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X-Shaped Radio Galaxies and the Gravitational Wave Background

David Roberts, Jake Cohen, & Jing LuBrandeis University

Lakshmi Saripalli & Ravi SubrahmanyanRaman Research Institute

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• Radio Galaxies• X-Shaped Radio Galaxies• Cheung’s Sample• Classification of XRGs• Galaxy Merger Rate & the GWB• Future Work

Outline

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Sources of GWs

• These include close binary star systems, supernova explosions, coalescing binary systems of compact objects (WDs, NSs, and stellar-mass BHs), and binary SMBH systems at the centers of galaxies.

• Pulsar timing is sensitive in the nanohertz range and thus to SMBH coalescenses.

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SMBH Binaries

• We can calculate the frequency of the signal from Kepler’s third law,

• GWs begin to dominate binary evolution when a ~ 0.001 pc = 200 AU ~ size of the solar system.

• If we put in M = 109 and a = 200 AU, we get P = 33 days = 2.8x106 s, or a frequency of 360 nHz.

• This is the domain of Pulsar Timing Arrays.

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SMBH Binaries

• How many such SMBH binaries are there?• What is their rate of formation in galaxy

mergers?• Look where we know there are SMBHs…• …and where we can learn about their spin

orientations.

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Radio Galaxies

• Typically linear in structure, with two extended “lobes” and sometimes a “core” coincident with the optical nucleus of the galaxy.

• The lobes are fed by a pair of oppositely-directed jets.

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Radio Galaxies

• Radio sources are far larger than their hosts, often spanning 100,000 parsecs or more.

• Fanaroff-Riley Type I (“FR-I”) – brightest in the center, fade away in the lobes. Low luminosity.

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Typical FR-I (3C 130)

NRAO Image Archive

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Radio Galaxies

• Fanaroff-Riley Type II (“FR-II”) – brightest in “hot spots” at the outside edges of the lobes. High luminosity.

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Typical FR-II (Cygnus A)

NRAO Image Archive

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Radio Galaxies

• The lobes in FR-IIs are formed by backflow from the interaction of the jets with the IGM.

• At the heart of each RG lies (at least) one Supermassive Black Hole.

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High Axial Ratios

• Typically sources are much “longer” than they are “wide,” that is, they have high axial ratios (major to minor axis).

• This is due to the pair of oppositely-directed jets with an axis orientation determined by the SMBH spin direction.

• There is, however, a small class of sources that look very different…

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X-Shaped Radio Galaxies (“XRGs”)

• A few sources have low axial ratios.• They can have two sets of lobes instead of

one.• They often have a high degree of inversion

symmetry.

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“Classical” XRG (NGC 326)

NRAO Image Archive

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“Double AGN” (3C 75)

NRAO Image Archive

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What Can We Learn from XRGs?

• Galaxies grow by mergers, and if XRGs represent the results of mergers we can determine the merger rate from the fraction of RG’s that are XRGs.

• This requires knowing the “lifetime” of an XRG.• We assume that the same statistics will apply to

the parent population of RGs, massive ellipticals.• This will lead to an estimate of the galaxy

merger rate (Merritt & Ekers 2002).

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What Makes XRG’s?

• 1) Backflow from lobes deflected by galaxy halo.

This has nothing to do with galaxy mergers.

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What Makes XRG’s?

• 2) Flip or drift of the BH axis due to interactions with a second BH or the accretion disk in the nucleus following a galaxy merger.

• Jets and lobes along original axis become a fading relic.

• New active axis generates a young radio source.

• Distinguishable by spectral aging or by VLBI.

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What Makes XRG’s?

• 3) Two active BH systems (“double AGN”).• Detectable by VLBI if the BH-BH separation is a

few parsecs or more.• Optical spectroscopy could lead to a complete

description of the SMBBH.

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What Makes XRG’s?

• Only those sources with two SMBHs are potential sources of the nanohertz GWB.

• Thus we search radio galaxy samples for such cases.

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X-Shaped Radio Galaxies

• We are investigating a sample of 100 low axial ratio sources, searching for what we call “true XRGs,” those possessing two distinct sets of lobes connected to the core.

• Thus we are trying to sort the low axial ratio sources into “true” XRGs (double SMBH systems) and the rest (with structures determined by other physics, especially deflected backflow).

