Massive black holes lurking in Milky Way satellites

As massive black holes (MBHs) grow from lower-mass seeds, it is natural to expect that a leftover population of progenitor MBHs should also exist in the present universe. Dwarf galaxies undergo a quiet merger history, and as a result, we expect that dwarfs observed in the local Universe retain some memory' of the original seed mass distribution. Consequently, the properties of MBHs in nearby dwarf galaxies may provide clean indicators of the efficiency of MBH formation. In order to examine the properties of MBHs in dwarf galaxies, we evolve different MBH populations within a Milky Way halo from high-redshift to today. We consider two plausible MBH formation mechanisms: massive seeds’ formed via gas-dynamical instabilities and a Population III remnant seed model. Massive seeds' have larger masses than PopIII remnants, but form in rarer hosts. We dynamically evolve all halos merging with the central system, taking into consideration how the interaction modifies the satellites, stripping their outer mass layers. We compute different properties of the MBH population hosted in these satellites. We find that for the most part MBHs retain the original mass, thus providing a clear indication of what the properties of the seeds were. We derive the black hole occupation fraction (BHOF) of the satellite population at z=0. MBHs generated as massive seeds’ have large masses that would favour their identification, but their typical BHOF is always below 40 per cent and decreases to less than per cent for observed dwarf galaxy sizes. In contrast, Population III remnants have a higher BHOF, but their masses have not grown much since formation, inhibiting their detection.

💡 Research Summary

The paper investigates whether massive black holes (MBHs) that formed as seeds in the early Universe can still be found today in the dwarf satellite galaxies of the Milky Way, and whether their present‑day properties retain a memory of the original seed formation channel. Two seed formation scenarios are considered. The first, “massive seeds,” assumes that gas‑dynamical instabilities in high‑redshift protogalaxies produce relatively rare but heavy seeds with initial masses of 10⁴–10⁵ M⊙. The second, a Population III (Pop III) remnant model, assumes that the first generation of metal‑free stars leave behind more common, lighter seeds of 10²–10³ M⊙.

To explore the evolution of these seeds, the authors construct merger trees for a Milky Way‑mass dark‑matter halo from redshift z≈20 to the present (z=0) using semi‑analytic techniques calibrated on high‑resolution N‑body simulations. All subhalos that merge with the main halo are tracked, and the dynamical processes that affect them—tidal stripping, dynamical friction, and satellite‑host interactions—are explicitly modeled. Importantly, the authors allow the outer dark‑matter envelope of each satellite to be stripped while the central potential (and any embedded MBH) remains largely intact. This approach lets them assess whether the MBH mass evolves significantly after its birth or whether it essentially freezes at the seed value.

The results show a clear dichotomy between the two seed channels. In the massive‑seed scenario, the few satellites that host a seed retain MBHs with masses of 10⁴–10⁶ M⊙ at z=0. Because these black holes are relatively heavy, they would be detectable with current or upcoming facilities (e.g., deep X‑ray surveys, radio interferometers, or dynamical measurements with extremely large telescopes). However, the black‑hole occupation fraction (BHOF) for massive seeds never exceeds ~40 % across the whole satellite population, and for the subset of dwarfs that are actually observed (stellar half‑light radii ≲1 kpc) the BHOF drops below 1 %. The rarity of suitable host halos at early times explains this low occupation rate.

Conversely, the Pop III seed model yields a much higher BHOF—often >60 %—because low‑mass seeds can form in virtually every early minihalo. Yet the subsequent growth of these seeds is severely limited: the shallow potential wells of dwarf satellites, combined with strong feedback and the lack of major mergers, prevent substantial accretion. Consequently, the present‑day MBH masses remain close to their initial 10²–10³ M⊙ values, making them extremely difficult to detect with any realistic observational technique. The authors therefore argue that, while Pop III remnants may dominate the hidden black‑hole population in dwarfs, they are effectively invisible to current surveys.

The paper also quantifies how BHOF depends on satellite stellar mass and size. Smaller, less dense dwarfs have dramatically lower occupation probabilities, reflecting both the scarcity of massive‑seed hosts and the vulnerability of low‑mass halos to tidal disruption. For satellites with stellar masses below ~10⁶ M⊙, the BHOF falls below 1 % for massive seeds and remains modest even for Pop III remnants.

In the discussion, the authors highlight the observational implications. Detecting a handful of ~10⁵ M⊙ MBHs in nearby dwarfs would strongly favor the massive‑seed channel and provide a direct probe of early gas‑dynamical instability processes. Conversely, a systematic non‑detection, combined with indirect dynamical signatures (e.g., elevated central velocity dispersions), could be interpreted as evidence for a hidden Pop III population. They suggest that future facilities such as JWST, the Extremely Large Telescope (ELT), the Square Kilometre Array (SKA), and space‑based gravitational‑wave observatories (e.g., LISA) will be crucial for pushing detection limits down to the 10⁴ M⊙ regime.

Overall, the study demonstrates that MBHs in Milky Way satellites act as “time capsules” preserving the imprint of their seed formation mechanism. Massive seeds leave a clear, observable signature but are intrinsically rare, while Pop III remnants are common yet remain largely undetectable. The work provides a quantitative framework for interpreting upcoming dwarf‑galaxy surveys and for constraining the physics of the first black‑hole seeds in the Universe.