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The galaxy CXOC J100043.1+020637, also known as CID-42, is a highly unusualobject. An apparent galaxy merger remnant, it displays signatures of both aninspiraling, kiloparsec-scale active galactic nucleus (AGN) pair and of arecoiling AGN with a kick velocity > 1300 km s^-1. Among recoiling AGNcandidates, CID-42 alone has both spatial offsets (in optical and X-ray bands)and spectroscopic offsets. In order to constrain the relative likelihood ofboth scenarios, we develop models using hydrodynamic galaxy merger simulationscoupled with radiative transfer calculations. Our gas-rich, major merger modelsare generally well matched to the galactic morphology and to the inferredstellar mass and star formation rate. We show that a recoiling supermassiveblack hole (SMBH) in CID-42 should be observable as an AGN at the time ofobservation. However, in order for the recoiling AGN to produce narrow-lineemission, it must be observed shortly after the kick while it still inhabits adense gaseous region, implying a large total kick velocity (v_k > 2000 kms^-1). For the dual AGN scenario, an unusually large broad-line offset isrequired, and the best match to the observed morphology requires a galaxy thatis less luminous than CID-42. Further, the lack of X-ray emission from one ofthe two optical nuclei is not easily attributed to an intrinsically quiescentSMBH or to a Compton-thick galactic environment. While the current data do notallow either the recoiling or the dual AGN scenario for CID-42 to be excluded,our models highlight the most relevant parameters for distinguishing thesepossibilities with future observations. In particular, high-quality,spatially-resolved spectra that can pinpoint the origin of the broad and narrowline features will be critical for determining the nature of this uniquesource.

Constraints on the Nature of CID-42: Recoil Kick or Supermassive Black Hole Pair?