It has long been assumed that Active Galactic Nuclei (AGN) reside at the centers of their host galaxies, but is this really true? A galaxy merger is expected to lead to the formation of a supermassive black hole (SMBH) binary, which can shrink through dynamical processes until it eventually coalesces through the emission of gravitational waves. Numerical relativity simulations show that, depending on the initial spin-orbit configuration of the binary, the merged SMBH receives a gravitational recoil kick that may reach several 1000 km s−1. The kick causes the merged SMBH to oscillate for up to ∼ 1 Gyr in the gravitational potential well of the galaxy, during which, the recoiling SMBH may be observed as a ‘displaced’ AGN. Displacements ∼ 10-100pc may be expected even in nearby elliptical galaxies and can be measured as spatial offsets in high resolution optical/infrared images.
In this dissertation, I present the results of a study of ∼100 nearby elliptical galaxies, that host AGN, using Hubble Space Telescope archival and new optical/infrared images, to analyze spatial offsets between the photo- center and AGN position. Evidence for significant spatial offsets has been found in about 20% of the sample of which six are considered to be robust displacements (∼5 - 40pc). Three other galaxies display a dual nucleus structure. These results are discussed in the context of the gravitational recoil hypothesis and alternative displacement mechanisms, including acceleration by radio jets, are considered. A second study described in this dissertation focusses on the luminous quasar E1821+643 which has previously been identified as an SMBH recoil candidate based on the large Doppler shifts of emission lines attributed to the retained gas. Follow-up spectroastrometry revealed a spatial displacement between the nucleus and the gas emitting the [OIII] lines. Hubble Space Telescope (HST) images were used to map the distribution of narrow-line emission on sub-arcsecond spatial scales which show that the [OIII] emission is intrinsically asymmetric. Spectroastrometry simulations reveal that the asymmetric [OIII] emission can account for only ∼25% of the observed displacement, which strengthens the case for a recoiling SMBH.
Library of Congress Subject Headings
Black holes (Astronomy)--Mathematics; Elliptical galaxies; Active galactic nuclei
Astrophysical Sciences and Technology (Ph.D.)
Department, Program, or Center
School of Physics and Astronomy (COS)
Jadhav, Yashashree, "Monsters on the Move: Gravitational Recoil of Supermassive Black Holes in Nearby Elliptical Galaxies" (2019). Thesis. Rochester Institute of Technology. Accessed from
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