Supermassive black hole growth from stellar binary encounters

TL;DR

The paper develops a kinematic Hills mechanism framework to quantify SMBH growth from binary star break-ups, linking encounter rates to central density, velocity dispersion, binary fraction, and SMBH mass. It presents a simple and a loss-cone–aware modified model, validates them against tidal disruption event theory, and applies them to 91 galaxies (emphasizing 30 well-measured cases) and to the LMC*, finding that stellar accretion from binary disruptions can substantially contribute to SMBH growth in many environments. The work highlights that in several galaxies, especially with high central densities, the Hills channel could meaningfully add to mass growth, while suggesting that the modified model better captures realistic rates for massive SMBHs. It also provides observationally testable predictions through hypervelocity star populations and forthcoming TDE datasets, offering a framework to assess non-gaseous growth channels for SMBHs across the local universe.

Abstract

The growth of supermassive black holes (SMBHs) remains a central problem in astrophysics, with current observations providing only limited constraints on the underlying mechanisms. One possible growth channel is stellar accretion via the Hills mechanism, wherein a SMBH tidally breaks up a passing binary star, capturing and eventually accreting a member of the binary. We adopt a framework based on kinematics to predict capture rates from parameters that include the central number density of stars, the stellar velocity dispersion, the binary fraction, and black hole mass. We then estimate the growth of SMBHs across a range of galactic environments. In a data set of 91 galaxies of various types and masses, we identify two candidates with SMBHs for which stellar accretion may be a driver of growth. Closer to home, a recent analysis of observed hypervelocity stars from the Large Magellanic Cloud (LMC) implicates binary star interactions with a massive black hole. Every hypervelocity star produced in this way leaves a bound partner that may be accreted, providing an active growth channel for the LMC's black hole.

Figures (5)

• Figure 1: The evolution of black hole mass from the Hills mechanism and stellar accretion. Each curve shows a unique starting mass, which we numerically evolve using the growth equation (Eq. (\ref{['eq:dmdt']})). We adopt the analytical relationship between black hole mass and central stellar density in Equation (\ref{['eq:massdens']}), and include the mass-dependence of other parameters based on the fiducial values in Equation (\ref{['eq:kfid']}). The analytical solutions (Eq. (\ref{['eq:msolve']}), dashed curves) are shown for comparison. Because the central density is near a maximum for $M_{\bullet} \approx 4\times 10^7$$\rm M_{\odot}$, the most rapid growth occurs for black holes around this size.
• Figure 2: Central density versus SMBH mass. The larger, violet points correspond to sources with more reliable black hole mass estimates, while the smaller, turquoise points represent galaxies with poorly constrained black hole masses. The gray line is a simple analytical model for the mass-density relation (Eq. (\ref{['eq:massdens']})). The low-mass end of this model is speculative. In all cases, the stellar densities are evaluated at 5 pc from the galactic centers.
• Figure 3: Black hole mass and central density evolution from the Hills mechanism growth channel predicted for the 30 best-measured galaxies with SMBHs in the hannah2024 sample. We begin with the observed masses and central densities, the use these values to predict future growth of the SMBH, as described in the main text (§\ref{['subsec:channel']}). The colors run from green hues (low-mass SMBHs) through reds (intermediate massed) to blue (high-mass SMBHs), and the same unique color is assigned to each galaxy. The thickness of the lines correlates with the relative increase in black hole mass over the 5 Gyr run. Clearly, growth favors the black hole in a high-density field of stars.
• Figure 4: Fraction of the black hole mass predicted to have come from stellar accretion through the Hills mechanism. The diamond symbols are the predictions for the 30 galaxies with high-quality SMBH masses in the hannah2025 data set, run backwards in time for 1 Gyr with our numerical integrator.
• Figure 5: Tidal disruption event rates from REPTiDEhannah2025 and the two kinematic models presented here, adapted to encounters between single stars and a SMBH. The black symbols are the predictions from REPTiDE, while the dark orange symbols are from our simple kinematic model. Our model overestimates the TDE rate at large SMBH mass, and underestimates it at low mass. The modified kinematic model predictions (light blue symbols) better track the REPTiDE estimates.