Merging Galaxies and Gravitational Waves: From Mpc to mpc

Special Session approved, and contributed papers welcome “Merging Galaxies and Gravitational Waves: From Mpc to mpc”

229th meeting of the American Astronomical Society
Grapevine, TX
6 January 2017

This Special Session will highlight advancements in astrophysics in the low frequency gravitational waveband and will feature a mix of invited and contributed oral presentations and posters. Galaxy mergers are key to galaxy assembly and dynamics, as large galaxies in the local Universe are thought to undergo multiple mergers during their development. It is also established that most, if not all, large galaxies in the local Universe host a supermassive black hole (SMBH). During a merger SMBHs sink, through dynamical friction, to the center of the merger product; this simple dynamical evolution model can replicate a variety of galaxy and quasar properties, including the M_BH-sigma relation, the quasar luminosity function, and the central brightness of galaxies. The two SMBHs will form a bound binary when their separation is of order 10 pc. Further interactions with stars in the central region, and possibly gas interactions, may drive the binary to a point at which gravitational wave emission dominates its dynamics. The nanohertz gravitational waves emitted by a binary SMBH should be detectable by precise timing of radio pulsars. The sensitivity of pulsar timing arrays has now breached the strength of gravitational-wave signals expected from the known population of merging galaxies.

The upper limits on nanohertz gravitational waves have a number of implications for galaxy dynamics
(i) masses of SMBH binaries could be systematically over-estimated, such that their gravitational waves are too;
(ii) SMBH binaries could “stall,” remaining at pc-scale separations and never emit gravitational waves; or
(iii) Binaries could evolve rapidly through the nanohertz regime because they couple strongly to the galactic environment.

This session will review what is known about the SMBH mass function, black hole-host relations, the galaxy merger process, and the influence of these on the expected gravitational wave signals. We will explore potential resolutions of the emerging mismatch between observed galaxy mergers and their not-yet-detected gravitational waves.

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