Constraints on the Nature of CID-42: Recoil Kick or Supermassive Black
  Hole Pair?
  The galaxy CXOC J100043.1+020637, also known as CID-42, is a highly unusual
object. An apparent galaxy merger remnant, it displays signatures of both an
inspiraling, kiloparsec-scale active galactic nucleus (AGN) pair and of a
recoiling AGN with a kick velocity > 1300 km s^-1. Among recoiling AGN
candidates, CID-42 alone has both spatial offsets (in optical and X-ray bands)
and spectroscopic offsets. In order to constrain the relative likelihood of
both scenarios, we develop models using hydrodynamic galaxy merger simulations
coupled with radiative transfer calculations. Our gas-rich, major merger models
are generally well matched to the galactic morphology and to the inferred
stellar mass and star formation rate. We show that a recoiling supermassive
black hole (SMBH) in CID-42 should be observable as an AGN at the time of
observation. However, in order for the recoiling AGN to produce narrow-line
emission, it must be observed shortly after the kick while it still inhabits a
dense gaseous region, implying a large total kick velocity (v_k > 2000 km
s^-1). For the dual AGN scenario, an unusually large broad-line offset is
required, and the best match to the observed morphology requires a galaxy that
is less luminous than CID-42. Further, the lack of X-ray emission from one of
the two optical nuclei is not easily attributed to an intrinsically quiescent
SMBH or to a Compton-thick galactic environment. While the current data do not
allow either the recoiling or the dual AGN scenario for CID-42 to be excluded,
our models highlight the most relevant parameters for distinguishing these
possibilities with future observations. In particular, high-quality,
spatially-resolved spectra that can pinpoint the origin of the broad and narrow
line features will be critical for determining the nature of this unique
source.
