## Constraints on primordial black hole dark matter from wide binaries in the galactic halo

Tyler, Emily Sarah (2022) Constraints on primordial black hole dark matter from wide binaries in the galactic halo. PhD thesis, University of Nottingham.

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## Abstract

Primordial black holes (PBHs), black holes that are formed in the early Universe from the collapse of over-densities, are a candidate for non-baryonic cold dark matter. The fraction of dark matter made up of PBHs has been constrained using multiple mechanisms including evaporation, gravitational lensing, gravitational waves from mergers, large-scale structure, accretion and the disruption of wide binaries. In this thesis we review the work on wide binaries and improve the reliability of these constraints, primarily by introducing a more physically motivated initial semi-major axis distribution for our simulated binaries.

We used Monte Carlo simulations to implement halo wide binary encounters with PBHs over a period of 10Gyr. The binaries are placed in a sea of perturbers and encounters are calculated using the impulse approximation. Broken binaries are kept in the simulations and used to calculate constraints, since their peculiar velocities are similar enough to be mistaken for binary stars. Our simulation results are consistent with previous work for a log flat initial semi-major axis distribution, and when broken binaries are removed from the simulations.

We calculate constraints by using a modified $\chi^2$ test to compare the simulated binary distribution with a catalog of observed binaries. A $\chi^2$ test provides p-values whilst also quantifying the goodness of the best fit parameters. We use a modified version in order to correct for the small number of binaries used in the comparison. Our constraints are re-scaled to take into account the non-uniform dark matter density along the binary Galactic orbits. To find the value by which to re-scale the constraints, we calculate the time-averaged dark matter density along each binary orbit for which we have sufficient data.

Our final constraints are much weaker than those calculated previously, primarily due to the inclusion of unbound binaries and a more physically motivated semi-major axis distribution. These constraints are subdominant to previously calculated constraints from other effects such as microlensing, gravitational waves and accretion in the PBH mass range $1M_\odot-1000M_\odot$, allowing PBHs to make up at least 20\% of dark matter.

More detailed simulations are needed to find tighter constraints on the PBH mass fraction: simulations of the Galactic potential would take into account the Galactic tide and disk and the variable dark matter density. These constraints could also be made more reliable by using a larger catalog of binary stars, such as a subset of GAIA DR2.

Item Type: Thesis (University of Nottingham only) (PhD) Green, Anne Primordial black hole, PBH, Dark matter, Wide binaries, Galactic halo Q Science > QB Astronomy UK Campuses > Faculty of Science > School of Physics and Astronomy 69110 Tyler, Emily 02 Aug 2022 04:40 02 Aug 2022 04:40 https://eprints.nottingham.ac.uk/id/eprint/69110

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