, hereafter S

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