Galaxy-scale consequences of tidal disruption events: extended emission-line regions, extreme coronal lines, and infrared-to-optical light echoes

ABSTRACT Stars in galactic centres are occasionally scattered so close to the central supermassive black hole that they are completely disrupted by tidal forces, initiating a transient accretion event. The aftermath of such a tidal disruption event (TDE) produces a bright-and-blue accretion flow that is known to persist for at least a decade (observationally) and can in principle produce ionizing radiation for hundreds of years. TDEs are known (observationally) to be overrepresented in galaxies that show extended emission-line regions (EELRs), with no pre-TDE classical active galactic nucleus activity, and to produce transient ‘coronal lines’, such as [Fe x] and [Fe xiv]. Using coupled cloudy-TDE disc simulations we show that TDE discs produce a sufficient ionizing radiation flux over their lifetimes to power both EELR of radial extents of $r \sim 10^4$ light years, and coronal lines. EELRs are produced when the ionizing radiation interacts with low-density ($n_H \sim 10^1\!-\!10^3 \, {\rm cm}^{-3}$) clouds on galactic scales, while coronal lines are produced by high-density ($n_H \sim 10^6\!-\!10^8 \, {\rm cm}^{-3}$) clouds near the galactic centre. High-density gas in galactic centres will also result in the rapid switching on of narrow-line features in post-TDE galaxies, and also various high-ionization lines, which may be observed throughout the infrared with James Webb Space Telescope. Galaxies with a higher intrinsic rate of TDEs will be more likely to show macroscopic EELRs, which can be traced to originate from the previous TDE in that galaxy.