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While most galaxies appear to host a central supermassive black hole (SMBH),they are expected to also contain a substantial population of off-center"wandering" SMBHs naturally produced by the hierarchical merger-driven processof galaxy assembly. This population has been recently characterized in ananalysis of the Romulus cosmological simulations, which correct for thedynamical forces on SMBHs without artificially pinning them to halo centers.Here we predict an array of electromagnetic signatures for these wanderers. Thepredicted wandering population of SMBHs from Romulus broadly reproduces theobserved spatial offsets of a recent sample of hyperluminous X-ray sources. Wepredict that the sources with the most extreme offsets are likely to arise fromSMBHs within satellite galaxies. These simulations also predict a significantpopulation of secondary active galactic nuclei (AGN) with luminosities at least10\% that of the central AGN. The majority of galaxies at $z=4$ that host acentral AGN with bolometric luminosity $L_\mathrm{bol}>10^{42} \ \mathrm{erg}\; \mathrm{s}^{-1}$ are predicted to host a companion off-center AGN ofcomparable brightness. We demonstrate that stacked X-ray observations ofsimilar mass galaxies may reveal a halo of collective emission attributable tothese wanderers. Finally, because wanderers dominate the population of SMBHswith masses of $\lesssim 10^7\,M_{\odot}$ in Romulus, they may dominate tidaldisruption event (TDE) rates at these masses if they retain a stellar component(e.g. a nuclear star cluster). This could warrant an order of magnitudecorrection to current theoretically estimated TDE rates at low SMBH masses.

Unveiling the Population of Wandering Black Holes via Electromagnetic Signatures