Demographics of Wandering Black Holes Powering Off-Nuclear Tidal Disruption Events

TL;DR

This paper investigates the demographics of wandering massive black holes that power off-nuclear tidal disruption events in the local universe. The authors combine the ROMULUS cosmological simulation with empirical constraints on local galaxies to infer the volumetric density of wandering black holes as a function of host stellar mass. They find a peak at $log_{10}(M_*/M_\odot)=11.10^{+0.05}_{-0.10}$, with more than half of WBHs residing in galaxies in the range $[10.7,11.2]$ in log stellar mass, and they show that the observed association between detectable stellar counterparts and halo-centric location can be explained by tidal stripping. These findings corroborate the predicted WBH population and provide a predictive framework for interpreting future off-nuclear TDE discoveries.

Abstract

The recent discovery of three off-nuclear tidal disruption events (EP240222a, AT2024tvd, and AT2025abcr) - following the first such source, 3XMM J2150$-$05 - reveals a small but robust population of off-nuclear, or `wandering', black holes (WBHs) with masses $M_\bullet > 10^4 M_\odot$. Two demographic trends are already apparent: (i) all events occur in massive, early-type parent galaxies with stellar masses $10.8 \lesssim \log_{10}(M_\star/M_\odot) \lesssim 11.1$; and (ii) events at larger halo-centric radii ($R_{\rm TDE}/R_{200}$) are associated with dwarf satellites ($M_\star \sim 10^7 M_\odot$), while those closer to halo centers lack detected stellar counterparts. Using results from the \texttt{ROMULUS} cosmological simulation, we show that both trends naturally arise from hierarchical galaxy formation. By combining the simulation with empirical constraints on the local galaxy population, we compute the volumetric density of WBHs, $φ_{\rm WBH}(M_\star)$, finding that it peaks at $\log_{10}(M_\star/M_\odot)=11.10^{+0.05}_{-0.10}$ and that more than half of all WBHs in the local Universe reside in galaxies with $10.7 \lesssim \log_{10}(M_\star/M_\odot) \lesssim 11.2$, explaining (i) and predicting its persistence as the sample grows. We further show that ii), i.e., the observed link between detection of stellar counterparts and $R_{\rm TDE}/R_{200}$, is also expected from tidal stripping. These results demonstrate that off-nuclear TDEs are powered by the population of WBHs long predicted by cosmological simulations.

Figures (1)

• Figure 1: Pre-TDE optical imaging of the parent galaxies of the four known off-nuclear tidal disruption events. For each source, the panel shows a color composite of the host galaxy and a zoomed-in view centered on the TDE location. Purple circles mark the most precise positions of the transients, derived from space-based X-ray or ultraviolet observations. Pronounced stellar overdensities consistent with bound dwarf satellites are detected for 3XMM J2150$-$05 and EP240222a, whereas no resolved stellar counterpart is visible at the locations of AT2024tvd and AT2025abcr above the smooth halo light. The stellar counterpart overdensities (or lack thereof) are quantified in Appendix \ref{['app:3']}, and the results are shown in Table \ref{['tab:hosts']}.