Abhishek Hegade and Zoe Haggard

From the calendar

Extreme-mass-ratio inspirals (EMRIs) are among the primary targets of space-based gravitational-wave detectors such as the Laser Interferometer Space Antenna (LISA). If the secondary orbits an actively accreting supermassive black hole (SMBH), torques exerted by the accretion disk on the EMRI can play an important role in both its formation and subsequent evolution. In this talk, I will describe a framework for modeling relativistic disk–EMRI interactions and show that relativistic effects become important for EMRIs in the LISA band. For circular EMRIs, the energy exchanged through disk–EMRI interactions is typically subdominant to gravitational-wave energy loss and primarily acts to accelerate the inspiral. For eccentric EMRIs, however, disk–EMRI interactions primarily slow the inspiral and can dominate over gravitational-wave energy loss during the early stages of evolution. These results highlight the importance of incorporating disk–EMRI interactions when modeling the formation and evolution of EMRIs around accreting SMBHs.

The central parsec of the Milky Way’s nuclear star cluster (NSC) is the only place where it is possible to resolve the individual stars surrounding a supermassive black hole (SMBH). For this reason, it is a unique laboratory that enables the direct study of the interplay between a SMBH and its surrounding stellar populations. With the advent of adaptive optics, of the most unexpected discoveries was population of at least 100 young stars residing within 0.4 parsecs of the central SMBH, a region where no star-formation was expected to occur due to low gas densities and strong tidal forces. Interestingly, many of these young stars have orbital motion consistent with belonging to a disk-like structure, and could be the product of an in-situ formation event within the past 4-8 Myr.

The current orbits and dynamics of these young stars, which are too young to be fully relaxed, provide insight into the unique environment surrounding a SMBH. With thirty years of high precision astrometric and spectroscopic measurements from W. M. Keck Observatory, we have doubled the number of stars for which we can measure orbits. Because of this improvement, we can now resolve sub-structures within the stellar disk and have discovered that the inner portion of the stellar disk -- closer to the SMBH -- appears warped and thickened. One possibility is that the observed sub-structure of the stellar disk could be the result of long-term interactions between the disk stars and the background density profile.