From Origins to Observables: Distinguishing Dark Compact Objects with Population-Level Microlensing Signatures

Abstract

While primordial black holes (PBHs) have long been a benchmark target for microlensing searches, the modern landscape of dark matter models suggests other, distinct, formation channels for compact objects made of dark matter. In the large class of self-interacting, dissipative models, dark matter has cooling channels that can enable fragmentation and gravitational collapse of some dark matter into compact objects, including black holes. The resulting populations have mass distributions, bias parameters, and abundance, spatial profile and velocity dispersion within the Milky Way that all differ from those of PBHs. We demonstrate that these population-level differences can leave imprints in the space of microlensing observables, with the differences in how the populations trace the dark matter giving the primary distinguishing lever. We discuss the possible overlap of microlensing signals from dark and baryonic lenses, and the complementarity of microlensing detection or constraints with other gravitational probes of novel populations of dark matter origin.

Figures (6)

• Figure 1: Primordial black holes (PBHs) and dark black holes (DBHs) have different formation channels, resulting in populations that differ in mass, abundance, and distribution. The PBH mass function is determined by very early universe cosmology. PBHs behave as collisionless dark matter, inheriting their spatial and kinematic distributions from the host dark matter halo. By contrast, the DBH mass function and abundance is determined by the dark sector cooling efficiency, and the spatial and kinematic distributions may reflect the formation or merger history of the host galaxy. The differences in formation processes leave signatures on microlensing observables.
• Figure 2: The sampled parameter distributions for the PBH population and the DBH population, including mass (top), distance from the observer (center), and velocity (bottom). PBHs are sampled using a Gaussian distribution centered at $0.5$$M_{\odot}$ tracing the Navarro-Frenk-White halo profile, while DBHs are sampled using a power-law distribution $P(M) \propto M^{-0.3}$ with distances following the de Vaucouleurs bulge profile. Both populations sample transverse velocities from radially binned Maxwellian distributions with characteristic velocities ranging from $120$ to $250$$\text{km/s}$. As each population is sampled from a distinct spatial distribution, the distance-dependent velocity prescription yields different effective kinematic distributions.
• Figure 3: The Einstein crossing time distribution of primordial black holes (PBHs) and dark black holes (DBHs). While the populations share similar timescale averages near $20$ days, their distribution also reveals statistical differences in the spread and tails, indicating distinct underlying population properties.
• Figure 4: The joint parameter spaces for the Einstein crossing time $t_{E}$, parallax $\pi_{E}$, and angular Einstein radius $\theta_{E}$, for primordial black holes (PBHs, orange) and dark black holes (DBHs, purple). The primary difference between the two populations comes from their spatial distribution: PBHs trace the halo, while DBHs are clustered in the bulge.
• Figure 5: An example distribution of mass of the primary (heavier) black hole in dark black hole binaries that merge within the Hubble time (red) compared to the distribution of all black holes (blue). The plot uses the atomic dark matter scenario, where DBH formation approximately follows the dynamics of Pop III star formation studied in shandera2018gravitational but with the binary parameter distributions adjusted to the mass function considered in this paper. A sample of 100,000 dark black holes was used to generate the comparison.