Where to search for supermassive binary black holes

Paola Marziani, Edi Bon, Natasa Bon, Mauro D'Onofrio

First listed 2025-02-18 | Last updated 2025-02-18

Abstract

Supermassive binary black holes (SMBBHs) are the anticipated byproducts of galaxy mergers and play a pivotal role in shaping galaxy evolution, gravitational wave emissions, and accretion physics. Despite their theoretical prevalence, direct observational evidence for SMBBHs remains elusive, with only a handful of candidates identified to date. This paper explores optimal strategies and key environments for locating SMBBHs, focusing on observational signatures in the broad Balmer lines. We present a preliminary analysis on a flux-limited sample of sources belonging to an evolved spectral type along the quasar main sequence, and we discuss the spectroscopic clues indicative of binary activity and highlight the critical role of time-domain spectroscopic surveys in uncovering periodic variability linked to binary systems.

Short digest

This note argues that the best hunting ground for supermassive binary black holes is a specific, evolved segment of the quasar main sequence—Population B/B1 sources with weak Fe II and broad Balmer lines—where BLR kinematics most clearly betray companions. On a flux-limited sample, the authors decompose Hβ and Mg II with broad/very-broad components and measure centroids at multiple fractional intensities, finding characteristic offsets and asymmetries suggestive of perturbed BLR dynamics. In the M•–L plane, these targets cluster at low Eddington ratios (Population B), reinforcing a focused search strategy. They advocate time-domain spectroscopy to confirm periodic profile changes, cautioning that photometric periodicities alone are vulnerable to red-noise false positives.

Key figures to inspect

• Figure 1: Use the optical-plane MS map (FWHM(Hβ) vs RFeII) to locate the B1 bin, note its prevalence and radio-loud fraction, and see why this corner is prioritized for SMBBH searches.
• Figure 2: Inspect the multi-component fits for Hβ and Mg II and compare their broad/very-broad components; weak Fe II and mismatched wings/centroids flag sources where a secondary compact object may perturb the BLR.
• Figure 3: Examine centroid distributions at multiple fractional intensities for Hβ and Mg II; check whether velocity offsets are consistent across lines and how radio-loud objects skew the distributions relative to the B1 reference range.
• Figure 4: Read the M•–L diagram to verify that the sample sits at low L/LEdd (Population B), clarifying the physical regime—longer dynamical times and stronger line-profile diagnostics—that motivates the targeted search.