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

Abstract

The supermassive black holes (SMBHs) with mass $M_{\bullet }>10^9{\mathrm{M}}_{\odot }$ hosted by high-redshift galaxies have challenged our understanding of black hole formation and growth, as several pathways have emerged attempting to explain their existence. The "heavy-seed" pathway eases the problem with the progenitors of these SMBHs having birth masses up to $\sim 10^5{\mathrm{M}}_{\odot }$. 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; with a mass of $\sim 10^{12}{\mathrm{M}}_{\odot }$ at redshift $z=0$). We search for Leo-like satellite halos among these merger trees, and investigate the probability that the progenitors of some 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, $T_{\mathrm{act}}$, has the largest impact on the number of HSSs. We find HSSs in a fraction $0.7\%$, $18.1\%$, and $96.5\%$ of MW-like halos when $T_{\mathrm{act}}$ is set to $9,000$K, $7,000$K, and $5,000$K respectively. This suggests that Leo I could be hosting a heavy seed and could provide an opportunity to disentangle heavy seeds from other SMBH formation mechanisms.

9 pages, 4 figures, 2 tables. Submitted to MNRAS. Comments are welcome