The first pieces of the gravitational-wave progenitor population puzzle

Abstract

The first gravitational wave (GW) observation marked a monumental moment in science and heralded the birth of a new field: GW astronomy. Since then, the field has rapidly developed, with about 90 mergers of binary black Hole (BBH), black hole neutron star (BHNS), and binary neutron star (NSNS) mergers observed to date. This number is set to triple in the next few years, with millions of detections anticipated in the following decade. How do these merging double compact objects form? Each detection adds a new piece to the puzzle of their origins, moving us from a phase of initial discovery into an era of population studies. Many different formation channels have been proposed to solve this ‘progenitor population puzzle’, but all depend crucially on their direct ancestors: massive stars. Massive binary stars impact nearly every part of modern astrophysics, as they shape our Universe through the elements and ionizing radiation they emit. However, these stars are challenging to study while alive as they are intrinsically rare and live short lives. This raises a second question: (what) can GW sources teach us about the lives and deaths of their stellar progenitors?

In this thesis, we aim to shed light on this question by analyzing the early population results of GW sources that make up the first pieces of the progenitor puzzle. We apply a combination of numerical population synthesis models and analytical models to develop an intuitive understanding of the complex phenomena that govern the evolution of massive stars, and ultimately lead to the formation of double compact objects.

The first puzzle piece is the notable structure that emerged in the mass distribution of merging BBHs. Observations have revealed a ‘bump’, followed by a low but non-zero rate of mergers with components ≳ 35 M⊙. This has been linked to the theoretical prediction of a ‘mass gap‘ caused by Pair Instability Supernovae (PISN). We show that the contribution of isolated binaries to form BBH mergers in this mass gap is negligible, even under extreme assumptions about mass accretion. This points towards dynamical formation channels for BBHs in this mass range. We furthermore provide the first quantitative study into the origin of the global peak of the BBH mass distribution, and find that it results naturally from the stable Roche-lobe overflow channel. The reason behind this lies in a characteristic of this channel: it cannot form BBH mergers below a certain mass. This also provides an alternative explanation for the much disputed ‘neutron star-black hole mass gap’ if observed in GW sources. More clues follow from the evolution of the BBH merger rate with redshift, which is determined by the delay-time distribution of its formation channel. We identify unique delay time-mass relationships for the two main isolated binary evolution channels and provide testable predictions for the redshift evolution for the BBH merger rates from each channel. Lastly, we investigate how our model predictions are affected by the metallicity-dependent cosmic star formation history. We present a new functional form for the latter and determine that it will not shift the location of features such as those discussed above. This is exciting as it suggests that these features indeed have the potential to reveal the underlying physics of their stellar progenitors.