• This will lead to a limit on the rate at which SMBH pairs are coalescing.

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Cheung’s Sample

• Cheung (2007) used the NRAO FIRST survey (VLA, 20 cm, B array, 5” resolution) to find candidate XRGs by selecting low axial ratio sources.

• He examined 1648 sources and found 100 candidate XRG’s (6.1%).

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100 XRG Candidates

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Typical FIRST Candidates

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Our Program

• We used data from the VLA archive to image every source in Cheung’s list for which there were data, a total of 52 sources.

• L band (20 cm), A array, and C band (6 cm), B array, each @ 1” resolution.

• This was done using standard techniques in AIPS.• Images to appear in Ap. J. Supplements,

interpretation to appear in Ap. J. Letters (both available now available on astro-ph).

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Classifying Candidates

• We divided our 52 sources into three categories based on possible mechanisms of formation.

• Classification used both the FIRST images and ours.• (1) Sources with deviations from linearity that occur close

to the source center (probable deflected backflow). Here the two sets of lobes are connected to each other.

• (2) Sources with deviations from linearity that occur at the outer edges of the source (possible axis drifts). Again, the two sets of lobes are connected.

• (3) Others. These include all of the possible “true” XRGs.

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“Inner Bend” Sources

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“Outer Bend” Sources

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“Other” Sources

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The Numbers

• “Inner bend” sources were 25, or 48%.• “Outer bend” sources were 8, or 15%.• “Other” sources were the remaining 19, or

37%.• Of the latter, we judged 11 (21%) to be

candidates for “true” XRG structure, that is, two independent pairs of lobes each connected to the core.

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“True” XRG Candidates

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“True” XRG Candidates

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“True” XRG Candidates

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The Numbers

• Combining Cheung’s and our detection frequencies, the fraction of extended radio sources that might be “true” XRGs is no greater than 1.3%. Adding “outer bend” sources takes it to 2.2%.

• The oft-quoted number in the literature (Leahy & Parma 1992) is 7%. This is compatible with Cheung’s numbers of low axial ratio sources without further classification (6%).

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The Numbers

• The lifetime of the “relic” lobes determines how long an axis-flip XRG will continue to have that structure.

• Merritt & Ekers estimated this from spectral steepening arguments to be about 108 yr.

• This leads to a merger rate for massive ellipticals of no more than 0.13 Gyr-1 galaxy-1 , or 0.22 Gyr-1 galaxy-1 with axis drift candidates included.

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The Bottom Line

• Our merger rate is compatible with estimates made by other techniques.

• Our numbers lead to a predicted GWB three to five times smaller than the previous estimate based on counting XRGs.

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Prospectus

• We have proposed a large JVLA project to observe all of Cheung’s candidates at 13 and 20 cm using the A, B, and C arrays (52 hrs total, resulting in ~0.6 TB of data).

• Using polarimetry we will employ the Laing-Garrington effect to get three-dimensional information on the orientations of the lobes.

• Superb frequency coverage (1 – 4 GHz) will enable detailed spectral index imaging of every system, and thus the histories of the lobes.

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Goals of the JVLA Observations

• First, to count “true” XRGs and estimate the galaxy merger rate more securely.

• Second, to study the 3D geometries and histories of the lobes.

• Third, to study the halos of galaxies using those sources that are convincingly created by backflow.

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Additional Observations

• Deep optical imaging of every source to delineate the visible halo structure.

• Optical spectroscopy to search for evidence of double AGN (emission line splitting).

• VLBA imaging of those with detectable VLA cores to examine parsec-scale jet structure and orientation and to search for double SMBH systems with parsec-scale separations.

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SUMMARY

• Radio galaxies provide valuable information on the spins of the SMBHs at their centers.

• Some X-shaped RG’s show evidence of spin-flips or drifts indicating the presence of a second SMBH.

• These data can be used to constrain the expected nanohertz gravitational wave background.

• Detailed examination of XRG candidates shows that the number of spin-flip sources is a factor of 3-5 times smaller than previously believed.

• Thus the nanohertz gravitational wave background may be smaller than previously expected.

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Acknowledgements

• The William R. Kenan, Jr. Charitable Trust for partial funding of my undergraduate students, and for providing computer resources.

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Questions and comments, please.

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