Wandering intermediate-mass black holes in Milky Way-mass galaxies in cosmological simulations: myth or reality?

Abstract In this work, we address the question“can we use the current cosmological simulations to identify intermediate-mass black holes (IMBHs) and quantify a putative population of wandering IMBHs?” and demonstrate that caution is necessary when drawing conclusions about wandering IMBHs in the Milky Way based on cosmological simulations, due to their relatively low resolution. We compare wandering-IMBH counts in different simulations with different sub-grid methods and post-processing recipes, the ultimate goal being to aid future wandering-IMBH detection efforts. In particular, we examine simulations in which IMBHs are identified as BH seeds forming at high redshift and those in which they are identified using star clusters as proxies, which implicitly appeals to a stellar dynamical formation channel. In addition, we employ the extremely high-resolution cosmological hydrodynamical “zoom-in” simulation of a Milky Way-sized galaxy, GigaEris, with the star cluster proxies method to identify IMBHs. Wandering IMBHs are defined using two methods: those within but originating beyond the virial radius, and “non-central” BHs. We find consistent counts of wandering high-redshift IMBHs across most of the different cosmological simulations employed so far in the literature, despite the different identification approaches, resulting in 5 to 18 wandering IMBHs per Milky Way-sized galaxy at z ≥ 3. Nevertheless, we argue this is only coincidental, as a significant discrepancy arises when examining the formation sites and the mass ranges of the wandering IMBHs. This raises questions about the extent to which current cosmological simulations can guide observational searches for wandering IMBHs.