Super-massive binary black holes and emission lines in active galactic nuclei

Abstract

It is now agreed that mergers play an essential role in the evolution of galaxies and therefore that mergers of supermassive black holes (SMBHs) must have been common. We see the consequences of past supermassive binary black holes (SMBs) in the light profiles of so-called 'core ellipticals' and a small number of SMBs have been detected. However, the evolution of SMBs is poorly understood. Theory predicts that SMBs should spend a substantial amount of time orbiting at velocities of a few thousand kilometers per second. If the SMBs are surrounded by gas observational effects might be expected from accretion onto one or both of the SMBHs. This could result in a binary Active Galactic Nucleus (AGN) system. Like a single AGN, such a system would emit a broad band electromagnetic spectrum and broad and narrow emission lines.The broad emission spectral lines emitted from AGNs are our main probe of the geometry and physics of the broad line region (BLR) close to the SMBH. There is a group of AGNs that emit very broad and complex line profiles, showing two displaced peaks, one blueshifted and one redshifted from the systemic velocity defined by the narrow lines, or a single such peak. It has been proposed that such line shapes could indicate an SMB system. We discuss here how the presence of an SMB will affect the BLRs of AGNs and what the observational consequences might be.We review previous claims of SMBs based on broad line profiles and find that they may have non-SMB explanations as a consequence of a complex BLR structure. Because of these effects it is very hard to put limits on the number of SMBs from broad line profiles. It is still possible, however, that unusual broad line profiles in combination with other observational effects (line ratios, quasi-periodical oscillations, spectropolarimetry, etc.) could be used for SMBs detection.Some narrow lines (e.g., [O III]) in some AGNs show a double-peaked profile. Such profiles can be caused by streams in the Narrow Line Region (NLR), but may also indicate the presence of a kilo-parsec scale mergers. A few objects indicated as doublepeaked narrow line emitters are confirmed as kpc-scale margers, but double-peaked narrow line profiles are mostly caused by the complex NLR geometry.We briefly discuss the expected line profile of broad Fe Kα that probably originated in the accretion disk(s) around SMBs. This line may also be very complex and indicate the complex disk geometry or/and an SMB presence.Finally we consider rare configurations where a SMB system might be gravitationally lensed by a foreground galaxy, and discuss the expected line profiles in these systems.

Evidence for Periodicity in 43 Year-Long Monitoring of NGC 5548

“…Further out, the circum-binary disk cools and may form a torus. Blending BLRs, have also been investigated (e.g., Shen & Loeb 2010;Popović 2012), or in case of a high-mass-ratio system, only one shifting BLR may be seen. A second black hole can give rise to a host of phenomena that can yield periodic signals (see Katz 1997;Sillanpaa et al 1988;Bogdanović et al 2008;Bon et al 2012;Kun et al 2014;Graham et al 2015a,b).…”

Spectroscopic Indication of a Centi-Parsec Supermassive Black Hole Binary in the Galactic Center of Ngc 5548

“…This motivates systematic searches of SMBHB candidates through periodic optical variability (e.g., Graham et al 2015a;Liu et al 2015). On the other hand, the motion of the BLR gas surrounding the black holes is influenced, if not governed, by the binary, and the broad emission lines should carry the information of the binary's orbital motion in their profiles (Eracleous et al 2012;Shen & Loeb 2010;Popović 2012;Liu et al 2016). Therefore, an SMBHB can naturally simultaneously explain all the periodicity in NGC5548, namely, the periodicity in the 5100 Å continuum, Hβ integrated fluxes, and Hβ profiles.…”

“…Below, we construct a simple toy model for a binary black hole pair, each having its own accretion disk and BLR, and use this model to decompose the observed profiles of the Hβ line (Popovic et al 2000;Popović 2012). …”

“…However, the identification of subparsec SMBHBs is particularly challenging because these small separations at cosmic distance are well below the angular resolving power of the current and even most future powerful telescopes. Searching for the spectroscopic signatures is certainly an ideal solution (e.g., Bon et al 2012;Eracleous et al 2012;Popović 2012;Shen et al 2013;Liu et al 2014;Yan et al 2015 and references therein), although robust, conclusive evidence for sub-parsec SMBHBs remains rare. If both components of the binary contain a broad-line region (BLR), monitoring secular changes in the intensity and velocity of the line-emitting clouds offers a promising avenue to identify gravitationally bound, sub-parsec SMBHB pairs in their last phase of dynamical evolution.…”

Wise J233237.05–505643.5: A Double-Peaked, Broad-Lined Active Galactic Nucleus With a Spiral-Shaped Radio Morphology

“…However, despite significant efforts searching for resolved binary AGNs (e.g., Komossa et al 2003;Rodriguez et al 2006;Civano et al 2010;Green et al 2010;Fu et al 2011aFu et al , 2012Comerford et al 2012;Barrows et al 2012) and late-stage (unresolved) merging SMBHs (Ju et al 2013;Shen et al 2013), such systems appear to be very rare. Two potential signatures of merging SMBHs are from their complicated radio morphology (see Baumgarte & Shapiro 2011) and unusual spectroscopic features such as velocity offsets between broad and narrow spectral line components (see the review by Popović 2012;Komossa 2012).…”