Weighing black holes across the mass scale with HARMONI
Supermassive black holes (SMBHs) are found at the centres of most massive galaxies and play a fundamental role in galaxy evolution through accretion and feedback. Despite their importance, how black holes form, grow, and co-evolve with their host galaxies remains one of the central open questions in astrophysics.
HARMONI on the ELT will provide unprecedented access to black hole demographics, extending dynamical measurements to regimes that are currently inaccessible:
- the most massive black holes in the Universe, and
- the elusive population of intermediate-mass black holes (IMBHs) in nearby galaxies.
Dynamical black hole mass measurements
The most robust way to measure black hole masses is through dynamical methods, using the motions of stars or gas in the immediate vicinity of the black hole. These measurements require resolving the sphere of influence of the black hole, where its gravitational potential dominates over that of the host galaxy.
This is fundamentally an angular-resolution problem.
Thanks to, the 39-m aperture of the ELT, Adaptive Optics correction, and Integral-field spectroscopy, HARMONI will resolve black hole spheres of influence that are well below the reach of current 8–10 m telescopes.
Supermassive black holes in massive galaxies
The most massive galaxies host the most massive black holes, but these systems are:
- rare, and
- typically distant (redshifts z ≈ 0.05–0.3).
Even though their spheres of influence can reach ~100 parsecs, their angular sizes are only a few tens of milliarcseconds, requiring ELT-class resolution.
With HARMONI, it will be possible to:
- Dynamically measure black hole masses in ultra-massive galaxies
- Test scaling relations between black hole mass and:
- stellar velocity dispersion
- bulge mass
- Discriminate between competing black hole growth scenarios (merger-driven vs. potential-well-regulated growth)
Low-resolution near-infrared spectroscopy combined with spatial scales of 20–25 mas provides optimal sensitivity for these high-velocity-dispersion systems, enabling efficient surveys with modest exposure times.
Intermediate-mass black holes: the missing link
Intermediate-mass black holes (∼10²–10⁵ solar masses) are a critical, but poorly constrained, population. They are expected to reside in:
- dwarf galaxies
- nuclear star clusters
Their detection is essential to understand:
- the initial seeding mechanisms of black holes
- whether early black holes formed from stellar remnants, direct collapse, or dense star cluster processes
However, IMBHs have extremely small spheres of influence, often below 20 milliarcseconds even in the nearest galaxies. Detecting their dynamical signatures requires:
- the finest spatial sampling
- medium spectral resolution to detect subtle deviations in stellar velocity dispersions
HARMONI’s angular resolution makes these measurements feasible for the first time beyond the very Local Group, opening a new observational window on black hole formation.
Why HARMONI is unique
HARMONI combines key capabilities that are essential for black hole science:
- Integral-field spectroscopy, providing full 2D maps of stellar and gas kinematics
- High spatial resolution, critical for resolving black hole spheres of influence
- Flexible spatial scales, enabling both compact nuclear studies and efficient surveys
- Near-infrared coverage, ideal for penetrating dust and accessing key spectral features
Together, these capabilities allow HARMONI to deliver benchmark black hole mass measurements across a wide range of galaxy masses and environments.
A cornerstone science case for the ELT era
Black hole science is one of the flagship drivers for ELT-class facilities. With HARMONI, astronomers will be able to:
- Extend black hole scaling relations into unexplored regimes
- Constrain black hole seed formation models
- Connect black hole growth to galaxy assembly across cosmic time
HARMONI will thus play a central role in building a complete, physically grounded picture of black hole evolution in the Universe.