A Large Systematic Search for Close Supermassive Binary and Rapidly Recoiling Black Holes

Abstract

We have carried out a systematic search for close supermassive black hole binaries (with sub-parsec separations) among z < ∼ 0.7 quasars observed spectroscopically in the Sloan Digital Sky Survey. Such binaries are predicted by models of supermassive black hole and host galaxy co-evolution, therefore their census and population properties constitute an important test of these models. Our working hypothesis is that one of the two black holes accretes at a much higher rate than the other and carries with it the only broad-emission line region of the system, making the system analogous to a single-lined spectroscopic binary star. Accordingly, we used an automatic technique based on spectroscopic principal component analysis to search for broad Hβ emission lines that are displaced from the rest-frame of the quasar by |∆v| > ∼ 1, 000 km s −1 (corresponding to binary periods and separations of P ∼ few × 100 yr and a ∼ few × 0.1 pc, respectively, for masses ∼ 10 8 M ⊙ ). This method can also yield candidates for rapidly recoiling black holes since their spectroscopic signature is similar. Our search yielded 88 candidates, several of which were previously identified and discussed in the literature. The widths of the broad Hβ lines are typical among quasars but the shifts are extreme. We found a correlation between the peak offset and skewness of the broad Hβ profiles (there is an extended wing on the opposite side of the profile from the shifted peak), which suggests that the profiles we have selected share a common physical explanation. The general properties of the narrow emission lines are typical of quasars. We carried out followup spectroscopic observations of 68 objects to search for changes in the peak velocities of the Hβ lines (the time interval in the observer's frame between the original and new observations is 1-10 yr and 5.7-10.0 yr in 2/3 of the cases). We measured statistically significant changes (at 99% confidence) in 14 objects, with resulting accelerations between −120 and +120 km s −1 yr −1 . The above results, taken at face value, are broadly consistent with predictions for the number of close supermassive binaries in the Sloan Digital Sky Survey quasar sample. However, such a comparison is complicated by several theoretical and observational uncertainties, such as the fact that the observable we employ to select objects depends on a combination of several degenerate intrinsic parameters of a binary. We emphasize that interpretation of the offset broad emission lines as signatures of supermassive binaries is subject to many significant caveats. Many more followup observations over a long temporal baseline are needed to characterize the variability pattern of the broad lines and test that this pattern is indeed consistent with orbital motion. The possibility that some of the objects in this sample are rapidly recoiling black holes remains open as the available data do not provide strong constraints for this scenario.

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.…”

“…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.…”

The puzzling case of the radio-loud QSO 3C 186: a gravitational wave recoiling black hole in a young radio source?

“…In addition, a few objects that show velocity offsets, but for which evidence for spatial offsets is yet to be found, are also present (Eracleous et al 2012;Kim et al 2016). But the best GW recoiling BH candidates are those that show both of these properties (Blecha et al 2016).…”

A Practical Guide to the Massive Black Hole Cosmic History

“…The presence of dual quasars indicates that at least in some cases accretion is triggered well before merger, however, that might not be the general case. Finally, evidence for bound MBH binaries remains circumstantial [131][132][133][134][135], and most of the proposed candidates have different explanations (see Dotti et al contribution to this volume for a detailed discussion [113]). …”