AGNs on the Move: A Search for Off-nuclear AGNs from Recoiling Supermassive Black Holes and Ongoing Galaxy Mergers with the Zwicky Transient Facility

Abstract

A supermassive black hole (SMBH) ejected from the potential well of its host galaxy via gravitational wave recoil carries important information about the mass ratio and spin alignment of the pre-merger SMBH binary. Such a recoiling SMBH may be detectable as an active galactic nucleus (AGN) broad-line region offset by up to 10 kpc from a disturbed host galaxy. We describe a novel methodology using forward modeling with The Tractor to search for such offset AGNs in a sample of 5493 optically variable AGNs detected with the Zwicky Transient Facility (ZTF). We present the discovery of nine AGNs that may be spatially offset from their host galaxies and are candidates for recoiling SMBHs. Five of these offset AGNs exhibit double-peaked broad Balmer lines, which may have arisen from unobscured accretion disk emission, and four show radio emission indicative of a relativistic jet. The fraction of double-peaked emitters in our spatially offset AGN sample is significantly larger than the 16% double-peaked emitter fraction observed for ZTF AGNs overall. In our sample of variable AGNs we also identified 52 merging galaxies, including a new spectroscopically confirmed dual AGN. Finally, we detected the dramatic rebrightening of SDSS 1133, a previously discovered variable object and recoiling SMBH candidate, in ZTF. The flare was accompanied by the reemergence of strong P Cygni line features, indicating that SDSS 1133 may be an outbursting luminous blue variable star.

Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster

“…Lacking an outward-pointing, comet-like appearance, we suspect that this active 15 star cluster is probably not an ejected nucleus from a gravitational wave recoil event (Blecha & Loeb 2008;Holley-Bockelmann et al 2008;Komossa & Merritt 2008;Askar et al 2021;Hogg et al 2021;Ward et al 2021). Although probably evident to many readers, we note that Nikhuli has a lopsided shape, unlike that of a (background) galaxy.…”

Variability-selected intermediate mass black hole candidates in dwarf galaxies from ZTF and WISE

“…It may also be that the use of the qsofit modeling technique based on the goodness of fit of a DPW to the light curve (Butler & Bloom 2011) used by (Baldassare et al 2020b) can allow for some supernova contamination. In Ward et al (2021) we found that qsofit could produce good DRW fits to supernovae light curves in ZTF and the σ QSO statistic needed comparison to a goodness of fit statistic from an SN model in order to separate SN and AGN populations.…”

“…Future work on these candidates will include forward modeling of ZTF and DECam images to fit for their positions relative to their host galaxies and determine the fraction which are wandering about their galaxy halo. The potential for forward modeling of ZTF images to determine the position of a variable point source relative to its host galaxy was demonstrated in Ward et al (2021) for recoiling SMBH candidates and will provide a way to confirm the positions of IMBH candidates without X-ray or radio detections. These candidates are just the tip of the iceberg in the search for optically variable AGN in dwarf galaxies, which will be greatly enhanced by the capabilities of the Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory (Ivezić et al 2019) over the next decade.…”

“…We produced a control sample of optically variable AGN with host galaxies of mass M * > 3 × 10 9 M from a parent sample of 5,493 variable ZTF AGN found by Ward et al (2021). This AGN sample was obtained from the ZTF alert stream (Patterson et al 2019) using the AMPEL alert broker AMPEL (Alert Management, Photometry and Evaluation of Lightcurves; Nordin et al 2019) to crossmatch ZTF transients to AGN catalogs using catsHTM (Soumagnac & Ofek 2018) and check for AGN-like variability with the Butler & Bloom (2011) quasar modeling routine.…”

“…In order to detect real variability amongst the sample of dwarf galaxy light curves we trialed a number of variability statistics including the Pearson correlation coefficient (as used by Secrest & Satyapal (2020)), the goodness of fit of the light curves to the quasar structure function (using qsofit (Butler & Bloom 2011), as used in Baldassare et al (2018Baldassare et al ( , 2020b; Ward et al (2021)), the excess variance (Sánchez et al 2017;Martínez-Palomera et al 2020) and the χ 2 /N in g and r bands. We found that a combination of the Pearson correlation coefficient and χ 2 produced the best separation between the AGN control sample and the majority of the non-variable dwarf galaxy population.…”