The present universe is in a phase in which both the star formation rate and Active Galactic Nuclei (AGN) activity are declining. In this after-noon cosmos LISA will observe quiescent massive black holes at the centers of galaxies within a volume of about 100 Mpc3.
LISA will offer the deepest view of galactic nuclei, exploring regions inaccessible to electromagnetic observations, by probing the dynamics of intrinsically dark, relic stars in the nearest environs of a massive black hole.
The probes used are the so-called Extreme Mass Ratio Inspirals EMRIs: a compact star (either a neutron star or a stellar-mass black hole) captured into a highly relativistic orbit around the massive black hole and spiralling through the strongest field regions a few Schwarzschild radii from the event horizon before plunging into the massive black hole. As the compact star weighs much less than the massive black hole, the mass ratio is extreme, and as the star-black hole pair is a binary, the inspiral phase is governed by the emission of gravitational waves.
Key questions can be addressed in the study of galactic nuclei with EMRIs:
- What is the distribution of stellar remnants at the galactic centres and what is the role of mass segregation and relaxation in determining the nature of the stellar populations around the nuclear Black holes in galaxies?
- What is the mass distribution of stellar relics?
- Are stellar-mass Black holes of 100 M⊙ or/and medium sized Black holes of 104 M⊙ present in galactic centres?
LISA will discover EMRI events, exploring the deepest regions of galactic nuclei, those near the horizons of black holes with masses close to the mass of the Black hole at our Galactic Centre, out to redshifts as large as z ~ 0.7.
In the Gravitational Universe, EMRIs are exquisite probes for testing stellar black hole populations in galactic nuclei. With LISA we will learn about the mass spectrum of stellar-mass black holes, which is largely unconstrained both theoretically and observationally. The measurement of even a few EMRIs will give astrophysicists a totally new and different way of probing dense stellar systems, determining the mechanisms that govern stellar dynamics in the galactic nuclei.