Direct Gravitational Imaging of Intermediate Mass Black Holes in Extragalactic Halos

TL;DR

This paper proposes a method to directly detect intermediate mass black holes in galaxy halos through gravitational lensing observations in the submillimeter band, supported by simulations of black hole distributions.

Contribution

It introduces a new observational approach using high-resolution submillimeter imaging to detect IMBHs in extragalactic halos, supported by dynamical modeling.

Findings

IMBHs follow a power-law distribution in galaxy halos.

Next-generation telescopes can map off-nuclear IMBHs of ~10^6 solar masses.

Most IMBHs near the center are stripped from their hosts.

Abstract

A galaxy halo may contain a large number of intermediate mass black holes (IMBHs) with masses in the range of 10^{2-6} solar mass. We propose to directly detect these IMBHs by observing multiply imaged QSO-galaxy or galaxy-galaxy strong lens systems in the submillimeter bands with high angular resolution. The silhouette of an IMBH in the lensing galaxy halo would appear as either a monopole-like or a dipole-like variation at the scale of the Einstein radius against the Einstein ring of the dust-emitting region surrounding the QSO. We use a particle tagging technique to dynamically populate a Milky Way-sized dark matter halo with black holes, and show that the surface mass density and number density of IMBHs have power-law dependences on the distance from the center of the host halo if smoothed on a scale of ~ 1 kpc. Most of the black holes orbiting close to the center are freely roaming as they have lost their dark matter hosts during infall due to tidal stripping. Next generation submillimeter telescopes with high angular resolution (< 0.3 mas) will be capable of directly mapping such off-nuclear freely roaming IMBHs with a mass of ~ 10^6 solar mass in a lensing galaxy that harbours a O(10^9) solar mass supermassive black hole in its nucleus.