The supermassive black hole coincident with the luminous transient ASASSN-15lh

TL;DR

This study links the luminous transient ASASSN-15lh to a supermassive black hole in an old galaxy, suggesting it resulted from a star disrupted by the black hole, with detailed host galaxy analysis supporting this scenario.

Contribution

It provides the first detailed analysis of the host galaxy of ASASSN-15lh, linking the transient to a supermassive black hole through spatial and spectral evidence.

Findings

The host galaxy's dominant stellar population is old, with no recent star formation.

The transient's position coincides with the galaxy's central supermassive black hole within 170 pc.

The transient is likely caused by a star disrupted by a spinning supermassive black hole.

Abstract

The progenitors of astronomical transients are linked to a specific stellar population and galactic environment, and observing their host galaxies hence constrains the physical nature of the transient itself. Here, we use imaging from the Hubble Space Telescope, and spatially-resolved, medium resolution spectroscopy from the Very Large Telescope obtained with X-Shooter and MUSE to study the host of the very luminous transient ASASSN-15lh. The dominant stellar population at the transient site is old (around 1 to 2 Gyr), without signs of recent star-formation. We also detect emission from ionized gas, originating from three different, time-invariable, narrow components of collisionally-excited metal and Balmer lines. The ratios of emission lines in the Baldwin-Phillips-Terlevich diagnostic diagram indicate that the ionization source is a weak Active Galactic Nucleus with a black hole mass of $M_\bullet = 5_{-3}^{+8}\cdot10^{8} M_\odot$, derived through the $M_\bullet$-$\sigma$ relation. The narrow line components show spatial and velocity offsets on scales of 1 kpc and 500 km/s, respectively; these offsets are best explained by gas kinematics in the narrow-line region. The location of the central component, which we argue is also the position of the supermassive black hole, aligns with that of the transient within an uncertainty of 170 pc. Using this positional coincidence as well as other similarities with the hosts of Tidal Disruption Events, we strengthen the argument that the transient emission observed as ASASSN-15lh is related to the disruption of a star around a supermassive black hole, most probably spinning with a Kerr parameter $a_\bullet\gtrsim0.5$.