An Enhanced Rate of Tidal Disruptions in the Centrally Overdense E+a Galaxy NGC 3156

Abstract

Time domain optical surveys have discovered roughly a dozen candidate stellar tidal disruption flares in the last five years, and future surveys like the Large Synoptic Survey Telescope will likely find hundreds to thousands more. These tidal disruption events (TDEs) present an interesting puzzle: a majority of the current TDE sample is hosted by rare post-starburst galaxies, and tens of percents of the galaxies are hosted in even rarer E+A galaxies, which make up~0.1% of all galaxies in the local universe. E+As are therefore overrepresented among TDE hosts by 1-2 orders of magnitude, a discrepancy unlikely to be accounted for by selection effects. We analyze Hubble Space Telescope photometry of one of the nearest E+A galaxies, NGC 3156, to estimate the rate of stellar tidal disruption produced as two-body relaxation diffuses stars onto orbits in the loss cone of the central supermassive black hole. The rate of TDEs produced by two-body relaxation in NGC 3156 is large when compared to other galaxies with similar black hole mass:˙~´- . This suggests that the preference of TDEs for E+A hosts may be due to central stellar overdensities produced in recent starbursts.

On the Mass and Luminosity Functions of Tidal Disruption Flares: Rate Suppression due to Black Hole Event Horizons

“…Our observations thus support a constant black hole occupation fraction for M • 10 5.5 M . Post-starburst galaxies (characterized as quiescent galaxies with strong Balmer absorption lines; Dressler & Gunn 1983;Zabludoff et al 1996) are overrepresented among TDF hosts (Arcavi et al 2014;French et al 2016French et al , 2017Law-Smith et al 2017;Graur et al 2017), which could be explained by a short relaxation time caused by a high central density in these galaxies (Stone & van Velzen 2016;Graur et al 2017). An overrepresentation of post-starburst galaxies is unlikely to significantly influence the distribution of total stellar mass of the mock TDF sample because the relative mass increase in the recent star-formation episode of these galaxies is modest, 10-50%, (Kaviraj et al 2007; D. French et al, in prep).…”

Stellar Dynamics and Stellar Phenomena Near a Massive Black Hole

“…TDEs are over-abundant in these post-starburst galaxies by factors of 33-190, and the implied TDE rate for this special class of hosts is 10 −3 yr −1 gal −1 (Stone & van Velzen 2016). This further exacerbates the discrepancy between the predicted and observed rates in all other galaxy types.…”

Rates of Stellar Tidal Disruption

“…Later works made virial estimates, starting with a galaxy's velocity dispersion σ, effective radius R eff , and luminosity L, then computing Υ vir = 2σ 2 R eff /(GL) (Wang and Merritt 2004;. Both of these methods make the large assumption that Υ is constant throughout the galaxy; a more self-consistent method would apply simple stellar population models to multiband photometry of the galactic nucleus to estimate Υ (R), as was done by Stone and van Velzen (2016). -Choice of PDMF, dN/dm : as we have seen, both the first and second moments of the PDMF enter into TDE rate calculations.…”