Abstract
Double-peaked broad emission lines in Active Galactic Nuclei (AGNs) may indicate the existence of a bound supermassive black hole (SMBH) binary where two distinct broad line regions (BLRs) contribute together to the line profile. An alternative interpretation is a disk emitter origin for the double-peaked line profile. Using simple BLR models, we calculate the expected broad line profile for a SMBH binary at different separations. Under reasonable assumptions that both BLRs are illuminated by the two active SMBHs and that the ionizing flux at the BLR location is roughly constant, we confirm the emergence of double-peaked features and radial velocity drifts of the two peaks due to the binary orbital motion. However, such a clear double-peaked feature only arises in a particular stage of the binary evolution when the two BHs are close enough such that the line-of-sight orbital velocity difference is larger than the FWHM of the individual broad components, while the two BLRs are still mostly distinct. Prior to this stage, the velocity splitting due to the orbit motion of the binary is too small to separate the emission from the two BLRs, leading to asymmetric broad line profiles in general. When the two BHs are even closer such that the two BLRs can no longer be distinct, the line profile becomes more complex and the splitting of the peaks does not correspond to the orbital motion of the binary. In this regime there are no coherent radial velocity drifts in the peaks with time. Asymmetric line profiles are probably a far more common signature of binary SMBHs than are double-peaked profiles. We discuss the temporal variations of the broad line profile for binary SMBHs and highlight the different behaviors of reverberation mapping in the binary and disk emitter cases, which may serve as a feasible tool to disentangle these two scenarios.
Highlights
Binary supermassive black holes (SMBHs) are generic outcomes of hierarchical galaxy mergers (e.g., Colpi & Dotti 2009, and references therein)
We consider the broad line profiles for bound binary SMBHs where the dynamics is dominated by the gravitational potential of the two BHs, assuming both BHs are active and have their own broad-line regions (BLRs)
The non-detection of coherent radial velocity drifts in some of the double-peaked broad line AGNs may rule out the existence of two distinct BLRs (e.g., Eracleous et al 1997; Gezari et al 2007), but cannot rule out the possibility of a close SMBH binary surrounded by a circumbinary BLR, as pointed out in Eracleous et al (1997)
Summary
Binary supermassive black holes (SMBHs) are generic outcomes of hierarchical galaxy mergers (e.g., Colpi & Dotti 2009, and references therein). Larger spectroscopic samples are starting to offer a unique opportunity to search for candidate binary SMBHs based on their spectral properties (e.g., Zhou et al 2004; Bonning et al 2007; Komossa et al 2008; Comerford et al 2009a; Boroson & Lauer 2009; Liu et al 2010b; Smith et al 2009; Shields et al 2009; Wang et al 2009; Xu & Komossa 2009) This is an important first step toward quantifying the frequency of binary SMBHs at various separations and providing constraints on hierarchical galaxy merger models and predictions for future low-frequency gravitational wave experiments such as the Laser Interferometer Space Antenna (LISA). Stated, we will use Hβ as the fiducial emission line since this is the best-studied broad line in reverberation mapping studies from which characteristic BLR properties are best determined
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