Constraining the mass density of free-floating black holes using razor-thin lensing arcs

Abstract

Strong lensing of active galactic nuclei in the radio can result in razor-thin arcs, with a thickness of less than a milli-arcsecond, if observed at the resolution achievable with very long baseline interferometry (VLBI). Such razor-thin arcs provide a unique window on the coarseness of the matter distribution between source and observer. In this paper, we investigate to what extent such razor-thin arcs can constrain the number density and mass function of 'free-floating' black holes, defined as black holes that do not, or no longer, reside at the centre of a galaxy. These can be either primordial in origin or arise as by-products of the evolution of super-massive black holes in galactic nuclei. When sufficiently close to the line of sight, free-floating black holes cause kink-like distortions in the arcs, which are detectable by eye in the VLBI images as long as the black hole mass exceeds ∼ 1000 Solar masses. Using a crude estimate for the detectability of such distortions, we analytically compute constraints on the matter density of free-floating black holes resulting from null-detections of distortions along a realistic, fiducial arc, and find them to be comparable to those from quasar milli-lensing. We also use predictions from a large hydrodynamical simulation for the demographics of free-floating black holes that are not primordial in origin, and show that their predicted mass density is roughly four orders of magnitude below the constraints achievable with a single razor-thin arc.

Unveiling the Population of Wandering Black Holes via Electromagnetic Signatures

“…In this work, we present observational predictions for detecting this population. Although previous work with these simulations concluded that detecting wanderers using razor-thin lensing arcs is infeasible (Banik et al 2019), we find that wanderers in Romulus can manifest as hyperluminous X-ray sources (HLXs) (e.g., King & Dehnen 2005;Barrows et al 2019), spatially-offset AGN (e.g., Comerford et al 2009;Koss et al 2012;Mezcua & Domínguez Sánchez 2020;Reines et al 2020), and a faint halo of potentially detectable emission around a typical galaxy. If wanderers retain their stellar counterparts, they may also produce offcenter tidal disruption events (TDEs).…”

Origins and Demographics of Wandering Black Holes

“…Here, we explore the mass functions of wandering SMBHs in Figure 5. Banik et al (2019) found a Schechter-like wandering SMBH mass function in Romulus25, and here we search for trends in halo mass and redshift. We again plot only the accreted portion of a SMBH's mass, excluding the initial seed mass.…”

Gravitational lensing effects of supermassive black holes in cluster environments

“…In addition to the wandering population of SMBHs detected in the high-resolution simulated cluster Romulus-C that includes the dark matter and the baryon component (Tremmel et al 2019 ), dark matter only simulations also reveal the existence of massive structures within the cluster-scale haloes, that could also be interpreted as freefloating black holes Banik et al ( 2019 ). Two mechanisms could originate the wandering SMBH population.…”