Probing the Origins of X-Shaped Radio Galaxies

David H. Roberts (1), Jake Cohen (1), Kevin Wang (1), Carly KleinStern (1), Christopher Morii-Sciolla (1), Lakshmi Saripalli (2), Mayuri SRao (2), Ravi Subrahmanyan (2), & Sanjay Bhatnagar (3)

(1) Brandeis University, (2) Raman Research Instutite, (3) NRAO

Introduction

Background: The X-shaped radio galaxies ("XRGs") are active galaxies that exhibit unusual extended radio structures, often in the shape of an "X" or a "Z." They are of interest because some of the possible mechanisms for their formation involve the presence of a pair of supermassive black holes ("SMBHs") at their centers. Binary SMBHs are thought to be the primary source of the as-yet-undetected low-frequency gravitational wave background. With the recent detection by Advanced LIGO of two discrete gravitational wave signals this has taken on increased importance.

Project: We are studying a sample of 100 XRG candidates assembled by Teddy Cheung (US Naval Research Laboratory) from the NRAO FIRST Survey (Cheung 2007, "FIRST "Winged" and X-Shaped Radio Source Candidates," Astron. J., 133, 2097).

Goals: The expansion in VLA capabilities, the recent creation of a large low axial-ratio sample by Cheung, and in particular the current scientific interest in predictions for GW detectors and separately the strides made in BH spin evolution models all make it timely to undertake a population study of sources showing signs of BH axis changes. The VLA observations of a large and complete sample of 99 low-axial ratio radio galaxies are designed to conduct three key studies (1) abundance rate estimation for different BH axis behaviors over time, with implications for GW astrophysics, BH growth, and BH evolution, (2) resolve the origin of XRGs (whether due to axis flips or drits, or due to backflow deflection caused by host thermal halos), and (3) the 3D structure of thermal halos of elliptical hosts of radio galaxies. As far as we know, this is the first time that such a population study has been proposed of the relative occurrence rates of the variety of off-axis distortions, and using them to derive BH coalescence phenomenology in the parent population of luminous ellipticals. For XRGs, trends in polarization and spectral index (SI) distributions are not established: the only program to date that examined these trends involved only 11 XRGs, and was inconclusive.

Collaboration: This project is a joint venture of the Brandeis Radio Astronomy Group, the Raman Research Institute, Bangalore, India, and the National Radio Astronomy Observatory.

The VLA Observations: Our sample is currently being observed at 1.4 and 3 GHz in the A, B, and C configurations of the Jansky Very Large Array. The observations to date are in the Log of Observations.

The VLBI Observations: Nine of the targets were observed with 13 antennas of the the EVN on November 1, 2015. The nine were selected from the subsample of 11 that were judged from their kiloparsec-scale structure to have the possibility of an independent transverse feature centered on the host (Roberts et al. 2015a). The goals of the observations were to (1) determine the current directions of the parsec-scale jet(s) and (2) search for double nuclei. We detected six of the sources, and found jets in two. Each of the parsec-scale jets points in the direction of the apparently-active kiloparsec-scale jet. Only one of the three sources without an obvious kiloparsec-scale jet was detected on the parsec scale.

The next VLBI targets will be "axis-shift" and "axis drift" candidates, sources that show distortions in their extended radio structures that are connected to the far ends of the sources.

Publications:

1. Roberts, Cohen, Lu, Saripalli, & Subrahmanyan 2015 Abstract: 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. Here, we present radio images of 52 of these sources that have been made from archival Very Large Array data with resolution of about 1″. Fifty-one of the 52 were observed at 1.4 GHz, 7 were observed at 1.4 and 5 GHz, and 1 was observed only at 5 GHz. We also present overlays of the Sloan Digital Sky Survey red images for 48 of the sources, and DSS II overlays for the remainder. Optical counterparts have been identified for most sources, but there remain a few empty fields. 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. The implications of this result for the gravitational wave background are discussed in Roberts et al.
2. Roberts, Saripalli, & Subrahmanyan 2015 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. In Roberts et al. we presented radio images of all 52 X-shaped radio source candidates out of the sample of 100 selected by Cheung for which archival VLA data were available. These images indicate that at most 21% 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. This suggests that fewer than 1.3% of extended radio sources appear to be candidates for genuine axis reorientations (“spin flips”), 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.

Scripted Polarization Calibration in CASA

XRG Images by Right Ascension.

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