Research Activities

I became interested in gravitational-wave astrophysics while I was working at my master thesis. At the end of 2018, the LIGO-Virgo collaboration released the first gravitational-wave transient catalog of compact binary mergers (Abbott et al., 2019). At the time, and still now, one the main open questions was the interpretation of the astrophysical merger rates behind the main sources of gravitational waves. Thus, I designed and tested a code capable to provide the merger rate density as a function of redshift, cosmo$\mathcal{R}$ate. I really got passionate about this project because it was broadly interdisciplinary: I had to combine the physics of the formation and evolution of compact objects as binary systems (Mapelli, 2018), with the metallicity-dependent star formation rate across redshift (Chruślinska, 2022).


During the first year of my PhD, I thus developed and publicly released cosmo$\mathcal{R}$ate. With this tool, I explored the astrophysical merger rates generated from compact objects formed in various formation channels (Santoliquido et al., 2020). I used populations of compact objects formed in young star clusters (Di Carlo et al., 2020; Rastello et al., 2020), which evolution was performed through N-body simulations, and populations formed in the so-called isolated scenario. In this case the evolution was implemented in our population-synthesis code mobse (Giacobbo et al., 2018).

click to see more Moreover, with the isolated formation channel, I explored the impact of several different parameters that govern the evolution of binary systems. For instance, I varied the parameter that determines the evolution of compact objects through the common envelope phase ($\alpha_{\rm{CE}}$). During this phase, the envelope of one massive star engulfs the entire binary system. A drag force is then exerted between the envelope and the binary system. $\alpha_{\rm{CE}}$ determines the efficiency of transferring energy from the binary system, whose semi-major axis shrinks, to the envelope, which eventually is ejected. I showed that binary black holes are mostly affected from the uncertainty of the metallicity-dependent star formation rate (right panel of Figure 1), instead of details on the binary evolution, such as $\alpha_{\rm{CE}}$ (left panel of Figure 1).


The last two years of my PhD have been devoted to the development of another tool: galaxy$\mathcal{R}$ate (Santoliquido et al., 2022). In this case, I wanted to update my previous code to include a complete galaxy treatment. This tool takes advantage of observational scaling relations, such as the main sequence of star-forming galaxies (Popesso et al., 2022) and the fundamental metallicity relation (Mannucci et al., 2011), to create an observation-based population of galaxies in which compact objects form. To retrieve the properties of the galaxies that host the mergers, I then adapted in a convenient and computationally efficient way the merger trees of cosmological simulations. A merger tree encodes the entire assembly history and property evolution of each single galaxy across cosmic time. Therefore, I condensed the information contained in the merger trees into a conditional probability, which links the properties of the formation galaxy, such as stellar mass and star-formation rate at the moment the compact object form, with the same properties of the host galaxy, where the compact objects merge. I am showing an example of it in the left panel of Figure 2. With galaxy$\mathcal{R}$ate, I showed, for instance, that binary black holes form at high redshift in low-mass metal-poor galaxies and merge in the local Universe in high-mass metal-rich galaxies (see right panel of Figure 2).

Figure 2 - Left panel: distribution of host galaxies properties in the local Universe ($z_{\rm{merg}} = 0$), for galaxies forming compact objects at $z_{\rm{form}} = 4$, with stellar mass $M_{\rm{form}} = 10^{7.2}$ M$_\odot$ and star-formation rate SFR$_{\rm{form}} = 10^{3.2}$ M$_\odot$ yr$^{-1}$. This conditional probability was evaluated adopting the merger trees of the EAGLE cosmological simulation (Schaye et al., 2015). Right panel: the mass distribution of formation galaxies (FGs, dashed lines) and host galaxies (HGs, solid lines). The different colours refer to redshift $z = 0, 1, 2,$ and 4. Binary black holes form in low-mass galaxies and merge in high-mass galaxies. The figure is adapted from Santoliquido et al. (2022).