A Chandra Study of the Radio Galaxy NGC 326: Wings, Outburst History, and AGN Feedback

📝 Abstract

NGC 326 is one of the most prominent X- or Z-shaped radio galaxies (XRGs/ZRGs) and has been the subject of several studies attempting to explain its morphology through either fluid motions or reorientation of the jet axis. We examine a 100 ks archival Chandra exposure and find several features associated with the radio galaxy: a high-temperature front that may indicate a shock, high-temperature knots around the rim of the radio emission, and a cavity associated with the eastern wing of the radio galaxy. A reasonable interpretation of these features in light of the radio data allows us to reconstruct the history of the AGN outbursts. The active outburst was likely once a powerful radio source which has since decayed, and circumstantial evidence favors reorientation as the means to produce the wings. Because of the obvious interaction between the radio galaxy and the ICM and the wide separation between the active lobes and wings, we conclude that XRGs are excellent sources in which to study AGN feedback in galaxy groups by measuring the heating rates associated with both active and passive heating mechanisms.

💡 Analysis

NGC 326 is one of the most prominent X- or Z-shaped radio galaxies (XRGs/ZRGs) and has been the subject of several studies attempting to explain its morphology through either fluid motions or reorientation of the jet axis. We examine a 100 ks archival Chandra exposure and find several features associated with the radio galaxy: a high-temperature front that may indicate a shock, high-temperature knots around the rim of the radio emission, and a cavity associated with the eastern wing of the radio galaxy. A reasonable interpretation of these features in light of the radio data allows us to reconstruct the history of the AGN outbursts. The active outburst was likely once a powerful radio source which has since decayed, and circumstantial evidence favors reorientation as the means to produce the wings. Because of the obvious interaction between the radio galaxy and the ICM and the wide separation between the active lobes and wings, we conclude that XRGs are excellent sources in which to study AGN feedback in galaxy groups by measuring the heating rates associated with both active and passive heating mechanisms.

📄 Content

arXiv:1112.2707v1 [astro-ph.HE] 12 Dec 2011 Accepted by ApJ Preprint typeset using LATEX style emulateapj v. 11/10/09 A CHANDRA STUDY OF THE RADIO GALAXY NGC 326: WINGS, OUTBURST HISTORY, AND AGN FEEDBACK Edmund J. Hodges-Kluck1 & Christopher S. Reynolds2,3 Accepted by ApJ ABSTRACT NGC 326 is one of the most prominent X- or Z-shaped radio galaxies (XRGs/ZRGs) and has been the subject of several studies attempting to explain its morphology through either fluid motions or reorientation of the jet axis. We examine a 100 ks Chandra X-ray Observatory exposure and find several features associated with the radio galaxy: a high-temperature front that may indicate a shock, high-temperature knots around the rim of the radio emission, and a cavity associated with the eastern wing of the radio galaxy. A reasonable interpretation of these features in light of the radio data allows us to reconstruct the history of the AGN outbursts. The active outburst was likely once a powerful radio source which has since decayed, and circumstantial evidence favors reorientation as the means to produce the wings. Because of the obvious interaction between the radio galaxy and the ICM and the wide separation between the active lobes and wings, we conclude that XRGs are excellent sources in which to study AGN feedback in galaxy groups by measuring the heating rates associated with both active and passive heating mechanisms. Subject headings: galaxies: active — galaxies: individual (NGC 326)

1. INTRODUCTION A small fraction of double-lobed extragalactic radio sources have long, centro-symmetric tails of radio emis- sion extending from the nucleus or primary lobes in a very different direction, thereby producing “X” or “Z”-shaped radio galaxies (XRGs/ZRGs). The tails, or “wings”, are faint, do not currently harbor a relativistic jet, and can be substantially longer (in projection) than the primary lobes. They have elicited considerable in- terest due to the suggestion by Merritt & Ekers (2002) that they result from a recent supermassive black hole (SMBH) merger that reoriented the jet, but their ori- gin is uncertain. It has also been suggested that the wings result from the deflection of lobe plasma by pres- sure fronts in the intergalactic or intracluster medium (IGM/ICM) (Leahy & Williams 1984; Worrall et al. 1995; Capetti et al. 2002; Gopal-Krishna et al. 2003; Zier 2005; Hodges-Kluck et al. 2010). Neither of these hy- potheses have yet been proven, even in a single source. B2 0055+26 is a dramatic XRG/ZRG, with wings longer (in projection) than the primary lobes (Figure 1). The wings are highly collimated over ∼100 kpc. Its host galaxy is the northern component of the dumbbell NGC 326 (Colla et al. 1975) and the entire system resides in the galaxy cluster Zw 0056.9+2636 (Zwicky & Kowal 1968). Hereafter, we adopt the convention in the litera- ture and refer to the whole system as NGC 326. The clus- ter atmosphere has been characterized by Worrall et al. (1995, hereafter, W95) on large scales using ROSAT, and has an average temperature of kT ∼2 keV. The cluster atmosphere is substantially asymmetric (Figure 2), sug- hodgeskl@umich.edu 1 Department of Astronomy, University of Michigan, Ann Ar- bor, MI, 48109 2 Department of Astronomy, University of Maryland, College Park, MD 20742 3 Joint Space Science Institute (JSI), University of Maryland, College Park, MD 20742 gesting that it is not in hydrostatic equilibrium and may be made up of two or more merging subclusters or that it is a composite of two or three poorer clusters seen in projection (W95). A detailed multifrequency study of the radio emission measured the spectral index α as a function of position in the lobes and wings, finding spec- tral steepening in the wings and away from the ends of the jets (Murgia et al. 2001, hereafter M01). Owing to its dramatic structure (Figure 1), NGC 326 is frequently used as the prototypical XRG/ZRG in for- mation models purporting to explain the X-shaped mor- phology. Ekers et al. (1978), who discovered the wings but not their true length, attributed them to ongoing regular precession of the relativistic jet inflating the pri- mary lobes. Later, Merritt & Ekers (2002) argued that the striking X-shaped structure necessitated rapid secu- lar precession. Noting that black holes are difficult to re- orient via external forces other than accretion disk insta- bilities, Merritt & Ekers (2002) suggested instead that the X-shaped structure is the result of the final coales- cence of a SMBH binary. Supposing that the components of the binary have misaligned angular momenta, the fi- nal spin axis (and jet axis) will be nearly instantaneously reoriented. Wirth et al. (1982) also argued in favor of a single rapid jet reorientation caused by a close encounter between the constituents of the dumbbell galaxy. Based on the large-scale, hot, asymmetric cluster at- mosphere, W95 found that the buoyancy of the lobe ma- terial could explain the wings provided that they wer

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