Detecting Intermediate-Mass Black Holes out to 20 Mpc with ELT/HARMONI: The Case of FCC 119

TL;DR

This work extends IMBH dynamical probing to the Virgo and Fornax clusters by constructing an 85-object NSC-bearing dwarf sample and performing end-to-end HARMONI/ELT simulations for FCC 119 at 20 Mpc. Using HSIM-generated mock datacubes and Jeans Anisotropic Modeling (JAM) within a Bayesian AdaMet framework, the authors demonstrate that IMBHs with $M_{ m BH}\gtrsim10^5~M_\odot$ can be dynamically detected and mass-measured with high precision when the NSC dominates the central potential and the sphere of influence is resolved by the 0.01″-scale IFU data. The study provides on-sky exposure benchmarks, shows a central kinematic signature contrast between BH-free and BH-present cases, and discusses mass-segregation effects and NSC brightness constraints that influence detectability. The results imply substantial potential for expanding the census of IMBHs in nearby dwarfs and for constraining IMBH formation channels, assembly histories, and occupation fractions with the ELT era. The methodology combines high-resolution imaging (HST), MGEs, MARCS/SPS spectra, JAM dynamics, and realistic HSIM data, establishing a practical framework for future IMBH surveys in local volume dwarfs.

Abstract

Intermediate-mass black holes (IMBHs; $M_{BH} \approx 10^{3-5} M_\odot$) play a critical role in understanding the formation of supermassive black holes in the early universe. In this study, we expand on Nguyen et al. simulated measurements of IMBH masses using stellar kinematics, which will be observed with the High Angular Resolution Monolithic Optical and Near-infrared Integral (HARMONI) field spectrograph on the Extremely Large Telescope (ELT) up to the distance of 20 Mpc. Our sample focuses on both the Virgo Cluster in the northern sky and the Fornax Cluster in the southern sky. We begin by identifying dwarf galaxies hosting nuclear star clusters, which are thought to be nurseries for IMBHs in the local universe. As a case study, we conduct simulations for FCC 119, the second faintest dwarf galaxies in the Fornax Cluster at 20 Mpc, which is also fainter than most of Virgo Cluster members. We use the galaxy's surface brightness profile from Hubble Space Telescope (HST) imaging, combined with an assumed synthetic spectrum, to create mock observations with the {\tt HSIM} simulator and Jeans Anisotropic Models (JAM). These mock HARMONI datacubes are analyzed as if they were real observations, employing JAM within a Bayesian framework to infer IMBH masses and their associated uncertainties. We find that ELT/HARMONI can detect the stellar kinematic signature of an IMBH and accurately measure its mass for $M_{BH} \gtrsim 10^5 M_\odot$ out to distances of $\sim$20 Mpc.

Figures (11)

• Figure S1: The distribution of our extended-HARMONI IMBH sample on distance vs. $K$s-band absolute magnitude comparing with the previous surveys: HARMONI MMBH Nguyen23, MASSIVE Ma14, ATLAS$^{\rm 3D}$Cappellari11 and HARMONI IMBH Nguyen2025b.
• Figure S2: Left: The $M_{\star}$--$M_{\rm NSC}$ relation shows the correlation between galaxy mass and NSC mass across various Hubble types Erwin12Georgiev16Spengler17Ordenes-Briceno18bSanchez-Janssen19. Right: The $M_{\star}$--$R_e$ relation illustrates the connection between galaxy mass and the NSC's effective radius Cote06Georgiev14Norris14Baldassare22. For comparison, we included the 10 Mpc HARMONI IMBH sample Nguyen2025b.
• Figure S3: Images from the HST, the Dark Energy Spectroscopic Instrument (DESI) Legacy Surveys DR10, The Two Micron All-Sky Survey (2MASS), and Sloan Digital Sky Survey (SDSS) of our extended HARMONI IMBH sample. Two red circles indicate radii of 12$^{\prime\prime}$ (inner) and 60$^{\prime\prime}$ (outer) from the galaxy center, marking the optimal region for selecting NGSs for AO correction. These stars are indicated by white crosses, with their respective $H$-band magnitudes estimated from Gaia.
• Figure S4: Left: The HST/ACS WFC F850LP image of FCC 119 ($15\hbox{$^{\prime\prime}$}\times15\hbox{$^{\prime\prime}$}$ or $1.5\times1.5$ kpc$^2$) shows its large-scale morphology. Right: A zoom-in to the central region ($5\hbox{$^{\prime\prime}$} \times 5\hbox{$^{\prime\prime}$}$) highlights its NSC.
• Figure S5: Top: The HST ACS/WFC F850LP radial SB profile of FCC 119 (blue dots) is shown simultaneously with the best-fit model (purple-thick line), which is a combination of a core-Sérsic (red-dotted line) and a Sérsic (red-dashed line) function, extrapolated into both the unresolved region of $0\hbox{$.\!\!^{\prime\prime}$}004$ and the extended region beyond 20$\hbox{$^{\prime\prime}$}$. The best-fit parameters of the best-fit model are shown in the legend. Bottom: The residual ( data-model) between the SB and the best-fit combined function.