[Paper Review] Astro2020 Science White Paper: The Local Relics of of Supermassive Black Hole Seeds

Jenny E. Greene, Aaron J. Barth+19 more|arXiv (Cornell University)|Mar 20, 2019

Adaptive optics and wavefront sensing4 citations

TL;DR

This white paper advocates for using 30-meter class extremely large telescopes (ELTs) to dynamically detect intermediate-mass black holes (IMBHs) in the 10³–10⁵ M☉ range through high-precision astrometry and integral-field spectroscopy in nearby galaxies and star clusters. ELTs will enable the first volume-limited census of IMBHs, distinguishing between heavy and light seed formation models and resolving the missing link in black hole mass evolution.

ABSTRACT

We have compelling evidence for stellar-mass black holes (BHs) of ~5-80 M_sun that form through the death of massive stars. We also have compelling evidence for so-called supermassive BHs (10^5-10^10 M_sun) that are predominantly found in the centers of galaxies. We have very good reason to believe there must be BHs with masses in the gap between these ranges: the first ~10^9 M_sun BHs are observed only hundreds of millions of years after the Big Bang, and all theoretically viable paths to making supermassive BHs require a stage of "intermediate" mass. However, no BHs have yet been reliably detected in the 100-10}^5 M_sun mass range. Uncovering these intermediate-mass BHs of 10^3-10^5 M_sun is within reach in the coming decade. In this white paper we highlight the crucial role that 30-m class telescopes will play in dynamically detecting intermediate-mass black holes, should they exist.

Motivation & Objective

• To close the observational gap in the black hole mass function between stellar-mass and supermassive black holes by detecting intermediate-mass black holes (IMBHs) in the 10³–10⁵ M☉ range.
• To determine the occupation fraction and mass distribution of IMBHs in low-mass galaxies, globular clusters, and galactic nuclei using next-generation telescopes.
• To test competing theoretical models of black hole seed formation—particularly heavy seeds via direct gas collapse or dynamical runaway—by measuring the observed IMBH mass function.
• To enable synergistic constraints with multi-messenger astronomy, including gravitational wave searches (e.g., LISA) and tidal disruption event surveys (e.g., LSST), by providing dynamical mass measurements.
• To develop open-source software tools for dynamical modeling of IMBH data to ensure scientific accessibility and reproducibility.

Proposed method

• Utilize 30-meter class ELTs with adaptive optics to achieve diffraction-limited imaging at angular resolutions below 10 mas (≈4 mas/pixel) for high-precision astrometry of stellar motions in globular clusters and galactic nuclei.
• Perform multi-epoch astrometric monitoring with 3–4 years baseline and 12 exposures per epoch to measure proper motions with errors ≤150 m s⁻¹, enabling 3σ detection of 10³ M☉ IMBHs via velocity dispersion enhancement.
• Conduct integral-field spectroscopy at R ≥ 8000 (up to 10,000) to map kinematics in the near-infrared (CO bandhead at 2.29 μm, Ca II triplet at 8500 Å), resolving the dynamical sphere of influence of IMBHs in low-mass systems.
• Target nearby galaxies within 5 Mpc, including Milky Way and M31 globular clusters, Local Group dwarfs (e.g., NGC 205, NGC 185), and low-mass spiral galaxies, using both northern and southern hemisphere ELTs.
• Apply high-resolution kinematic modeling to infer black hole masses from stellar velocity dispersion and line-of-sight velocity profiles in nuclear star clusters.
• Develop and support open, shared-use software for dynamical modeling of IMBH data to ensure reproducibility and community-wide access.

Experimental results

Research questions

• RQ1 What is the true occupation fraction of intermediate-mass black holes (IMBHs) in low-mass galaxies and globular clusters within the Local Volume?
• RQ2 Can 30-meter class telescopes detect IMBHs in the 10³–10⁵ M☉ range through high-precision astrometry and spectroscopy?
• RQ3 How do the observed mass functions and spatial distributions of IMBHs distinguish between heavy-seed (e.g., direct collapse) and light-seed (e.g., stellar remnants) formation mechanisms?
• RQ4 To what extent do dynamical measurements of IMBHs in nearby systems constrain the merger rate and detectability of IMBHs by space-based gravitational wave detectors like LISA?
• RQ5 What is the role of gravitational wave recoil and environmental effects in removing IMBHs from clusters, and how can ELT observations help test these predictions?

Key findings

• ELTs will achieve angular resolution below 10 mas, enabling detection of the dynamical influence radius of 10⁴–10⁵ M☉ black holes in galaxies up to 5 Mpc away.
• A 3σ detection of a 10³ M☉ IMBH in a globular cluster requires proper motion errors ≤150 m s⁻¹, achievable with 3 epochs over 4 years and 12 exposures per epoch.
• Spectral resolving power R ≥ 8000 is required to measure velocity dispersions down to 15–20 km s⁻¹ in nuclear star clusters, enabling detection of IMBHs as low as ~10⁴ M☉.
• The dynamical sphere of influence of IMBHs in globular clusters is ~1–2 arcsec, well within the field of view of current AO imagers (0.5–1 arcmin), making them accessible targets.
• High-precision astrometry with 30-m ELTs will improve detection limits by nearly an order of magnitude compared to current 8–10 m telescopes.
• The combination of ELT astrometry and spectroscopy will provide the first volume-limited census of IMBHs, enabling direct comparison with theoretical seeding models and constraints on early black hole growth.