Session S12: Black Hole Populations

Simulating the Primordial Black Hole Binaries Formed in Dark Matter Halos

Primordial Black Holes (PBHs) might be responsible for a number of gravitational-wave events detected by LIGO, possibly constituting a significant fraction of dark matter. By modifying "Rapster", a semi analytic code for rapid population synthesis of binary black-hole mergers in environments rich in dynamic interactions, we simulate the formation and evolution of PBH binaries within dark matter halos. We focus on their merger properties such as mass and spin. Assuming that dark matter halos are only made of PBHs, we highlight how halo characteristics such as concentration and scale radius might affect binary formation and mergers. We compare our findings with BBHs formed in dense environments, such as globular and nuclear star clusters, revealing their distinct properties. Our work provides important insights into the dynamics of PBHs in dark matter halos, advancing our understanding of gravitational-wave events detected in LIGO.   

The Binary Black Hole Merger Rate Deviates from a Simple Delayed Cosmic Star Formation Rate: The Impact of Metallicity and Delay Times

Gravitational-wave detectors are making it possible to investigate how the merger rate of binary black holes (BBHs) evolves with redshift (z). We examine whether the merger rate R_merge(z) of isolated binaries deviates from the form assumed in state-of-the-art research: a scaled star formation rate density (SFRD). To address this, we conduct population synthesis simulations using COMPAS for a two-dimensional grid of stellar evolution prescriptions, convolve results with a metallicity-dependent SFRD, and compare the simulated R_merge(z)'s to "toy models" of R_merge(z) in the form of scaled SFRDs. We find that simulated R_merge(z)'s deviate from toy model rates by factors up to 5× for BBHs. Deviations are caused by two effects:

(i) The formation efficiency of BBHs is an order of magnitude higher at lower metallicity (Z < Z_⊙/5), corresponding to high z, due to weaker stellar winds.

(ii) BBH mergers can be significantly delayed (e.g., >1 Gyr) from BBH formation.

Combined, these effects cause R_merge(z) to be up to 3.5× higher at z=0 and 5× lower at z~9 compared to a scaled SFRD. We also find that scaling toy models to the inferred local R_merge causes deviations from simulated R_merge(z)'s beyond factors 10× at z~9. Interestingly, some of our simulations find z-dependence in BBH delay time distributions, increasing the complexity of delay time effects on R_merge(z).   

Which came first? Black-hole Spin or Supernova Kick

While the origins of black hole spins remain a mystery, it's commonly assumed that if black holes come from isolated massive star binaries, their spins should align with orbital angular momentum. However, this notion is often in conflict with observations. We will question this long-held viewpoint and explore various mechanisms that can spin up BHs before or during supernovae. In addition to natal spins, we will discuss methods that can spin BHs isotropically, parallel to supernova kicks, and perpendicular to supernova kicks. These different mechanisms leave behind distinct imprints in the observable distributions of spin magnitudes, spin-orbit misalignments and the effective inspiral spin of merging binaries. In particular, these mechanisms allow even the binaries originating in the field to exhibit precession and retrograde spin. This broadens the parameter space allowed for isolated binary evolution, which was previously thought to be exclusive to dynamically assembled binaries.   

Constraining a companion of the galactic center black hole, Sgr A*

We use 23 years of astrometric and radial velocity data on the orbit of the star S0-2 to constrain a hypothetical intermediate-mass black hole orbiting the massive black hole Sgr A* at the Galactic center.  The data place upper limits on variations of the orientation of the stellar orbit at levels between 0.02 and 0.07 degrees per year.  We use a combination of analytic estimates and full numerical integrations of the orbit of S0-2 in the presence of a black-hole binary.  For a companion IMBH outside the orbit of S0-2 (a > 1020 a.u.), we find that a companion black hole with mass between 10³ and 10⁵  solar masses is excluded, with a boundary behaving as a ~ m^1/3.  For a companion with semimajor axis < 1020 a.u., a black hole with mass between 10³ and 10⁵ solar masses is excluded, with a ~ m^-1/2. These bounds arise from quadrupolar perturbations of the orbit of S0-2.  Significantly stronger bounds on an inner companion arise from the fact that the location of S0-2 is measured relative to the bright emission of Sgr A*, and that separation is perturbed by the wobble of Sgr A* about the center of mass between it and the companion.  The result is a set of bounds as small as 400 solar masses at 200 a.u.; numerical simulations suggest a bound from these effects varying as a ~ m^-1.     

Galaxy lens reconstruction based on strongly lensed gravitational waves: the mass-sheet and similarity transformation degeneracy

Einstein's general relativity posits that gravitational waves, like light, can be gravitationally lensed by intervening mass distributions. If lensing by galaxies is detected in the future, these gravitational waves allow us to reconstruct the strong lenses that produced them, potentially allowing for several new science applications. However, such gravitational-wave-based strong lensing reconstructions suffer from the well-known mass sheet degeneracy and the less well-known similarity transformation degeneracy. We review these two degeneracies and discuss their implications for gravitational wave-based lens reconstructions and two notable gravitational wave lensing science cases: the Hubble constant measurement and tests for modified gravitational wave propagation. Furthermore, we review methods to break these degeneracies: although binary black holes are standard sirens, they cannot break the mass-sheet degeneracy with electromagnetic lens reconstructions and photometric redshift measurements of the lens and source galaxy in cosmological applications. Indeed, any science application measuring the Hubble constant will be affected by the mass-sheet degeneracy unless complementary measurements, such as the lens galaxy's velocity dispersion, are available. Fortunately, modified gravitational-wave propagation tests are unaffected by the mass-sheet degeneracy. However, both applications suffer from the similarity transformation degeneracy, which can only be broken with complementary electromagnetic observation, which provides redshifts and Einstein radius of the lens system. Breaking these degeneracies is critical to unlocking the full potential of gravitational wave-based lens reconstructions for future scientific applications.