Speaker
Description
A new 4-dimensional model describing the Galactic distribution of neutron stars (NSs) and black holes (BHs). The compact remnants created as stars die in supernova explosions — the Galactic Underworld — are found to exhibit a fundamentally different distribution and structure to the visible Galaxy. Our simulated distribution of NSs and BHs is significantly less concentrated into a thin disk, with the scale height more than triple that of the visible Galaxy. The differences of form primarily arise from two effects.
Firstly, the distribution of remnants inherits structure from the changing distribution of their parent stars over cosmic time. Secondly, an even larger effect arises from the natal kick received by the remnants during their supernova explosions. We find almost a third of remnants acquire sufficient kinetic energy to entirely escape the Galactic potential, leading to a small Galactic mass loss. The BH–NS fraction increases near the Galactic centre: a consequence of smaller kick velocities expected for the former. In the solar neighbourhood, our simulated remnant distribution yields probable distances of 19 pc and 21 pc to the nearest NS and BH, respectively. Although the underworld only contains of order ∼1% of the Galaxy's mass, observational signatures and physical traces of its population, such as microlensing, will become increasingly present in data ranging from gravitational wave detectors to high precision surveys such as Gaia. We present a prediction for the rate of microlensing events and hope that models such as this represent an important step in understanding the expected signals, and the interpretation of their implications.
