Accessing a New Population of Supermassive Black Holes with Extensions to the Event Horizon Telescope

TL;DR

This paper identifies 12 promising supermassive black hole candidates beyond Sgr A* and M87*, predicts their observable properties using models, and discusses how future EHT upgrades can enable studies of a larger black hole population.

Contribution

The paper provides theoretical predictions for new black hole targets and assesses how upcoming EHT enhancements can facilitate their imaging and analysis.

Findings

Predicted larger optical and Faraday depths for new sources.

Visibility size constraints can estimate masses within a factor of 2.

Polarization structure varies with Eddington ratio and inclination.

Abstract

The Event Horizon Telescope has produced resolved images of the supermassive black holes Sgr A* and M87*, which present the largest shadows on the sky. In the next decade, technological improvements and extensions to the array will enable access to a greater number of sources, unlocking studies of a larger population of supermassive black holes through direct imaging. In this paper, we identify 12 of the most promising sources beyond Sgr A* and M87* based on their angular size and millimeter flux density. For each of these sources, we make theoretical predictions for their observable properties by ray tracing general relativistic magnetohydrodynamic models appropriately scaled to each target's mass, distance, and flux density. We predict that these sources would have somewhat higher Eddington ratios than M87*, which may result in larger optical and Faraday depths than previous EHT targets. Despite this, we find that visibility amplitude size constraints can plausibly recover masses within a factor of 2, although the unknown jet contribution remains a significant uncertainty. We find that the linearly polarized structure evolves substantially with Eddington ratio, with greater evolution at larger inclinations, complicating potential spin inferences for inclined sources. We discuss the importance of 345 GHz observations, milli-Jansky baseline sensitivity, and independent inclination constraints for future observations with upgrades to the Event Horizon Telescope (EHT) through ground updates with the next-generation EHT (ngEHT) program and extensions to space through the Black Hole Explorer (BHEX).