Soheb Mandhai
The evolution of two giant stars in a binary (a two-star system) can lead to the formation of some of the most violently interacting objects in the universe, such as a combination of neutron star and black hole pairs. The joint orbit of these objects can distort space-time, creating ripples known as gravitational waves. Over time the orbital distance between the two objects decays leading to an extreme collision. In some cases, for systems with at least one neutron star, a short burst of highly energetic light, known as gamma-rays, can be released.
These bursts of radiation can be isolated allowing us to determine the approximate location in space. Understanding the origin of these events provides some insight into the type of galaxies and environments that these progenitors were formed in. In turn, allowing us to build on our understanding of how systems like these evolve from birth.
In some cases, the binaries may eject from their host galaxy into intergalactic space. As such we may see some of these events occurring at some distance from their original host galaxy. A key aim of ongoing studies is to determine how far these events occur from us – an aim that is usually achieved by finding a likely host with a known distance.
In our work, we present predictive models that have been produced using a simulated population of similar binaries paired with galaxies from a cosmological (virtual universe) simulation. We explore the motion of these binaries and gauge the distances at which they merge relative to their host’s galactic centre. We are also able to extract the properties of the simulated host and thus, observe the preferred galaxy environments of these systems.
What are compact binaries? What makes them special? –
Compact binaries consist of two stellar objects that are formed following the death of a star. In context of this work, we are interested in binaries that consist of a neutron star that is paired with another neutron star or black hole.
These systems can produce distortions in space-time known as gravitational waves. Which can be used to probe and understand the neutron star (NS)/black-hole (BH) components at an unprecedented level.
Upon merging, systems with at least one neutron star may produce an electromagnetic counterpart such as a short-duration gamma-ray burst (SGRB) or a kilonova.
The first coincident detection of a gravitational wave resulting from the merger of two neutron stars, GW170817A, with a short-duration gamma-ray burst, GRB 1708017A, sparked the multi-messenger era in Astrophysics!
Slide 2:
The Associativity Issue:
Whilst GW detections for NS-NS and BH-NS are lacking, there are a plethora of SGRB detections. The localisation of these events provides us with an insight into the site of the progenitor compact binary merger.
There are relatively few events that have been successfully associated with the galaxy of origin. Without knowing this, we have little insight into the properties of the host galaxies and distances at which these objects are occurring.
Galaxies may also be distant or too faint to observe. Therefore, associations to these galaxies, should they be the true host may be missed. However, for nearby galaxies, we can calculate the distance between SGRB location and the galaxy. This can be used to gauge the feasibility of an association.
Generating Predictive Models Using Simulations:
The models produced in this work use synthetic binaries that are seeded into hydrodynamically simulated galaxies. For each system, we evolve the binaries within their host galaxy and trace their orbits. At the point of coalescence, we can extract the properties of the host and the relative position of the binary.
During this process, we consider the likelihood of binaries forming within the galaxy environments available; the effect of galaxy mass on the orbit of binaries; the effect of natal kick velocities imparted by the supernovae during the formation of the compact binary.
Slide 3:
Binary Migration Relative To Their Host Galaxies:
Typically, our results show that ~50% of binaries tend to merge within a ~ (few) 10 ly from the centre of their host galaxy. For dwarf galaxies, binaries tend to travel between a (few) 100 – 10000 ly, with most binaries merging a (few) 1000 ly away. For larger galaxies such as spirals, much like our Milky Way, we find the largest number of binaries forming. These systems tend to migrate between a (few) 1-1000 ly, with most mergers occurring within a ~ (few) 10 ly. For massive galaxies such as ellipticals, binaries are bound more strongly to their host. Mergers tend to occur between (few) 1-100 ly with most mergers occurring within ~ (few) 10 ly.
Depending on the binary type, between 40-45% of binaries will be ejected from their host galaxy. SGRBs produced by these systems would be considered host-less.
Slide 4:
Research Applications:
We have shown a non-negligible fraction of binaries may appear host-less if observed. This can in turn be used to gauge the likelihood of nearby tentative hosts. Future missions and surveys will enable us to constrain the distances at which mergers are occurring (based on improved GW detections); uncover distant galaxies and SGRBs, which will enable more successful associations; enable coincident GW-SGRB detections. These factors will allow us to test and improve the constraints that are placed on search campaigns for host galaxies using this work.