Paper Review: Gravitational Wave Sources from Pop III Stars are Preferentially Located within the Cores of their Host Galaxies

This paper, authored by Fabio Pacucci, Abraham Loeb and Stefania Salvadori, presents the results of a simulation run in order to determine the likelihood of the binary black holes (BBHs) that produced the gravitational waves famously detected by LIGO having been formed from Population III stars (stars in the early universe with very low metallicity (amounts of metal)), rather than being formed more recently, given the observations that have been made. The paper makes the case that it is most likely that the BBHs were formed from Population III stars.

I find this to be an interesting paper because it explores a new observational technique and how it could be used in the future to find evidence of stars in the early universe. It is also interesting to read about the likely origins of something as seemingly strange as a binary black hole system and to think that the remnants of stars from the early universe are still around in this form today.

The paper begins by referencing the gravitational wave observations made by LIGO, which show that gravitational waves are emitted when the black holes in a BBH merge. The observations suggest that the merging black holes are of similar masses to one another (about 30 solar masses). It is also predicted that a lot more gravitational waves will be detected in the future, after new detectors are in use. The paper goes on to discuss the two ways in which the type of BBHs that could produce gravitational waves could have formed. The high mass of the stars that would have been needed to produce these BBHs means that the stars must have had a low metallicity, and so either the BBHs formed recently from stars that were far away from (or isolated in some other way from) other stars and so had an unusually low metallicity, or they were formed from stars that were around before the metals were produced in high quantities- population III stars. Being formed from population III stars would also imply that the BBHs took about 10 billion years to merge after they had been formed, which is quite likely to be the case. The authors of this paper used an N-body (gravitational) and chemical evolution simulation to track where the remnants of population III stars should be found, within the bulge (bright star forming region) of a galaxy like the Milky Way; this can then be used with the locations of BBHs, found using gravitational wave astronomy, in the future to find out whether they are the remnants of population III stars.

The second section of the paper describes the simulations that were used to gather the results. As aforementioned, the simulations used were an N-body simulation and a chemical evolution simulation to simulate the Milky Way. The simulation reproduced the relationship between the age and metallicity of stars found in the universe and the known properties of low metallicity stars and galaxies in the early universe, which is a sign that the simulation could be accurately representing the evolution of such galaxies through time. The simulation is used to pinpoint the locations of Population III stars in the Milky Way as they evolve through time.

The third section analyses the results gathered from the simulation about the locations of population III stars and their remnants.

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This figure shows the results that were gathered about the distribution of population III star remnants in the Milky Way today compared to the distribution of other stars and their remnants in the Milky Way in the simulation.

From these results, it was found that about 80% of population III star remnants are found at less than about 3 kiloparsecs (about 9×1019m) from the centre of the Milky Way; whereas the other stars and their remnants were more evenly distributed throughout the galaxy.

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This figure is a plot of the density of each type of star against the distance from the centre of the simulated Milky Way.

The figure above shows that the density of population III remnants deceases much faster with distance from the centre of the galaxy than that of other stars and their remnants. It was also found that the density of population III remnants divided by the density of other stars and their remnants was inversely proportional to distance from the centre of the galaxy, meaning the further away from the centre of the galaxy you get, the less population III remnants you will find compared to other stars and their remnants.

An expression for probability of actually detecting and observing a gravitational wave signal coming from the remnants of population III stars at a given position was then found from the simulations.  Then, the rate of BBH mergers in the Milky Way was computed by multiplying the density of galaxies in the universe, the average number of BBHs per galaxy and the average rate of BBH mergers in general.  It was found that the probability of detecting gravitational waves from population III remnants increased towards the centre of the Milky Way and tended to 100%.

The main conclusions that are drawn from this paper is that it is most likely that the gravitational waves being observed currently are more likely to be from the remnants of population III stars than from BBHs formed by other means and that the remnants of population III stars are more likely to be found in the centre of the Milky Way (or another galaxy similar to the Milky Way) than in the outer regions of the galaxy. This could be used to guide observers in the future about where to search for gravitational waves in order to find the remnants of population III stars in the Milky Way.

-Jess