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

Intermediate-mass black holes (IMBHs; MBH≈103–5 M⊙) 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.’s 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 a 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 galaxy in the Fornax Cluster at 20 Mpc, which is also fainter than most of the 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 HSIM simulator and Jeans Anisotropic Models (JAMs). These mock HARMONI data cubes are analyzed as if they were real observations, employing JAMs 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 MBH≳105M⊙ out to distances of ∼20 Mpc.