Constraints on Binary Black Holes
The formation of SMBH binaries following galaxy merging has been suggested as a natural consequence of galaxy formation for quite some time (Begelman, Blandford and Rees 1980). Among the possible observational indications that coalescence of binary SMBHs has occurred are the peculiar properties of some jets/lobes in radio galaxies (Merritt and Ekers 2002; Liu, Wu and Cao 2003). In a recent review of supermassive black hole (SMBH) studies, Ferrarese and Ford (2005) presented crucial areas of future research. Among these was the determination of the prevalence of SMBH binary systems. X-ray observations of SMBH binaries have already shown great success with in the nearby galaxy NGC 6240 two SMBHs about 1 kpc apart are detected (Komossa et al. 2003; see figure).
Left: Chandra X-ray image of two SMBH in the nucleus of the galaxy NGC 6240 (Komossa et al. 2003). Right: Chandra X-ray spectrum of the southern nucleus in NGC 6240 (the northern nucleus is very similar)
Constellation-X will search for the dual Fe-K lines which may betray the presence of the binary SMBH (see figure). The detection of such features will effectively mimic the resolving power of Chandra with high-resolution X-ray spectroscopy by spectrally resolving the narrow part of the iron line into two components in "closer" SMBH binaries. Interestingly, candidate binary black holes are currently being identified with optical spectroscopy in e.g., the DEEP2 survey (Gerke et al. 2007). By 2017, with ground-based observatories such as LSST operating, and a multitude of wider-field optical spectroscopic and X-ray surveys (e.g., XBOOTES; Murray et al. 2006), there should be more candidates for Constellation-X to follow-up.
For the case of NGC 6240, for instance, the velocity difference between the two cores is approx 20 km/s as the nuclei are separated by ~1 kpc and still very early in the merger process. However, for black holes farther along in their evolution towards merger (300 km/s will be easily detected), Constellation-X could well detect the two peaks in the (narrow) iron line.
