Abstract
ABSTRACT With projects such as Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays (PTAs) expected to detect gravitational waves from supermassive black hole mergers in the near future, it is key that we understand what we expect those detections to be, and maximize what we can learn from them. To address this, we study the mergers of supermassive black holes in the Illustris simulation, the overall rate of mergers, and the correlation between merging black holes and their host galaxies. We find these mergers occur in typical galaxies along the MBH−M* relation, and that between LISA and PTAs we expect to probe the full range of galaxy masses. As galaxy mergers can trigger star formation, we find that galaxies hosting low-mass black hole mergers tend to show a slight increase in star formation rates compared to a mass-matched sample. However, high-mass merger hosts have typical star formation rates, due to a combination of low gas fractions and powerful active galactic nucleus feedback. Although minor black hole mergers do not correlate with disturbed morphologies, major mergers (especially at high-masses) tend to show morphological evidence of recent galaxy mergers which survive for ∼500 Myr. This is on the same scale as the infall/hardening time of merging black holes, suggesting that electromagnetic follow-ups to gravitational wave signals may not be able to observe this correlation. We further find that incorporating a realistic time-scale delay for the black hole mergers could shift the merger distribution towards higher masses, decreasing the rate of LISA detections while increasing the rate of PTA detections.
Highlights
We have taken advantage of the cosmological galaxy formation simulation Illustris to investigate the mergers of SMBHs and the connection between black hole (BH) mergers and the morphologies of the galaxies in which they are found
Gravitational wave detections of supermassive black hole mergers should probe a large range of BH masses, with Pulsar Timing Arrays (PTA) probing the massive end of the MBH − M∗ relation and Laser Interferometer Space Antenna (LISA)’s sensitivity probing the low mass end of the MBH − M∗ relation
Most BH mergers involve the central black hole of a given galaxy, with Mchirp being within ∼0.5 dex of MBH,cen
Summary
It is well established that supermassive black holes (SMBHs) are found at the center of massive galaxies (Kormendy & Richstone 1995), and that the black hole (BH) masses correlate strongly with host galaxy properties, suggesting an evolutionary link between galaxy formation and BH growth (e.g. Magorrian et al 1998; Gebhardt et al 2000; Graham et al 2001; Ferrarese 2002; Tremaine et al 2002; Haring & Rix 2004; Gultekin et al 2009; McConnell & Ma 2013; Kormendy & Ho 2013; Reines & Volonteri 2015; Greene et al 2016; Schutte et al 2019). Cosmological hydrodynamical simulations which incorporate both galaxy formation and SMBHs provide an excellent tool to investigate the connection between BH mergers detectable by gravitational waves and the properties and histories of the host galaxies in which they are found. Using the detailed BH data in the Illustris simulation, we are able to connect each BH merger to the galaxy in which it occurs, and generate a list of every BH merger each galaxy has hosted throughout its history This allows us to analyze how recent BH mergers correlate with galaxy properties (such as morphology and star formation rate), the timescale over which these correlations survive, and the implications this has for electromagnetic followup observations to GW detections.
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