arXiv subjects

Federica Fastidio

Publications and source records attributed to Federica Fastidio.

3 recordsLinked to original sources

arXiv2026

Converging on the eccentricity of massive black hole binaries in galactic mergers

Binaries of supermassive black holes (MBHBs) represent the primary sources of the gravitational wave background (GWB) detectable by Pulsar Timing Arrays (PTAs). The eccentricity with which binaries form in galactic mergers is the key parameter determining their evolutionary timescale from pairing to coalescence. However, accurately determining the binary eccentricity at formation is difficult in simulations due to stochastic effects. We present a numerical study of the formation and evolution of MBHBs that are potential PTA sources. We simulate mergers of equal-mass galaxies on different initial orbits and follow the dynamics of the MBHBs through the hardening phase. We find that low-resolution simulations are affected by stochasticity due to torques from the stellar distribution acting at pericentre passages. The dispersion in binary eccentricity decreases with increasing central resolution, as expected for a Poisson process. We provide a fitting formula for the resolution requirement of an N-body simulation of MBHB formation and evolution as a function of the initial eccentricity of the merger, e0, and the required accuracy in the binary eccentricity, eb. We find that binaries experience a torque at first pericentre that is approximately independent of initial eccentricity, producing a general trend in which the binary eccentricity decreases above sufficiently large initial orbital eccentricities. While this behaviour is generic, the precise cross-over eccentricity (e0~0.97 in our models) and the sharpness of the drop-off depend on the galaxy initial conditions. We provide a fitting formula for eb(e0) that can be used in semi-analytical models to determine the merger timescales of MBHBs as well as the amplitude and slope of the GWB.

astro-ph.GA↗

arXiv2025

Realistic consecutive galaxy mergers form eccentric PTA sources

Results from pulsar timing arrays (PTAs) show evidence of a gravitational wave background (GWB) consistent with a population of unresolved supermassive black hole binaries (BHBs). The observed spectrum shows a flattening at lower frequencies that can be explained by a population of eccentric BHBs. This study aims to determine the dynamical evolution and merger timescales of the most massive BHBs, which are potential sources of the GWB. We select successive galactic major mergers from the IllustrisTNG100-1 cosmological simulation and re-simulate them at high resolution with the N-body code Griffin, down to binary separations of the order of a parsec. Coalescence timescales are estimated using a semi-analytical model that incorporates gravitational wave emission and stellar hardening. Throughout our investigation, we consider the impact of prior mergers on the remnant galaxy in the form of core scouring and anisotropy, which can influence the subsequent formation and evolution of BHBs. We find that all the binaries in our sample enter the PTA band with an eccentricity e>0.85: such a large eccentricity can impact the shape of the PTA observed GWB spectrum, and it highlights the importance of including the eccentricity of binaries when interpreting the PTA signal. Furthermore, we find that: (i) starting from initial separations of a few tens of kpcs, the dynamical friction phase lasts for a few hundred Myrs; (ii) the binary formation time is not resolution dependent; (iii) the scatter on the eccentricity at binary formation decreases with increasing resolution; (iv) triple systems form whenever a third galaxy interacts with a binary which hasn't yet reached coalescence.

astro-ph.GA↗

arXiv2024

Eccentricity evolution of PTA sources from cosmological initial conditions

Recent results from pulsar timing arrays (PTAs) show evidence for a gravitational wave background (GWB) consistent with a population of unresolved supermassive black hole (SMBH) binaries (BHBs). While the data do not yet constrain the slope of the spectrum, this appears to flatten at the lowest frequencies, deviating from the power-law shape expected for circular binaries evolving solely due to gravitational wave (GW) emission. Interestingly, such flattening can be explained with a population of eccentric rather than circular binaries. The eccentricity of BHBs is notoriously difficult to predict based simply on the parameters of the host galaxies and the initial galactic orbit, as it is subject to stochastic effects. We study the evolution of the eccentricity of BHBs formed in galactic mergers with cosmological initial conditions from pairing to coalescence, with a focus on potential PTA sources. We select galactic mergers from the IllustrisTNG100-1 simulation and re-simulate them at high resolution with the N-body code Griffin down to binary separations of the order of a parsec. We then estimate coalescence timescales with a semi-analytical model of the evolution under the effects of GW emission and stellar hardening. We find that most mergers in IllustrisTNG100-1 occur on highly eccentric orbits, and that the eccentricity of BHBs at binary formation correlates with the initial eccentricity of the merger, if this is no larger than approximately 0.9. For extremely eccentric mergers, the binaries tend to form with modest eccentricities. We discuss the implications of these results on the interpretation of the observed GWB.

astro-ph.GA↗