Solving the "host galaxy problem" for tidal disruption events
Lead Research Organisation:
QUEEN'S UNIVERSITY BELFAST
Abstract
Luminous, supernova-like flares from tidal disruption events (TDEs) of stars by supermassive black holes are now discovered regularly by wide-field transient surveys. These unique transients are powerful probes of accretion physics, black hole growth, galaxy evolution and more. From a decade of observations, one of the most exciting challenges to emerge in this field is the "TDE host galaxy problem": if virtually every galaxy harbours a SMBH, why are a third of TDEs found in rare post-starburst galaxies, which make up only ~1% of the galaxy population?
Answering this question is fundamental to understanding the processes which govern TDEs. Despite the success in finding TDEs over the past decade, their measured rate seems to be converging on a lower value than expected, indicating crucial gaps in existing theories of the galactic dynamics that drive TDEs. Explanations for an elevated TDE rate in certain galaxies include disturbed radial density profiles, nuclear starbursts, and recent mergers, but distinguishing between these channels is only possible by resolving structure on scales <100 pc. It cannot be done using the data from existing or planned wide-field surveys (even including LSST), but can be achieved using a high-resolution space-based observations of known TDE host galaxies.
To solve this problem, the Project Lead has been awarded 57 orbits with HST to survey all known TDE host galaxies out to z~0.05, forming a volume-complete sample with the multi-colour, high-resolution imaging needed to understand the faint and small-scale structures in these important galaxies. Our sample of 19 new objects (plus 4 from the literature) is the first data set with (i) the statistical power to determine whether these galaxies have higher central densities than typical quiescent galaxies, (ii) the resolution to spatially map the stellar ages and determine whether quenching occurs from the inside-out or outside-in, and (iii) the sensitivity to faint tidal features and shells needed to determine whether minor mergers are responsible for quenching the star-formation and altering the central density profiles.
We are also part of a team with approved XMM-Newton and NICER time to study the TDE accretion disks and search for Quasi-Periodic Eruptions in 13 out of 19 host galaxies. Assuming that QPEs are driven by interactions between orbiting stars and TDE disks, the fraction of TDEs with QPEs will constrain the number density of stars on bound nuclear orbits with periods of hours-days (so-called extreme mass-ratio inspirals, or EMRIs). We will combine the X-ray, HST UV, and long-term optical data to measure the physical sizes of the TDE disks. Thus our project will constrain the density of stars in these galaxies both on scales of ~100 pc and at the accretion disk scales ~1 AU.
Our unique dataset and carefully designed work plan provide an opportunity to tackle the host galaxy problem and determine the physical factors that lead to TDEs -- examining and connecting both the central densities and merger histories of these galaxies. This project simultaneously provides a detailed snapshot of the short-lived but critical post-starburst phase in galaxy evolution, and the first electromagnetic constraints on the rate of EMRIs, using QPEs as a probe many years before space-based GW detectors.
Answering this question is fundamental to understanding the processes which govern TDEs. Despite the success in finding TDEs over the past decade, their measured rate seems to be converging on a lower value than expected, indicating crucial gaps in existing theories of the galactic dynamics that drive TDEs. Explanations for an elevated TDE rate in certain galaxies include disturbed radial density profiles, nuclear starbursts, and recent mergers, but distinguishing between these channels is only possible by resolving structure on scales <100 pc. It cannot be done using the data from existing or planned wide-field surveys (even including LSST), but can be achieved using a high-resolution space-based observations of known TDE host galaxies.
To solve this problem, the Project Lead has been awarded 57 orbits with HST to survey all known TDE host galaxies out to z~0.05, forming a volume-complete sample with the multi-colour, high-resolution imaging needed to understand the faint and small-scale structures in these important galaxies. Our sample of 19 new objects (plus 4 from the literature) is the first data set with (i) the statistical power to determine whether these galaxies have higher central densities than typical quiescent galaxies, (ii) the resolution to spatially map the stellar ages and determine whether quenching occurs from the inside-out or outside-in, and (iii) the sensitivity to faint tidal features and shells needed to determine whether minor mergers are responsible for quenching the star-formation and altering the central density profiles.
We are also part of a team with approved XMM-Newton and NICER time to study the TDE accretion disks and search for Quasi-Periodic Eruptions in 13 out of 19 host galaxies. Assuming that QPEs are driven by interactions between orbiting stars and TDE disks, the fraction of TDEs with QPEs will constrain the number density of stars on bound nuclear orbits with periods of hours-days (so-called extreme mass-ratio inspirals, or EMRIs). We will combine the X-ray, HST UV, and long-term optical data to measure the physical sizes of the TDE disks. Thus our project will constrain the density of stars in these galaxies both on scales of ~100 pc and at the accretion disk scales ~1 AU.
Our unique dataset and carefully designed work plan provide an opportunity to tackle the host galaxy problem and determine the physical factors that lead to TDEs -- examining and connecting both the central densities and merger histories of these galaxies. This project simultaneously provides a detailed snapshot of the short-lived but critical post-starburst phase in galaxy evolution, and the first electromagnetic constraints on the rate of EMRIs, using QPEs as a probe many years before space-based GW detectors.
Organisations
People |
ORCID iD |
| Matt Nicholl (Principal Investigator) |