A Dependence of the Tidal Disruption Event Rate on Global Stellar Surface Mass Density and Stellar Velocity Dispersion

Abstract

The rate of tidal disruption events (TDEs), R TDE , is predicted to depend on stellar conditions near the super-massive black hole (SMBH), which are on difficult-to-measure sub-parsec scales. We test whether R TDE depends on kpcscale global galaxy properties, which are observable. We concentrate on stellar surface mass density,  S M , and velocity dispersion, s v , which correlate with the stellar density and velocity dispersion of the stars around the SMBH. We consider 35 TDE candidates, with and without known X-ray emission. The hosts range from starforming to quiescent to quiescent with strong Balmer absorption lines. The last (often with post-starburst spectra) are overrepresented in our sample by a factor of - 2 , higher on average than for a volume-weighted control sample of Sloan Digital Sky Survey galaxies with similar redshifts and stellar masses. This is because (1) most of the TDE hosts here are quiescent galaxies, which tend to have higher  S M than the star-forming galaxies that dominate the control, and (2) the star-forming hosts have higher average  S M than the star-forming control. There is also a weak suggestion that TDE hosts have lower s v than for the quiescent control. Assuming that, and applying a statistical model to the TDE hosts and control sample, we estimate a =  0.9 0.2 and b = - 1.0 0.6. This is broadly consistent with R TDE being tied to the dynamical relaxation of stars surrounding the SMBH.

The tidal disruption event AT2017eqx: spectroscopic evolution from hydrogen rich to poor suggests an atmosphere and outflow

“…We find an excellent fit with a Sérsic index of only 0.7 (Figure 15), indicating a less sharply-peaked light distribution than typical TDE hosts. Graur et al (2018) showed that TDE hosts generally have a stellar mass surface density log(Σ M * /(M kpc −2 )) > 9, which is consistent with typical quiescent galaxies, but high for star-forming TDE hosts. This can be interpreted as evidence for a high density of stars around the central SMBH, which could naturally lead to a higher rate of TDEs (Graur et al 2018), as earlier proposed by Stone & van Velzen 2016).…”

“…We also detect narrow absorption lines from the host galaxy, from which we measure the redshift z = 0.1089± 0.0005. The host is a Balmer-strong absorption galaxy -a rare type of galaxy that is greatly over-represented among TDE hosts (Arcavi et al 2014;French et al 2016;Law-Smith et al 2017;Graur et al 2018).…”

“…A surprisingly large fraction of TDEs have been found in a specific class of quiescent Balmer-strong absorption galaxies (Arcavi et al 2014;French et al 2016;Graur et al 2018;Law-Smith et al 2017), defined spectroscopically by the presence of Balmer lines in absorption and a lack of nebular emission lines. This combination signifies that star formation was likely significant up to ∼ 1 Gyr ago but has now largely ceased, leaving behind A type stars that dominate the stellar light (whereas the ionizing O and B stars have already died).…”

Rates of Stellar Tidal Disruption

“…A subsequent analysis (Graur et al 2018) of a larger TDE host sample found somewhat smaller but still significant rate enhancements (R = 35 +21 −17 and R = 18 +8 −7 for the same categories of Balmer-line strength). The Graur et al (2018) rate enhancements were computed from a larger but more heterogeneous sample of 33 TDE host galaxies, and vary significantly if one examines different sub-samples. TDE host galaxies display some other peculiarities as well, such as an unusually high surface brightness on ∼ kpc scales (Law-Smith et al 2017b).…”

On Post-starburst Galaxies Dominating Tidal Disruption Events

“…(3) We convolve P (r, ∆t) with Eq (20) In Figure 2 we place contours of P (∆t) for ∆t = 1yr (blue dotted contours) and ∆t = 5yr (red dot-dashed contours), respectively, on the two-dimensional parameter plane of (η, σ/v c ). The two black contours are the current observational constraint of N tde = 10 5 − 10 6 per PSB, corresponding to 10 −4 − 10 −3 yr −1 per PSB with a time span of 1Gyr (French et al 2016) [also see Law-Smith et al (2017); Graur et al (2018)]. Also shown as horizontal magenta dot-dashed line is an indicative case where the vertical velocity dispersion is equal to sound speed of atomic cooling gas gas of temperature 10 4 K, out of which stars in the disk may have formed.…”