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Abstract Post-merger galaxies are unique laboratories to study the triggering and interplay of star formation and active galactic nucleus (AGN) activity. Combining new, high-resolution Jansky Very Large Array (VLA) observations with archival radio surveys, we have examined the radio properties of 28 spheroidal post-merger galaxies. We detect 18 radio sources in our post-merger sample and find a general lack of extended emission at (sub)kiloparsec scales, indicating the prevalence of compact, nuclear radio emission in these post-merger galaxies, with the majority (16/18; 89%) characterized as low luminosity. Using multiwavelength data, we determine the origin of the radio emission, discovering 15 new radio AGNs and three radio sources likely associated with star-forming (SF) processes. Among the radio AGNs, almost all are low luminosity (13/15; 87%), inconsistent with a relativistic jet origin. We discover a new dual AGN (DAGN) candidate, J1511+0417, and investigate the radio properties of the DAGN candidate J0843+3549. Five of these radio AGNs are hosted by a SF or SF-AGN composite emission-line galaxy, suggesting that radio AGN activity may be present during periods of SF activity in post-mergers. The low-power jets and compact morphologies of these radio AGNs also point to a scenario in which AGN feedback may be efficient in this sample of post-mergers. Lastly, we present simulated, multifrequency observations of the 15 radio AGNs with the Very Long Baseline Array and the very-long-baseline interferometry capabilities of the Next-Generation VLA to assess the feasibility of these instruments in searches for supermassive black hole binaries. 

Abstract Mrk 1018 is a nearby changing-look active galactic nucleus (AGN) that has oscillated between spectral Type 1.9 and Type 1 over a period of 40 yr. Recently, a recoiling supermassive black hole (rSMBH) scenario has been proposed to explain the spectral and flux variability observed in this AGN. Detections of rSMBHs are important for understanding the processes by which SMBH binaries merge and how rSMBHs influence their galactic environment through feedback mechanisms. However, conclusive identification of any rSMBHs has remained elusive to date. In this paper, we present an analysis of 6.5 yr of multifrequency Very Long Baseline Array monitoring of Mrk 1018. We find that the radio emission is compact down to 2.4 pc, and it displays flux density and spectral variability over the length of our campaign, typical of a flat-spectrum radio core. We observe proper motion in RA of the radio core at −36.4 ± 8.6μas yr⁻¹(4.2σ), or 0.10c± 0.02cat the redshift of Mrk 1018. No significant proper motion is found in DEC (31.3 ± 25.1μas yr⁻¹). We discuss possible physical mechanisms driving the proper motion, including an rSMBH. We conclude that the apparent velocity we observe of the VLBI radio core is too high to reconcile with theoretical predictions of rSMBH velocities and that the proper motion is most likely dominated by an unresolved, outflowing jet component. Future observations may yet reveal the true nature of Mrk 1018. However, our observations are not able to confirm it as a true rSMBH. 

Abstract Pulsar timing array collaborations, such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), are seeking to detect nanohertz gravitational waves emitted by supermassive black hole binaries formed in the aftermath of galaxy mergers. We have searched for continuous waves from individual circular supermassive black hole binaries using NANOGrav’s recent 12.5 yr data set. We created new methods to accurately model the uncertainties on pulsar distances in our analysis, and we implemented new techniques to account for a common red-noise process in pulsar timing array data sets while searching for deterministic gravitational wave signals, including continuous waves. As we found no evidence for continuous waves in our data, we placed 95% upper limits on the strain amplitude of continuous waves emitted by these sources. At our most sensitive frequency of 7.65 nHz, we placed a sky-averaged limit ofh₀< (6.82 ± 0.35) × 10⁻¹⁵, andh₀< (2.66 ± 0.15) × 10⁻¹⁵in our most sensitive sky location. Finally, we placed a multimessenger limit of $\mathit{}<(1.41\pm 0.02)\times {10}^9{M}_{\odot }$on the chirp mass of the supermassive black hole binary candidate 3C 66B.