Heavy black hole seed survivors in dwarf galaxies: a case study of Leo I

Abstract

High-redshift supermassive black holes (SMBHs) have challenged our understanding of black hole formation and growth. The “heavy-seed” pathway eases the problem with the progenitors of these SMBHs having birth masses up to ∼105M⊙. Here, we investigate the possibility that a local dwarf galaxy, Leo I, harbors a heavy-seed descendant. Using Monte-Carlo merger trees to generate the merger histories of 1,000 dark matter halos similar to the Milky Way (MW), we search for Leo-like satellite halos and investigate the probability that the progenitors of these satellites formed a heavy seed. We derive the likelihood of such “heavy seed survivors” (HSSs) across various formation and survival criteria as well as Leo-similarity criteria. We find that the virial temperature for the onset of atomic cooling and rapid gas infall that yields heavy seeds, Tact, has the largest impact on HSS frequency. We find HSSs in a fraction 0.7 %, 18.1 %, and 96.5 % of MW-like halos when Tact is set to 9,000K, 7,000K, and 5,000K respectively. For the canonical atomic-cooling threshold (Tact ≳ 8, 000K), the HSS fraction is ≲ 1%, indicating that Leo I hosting a heavy-seed descendant is unlikely under well-motivated physical assumptions. Higher HSS fractions occur only under optimistic and currently unsubstantiated assumptions on Tact. Nevertheless, these results motivate future work that clarifies the possibility of heavy-seed BH formation in pristine halos with nominal virial temperatures below 8,000K. This may motivate Leo I as a target for disentangling heavy seeds from other SMBH formation mechanisms.