An unexpected population of quenched galaxies harbouring under-massive SMBHs revealed by tidal disruption events

TL;DR

This study uses tidal disruption events to reveal a surprising population of low-mass, quenched galaxies with under-massive SMBHs, challenging existing models of galaxy and black hole co-evolution.

Contribution

It uncovers a new population of low-mass, quenched galaxies with under-massive SMBHs, suggesting different quenching mechanisms than in more massive galaxies.

Findings

Quenched galaxies host more massive SMBHs at fixed stellar mass in general.

A reversal trend at low masses shows quenched galaxies have less massive SMBHs than star-forming ones.

Selection biases imply the true fraction of such galaxies might be higher.

Abstract

Restricted by event horizon suppression, tidal disruption events (TDEs) provide a unique window into otherwise hidden supermassive black holes (SMBHs) at the lower end of the mass spectrum, allowing the connection between star formation and SMBH mass to be explored across a broad stellar mass range. We derive stellar masses and specific star formation rates using Prospector fits to UV-MIR broadband spectral energy distributions (SEDs) for 42 TDE hosts, together with a high-mass comparison sample, and combine these with SMBH mass estimates from the literature. We first verify our approach by reproducing the established result that quenched galaxies host more massive SMBHs than star-forming systems at fixed stellar mass, a result widely interpreted as evidence for SMBH growth driving the blue-to-red sequence transition. However, examining the TDE sample in isolation reveals a trend reversal at lower masses, uncovering a surprising population of low-mass ($10^{9.6} \lesssim M_{\rm gal} \lesssim 10^{10.5}$ M$_\odot$), quenched galaxies hosting SMBHs systematically less massive ($M_{\rm BH} \lesssim 10^{6.5}$ M$_\odot$) than those in star-forming galaxies of comparable stellar mass. After ruling out degeneracies in our SED fits, we conclude that this reflects a physical difference in the quenching mechanism between these TDE hosts and the more massive galaxies. This is unlikely to be driven by AGN feedback, and could instead result from environmental processes, which can end star formation and hinder SMBH growth. We also show that the quenched and post-starburst population within the TDE sample is likely under-represented due to selection biases, suggesting the true fraction could be even higher than observed.