Title:	X-Shaped Radio Galaxies and the Nanohertz gravitational wave background
Authors:	Roberts, David H Saripalli, Lakshmi Subrahmanyan, Ravi
Keywords:	supermassive back holes
Issue Date:	27-Oct-2016
Publisher:	Cambridge University Press for International Astronomical Union
Citation:	Proceedings of the IAU General Assembly, Astronomy in Focus meeting Volume 29B, held in 2015, Edited by Piero Benvenuti pp. 319-320, 2016
Abstract:	Coalescence of supermassive black holes (SMBHs) in galaxy mergers is potentially the dominant contributor to the low frequency gravitational wave background (GWB). It was proposed by Merritt & Ekers that X-shaped radio galaxies are signposts of such coalescences and that their abundance might be used to predict the magnitude of the GWB. Cheung identified a sample of 100 candidate X-shaped radio galaxies using the NRAO FIRST survey; these are small-axial-ratio extended radio sources with off-axis emission. In Roberts et al. we made radio images of 52 of these sources with resolution of about 1 arcsecond using archival Very Large Array data. Fifty-one of the 52 were observed at 1.4 GHz, seven were observed at 1.4 and 5 GHz, and one was observed only at 5 GHz. Our higher resolution VLA images along with FIRST survey images of the sources in the sample reveal that extended extragalactic radio sources with small axial ratios are largely (60%) cases of double radio sources with twin lobes that have off-axis extensions, usually with inversion-symmetric structure. The available radio images indicate that at most 20% of sources might be genuine X-shaped radio sources that could have formed by a restarting of beams in a new direction following an interruption and axis flip. The remaining 20% are in neither of these categories. These images indicate that at most a small fraction of the candidates might be genuine X-shaped radio sources that were formed by a restarting of beams in a new direction following a major merger, or by spin drift caused by BH-BH interaction. This suggests that fewer than 1.3% of extended radio sources appear to be candidates for genuine axis reorientations (“spin flips”), or 2.2% if possible “axis drift” sources are included, much smaller than the 7% suggested by Leahy & Parma. Thus, the associated GWB may be substantially smaller than previous estimates. These results can be used to normalize detailed calculations of the SMBH coalescence rate and the GWB.
Description:	Restricted Access.
URI:	http://hdl.handle.net/2289/6682
ISSN:	1743-9213
Alternative Location:	http://adsabs.harvard.edu/abs/2016IAUFM..29B.319R https://www.cambridge.org/core/journals/proceedings-of-the-international-astronomical-union/article/xshaped-radio-galaxies-and-the-nanohertz-gravitational-wave-background/E9FF32F04DFA5174A39712335434DDF2
Copyright:	2016 International Astronomical Union
Appears in Collections:	Research Papers (A&A)

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