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

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.

💡 Research Summary

The authors investigate the low‑mass end of the supermassive black‑hole (SMBH) – galaxy connection by exploiting tidal‑disruption events (TDEs) as a selection tool. TDE flares are only observable for black holes below ≈10⁸ M☉ because above this mass the tidal radius lies inside the event horizon; consequently, TDEs uniquely reveal otherwise dormant low‑mass SMBHs. The study assembles a sample of 42 TDE host galaxies for which UV‑to‑mid‑infrared broadband photometry is available. Using the Galfetch pipeline they construct spectral energy distributions (SEDs) and fit them with the Prospector‑α framework, employing a non‑parametric Dirichlet star‑formation‑history prior (α = 0.2) and a log‑uniform prior on total stellar mass. From the posterior distributions they extract total stellar mass, metallicity, dust optical depth, and, crucially, the recent star‑formation rate (SFR) in the last ≈7.5 Myr. Specific star‑formation rate (sSFR) is defined as SFR divided by stellar mass; galaxies with sSFR < 10⁻¹⁰·¹⁵ yr⁻¹ are classified as quenched.

SMBH masses are taken from Mummery et al. (2024), who calibrated a tight relation between the late‑time optical/UV plateau luminosity of a TDE and the mass of the disrupting black hole. This method yields robust SMBH estimates for the 42 events that show a detectable plateau (49 of 63 total TDEs). To broaden the mass baseline, the authors add a high‑mass comparison set (42 galaxies) from Kormendy & Ho (2013), for which SMBH masses are measured dynamically (stellar, gas, or maser kinematics). The same photometric fitting procedure is applied to these galaxies, ensuring a homogeneous analysis.

When plotting SMBH mass versus total stellar mass, the high‑mass sample is dominated by red, passive galaxies and follows the well‑known trend that quenched systems host more massive black holes at fixed stellar mass. The TDE hosts occupy a lower‑mass regime (10⁹·⁶–10¹⁰·⁵ M☉) and include both star‑forming and passive objects. Fitting three separate least‑squares lines (high‑mass passive, TDE star‑forming, TDE passive) reproduces the classic offset: star‑forming TDE hosts have larger SMBHs than quenched TDE hosts of the same stellar mass. This reversal of the canonical trend is the central result.

The authors test whether the observed offset could be an artefact of SED fitting degeneracies (dust‑SFR, age‑metallicity). Because Prospector uses a flexible non‑parametric SFH and broad priors, the posterior distributions show no strong covariance between current SFR and dust attenuation, metallicity, or other SFH bins. Moreover, the high‑mass control sample, fitted with the identical pipeline, does not display a similar bias, reinforcing that the trend reversal is physical.

To interpret the result, the paper examines possible mechanisms that could suppress both star formation and SMBH growth in low‑mass quenched galaxies. AGN feedback is deemed unlikely because the SMBHs are already under‑massive (M_BH ≲ 10⁶·⁵ M☉) and would not generate sufficient energetic outflows. Instead, environmental processes—such as ram‑pressure stripping, tidal interactions, or strangulation in groups and clusters—are proposed. These mechanisms can rapidly remove or heat the cold gas reservoir, quenching star formation while simultaneously starving the central black hole of fuel, leading to the observed population of “under‑massive” SMBHs in quenched dwarfs.

The paper also discusses selection effects inherent to TDE surveys. TDEs are preferentially discovered in post‑starburst (E+A) or green‑valley galaxies because of their high central stellar densities and characteristic spectra (strong Balmer absorption, weak emission). However, the requirement of a detectable late‑time plateau further biases the sample toward events with certain light‑curve properties, potentially under‑representing the true fraction of quenched and post‑starburst hosts. The authors argue that correcting for these biases would likely increase the observed prevalence of quenched galaxies among TDE hosts.

In summary, this work provides the first empirical evidence that, at stellar masses below ~10¹⁰ M☉, quenched galaxies can host SMBHs that are systematically lighter than those in star‑forming counterparts—a reversal of the well‑established high‑mass trend. The finding challenges models that attribute quenching primarily to SMBH‑driven feedback and instead highlights the role of external environmental factors in simultaneously halting star formation and limiting black‑hole growth. Future larger TDE samples, deeper spectroscopic follow‑up, and refined selection‑function modeling will be essential to confirm the result and to integrate it into a comprehensive picture of galaxy evolution across the full mass spectrum.