Toward Determining the Number of Observable Supermassive Black Hole Shadows
Dominic W. Pesce, Daniel C. M. Palumbo, Ramesh Narayan, Lindy Blackburn, Sheperd S. Doeleman, Michael D. Johnson, Chung‐Pei Ma, Neil M. Nagar, Priyamvada Natarajan, Angelo Ricarte
Abstract
Abstract We present estimates for the number of shadow-resolved supermassive black hole (SMBH) systems that can be detected using radio interferometers, as a function of angular resolution, flux density sensitivity, and observing frequency. Accounting for the distribution of SMBHs across mass, redshift, and accretion rate, we use a new semianalytic spectral energy distribution model to derive the number of SMBHs with detectable and optically thin horizon-scale emission. We demonstrate that (sub)millimeter interferometric observations with ∼0.1 μ as resolution and ∼1 μ Jy sensitivity could access >10 6 SMBH shadows. We then further decompose the shadow source counts into the number of black holes for which we could expect to observe the first- and second-order lensed photon rings. Accessing the bulk population of first-order photon rings requires ≲2 μ as resolution and ≲0.5 mJy sensitivity, whereas doing the same for second-order photon rings requires ≲0.1 μ as resolution and ≲5 μ Jy sensitivity. Our model predicts that with modest improvements to sensitivity, as many as ∼5 additional horizon-resolved sources should become accessible to the current Event Horizon Telescope (EHT), whereas a next-generation EHT observing at 345 GHz should have access to ∼3 times as many sources. More generally, our results can help guide enhancements of current arrays and specifications for future interferometric experiments that aim to spatially resolve a large population of SMBH shadows or higher-order photon rings.