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

TL;DR

This work uses Prospector-based SED fitting of $UV$–$MIR$ photometry to derive stellar masses and sSFR for 42 TDE host galaxies, augmented by a high-mass comparison sample, to study the SMBH–star formation connection across a wide mass range. SMBH masses are inferred for TDE hosts from plateau emission and compared to literature BH masses, revealing that in the TDE regime, quenched hosts tend to harbor under-massive BHs ($M_{ m BH} \lesssim 10^{6.5} M_{\odot}$) within $M_{ m gal} \sim 10^{9.6}$–$10^{10.5} M_{\odot}$, in contrast to star-forming hosts of similar mass. The authors find that this reversal cannot be explained by modelling degeneracies and argue that environmental quenching, rather than AGN feedback, suppresses star formation and SMBH growth in these low-mass systems, with a connection to E+A/post-starburst hosts. They also show redshift-dependent selection biases likely under-represent these passive systems, suggesting the true fraction could be higher in a volume-limited sample; upcoming surveys like Rubin LSST will be crucial to characterising this population and its environments. Overall, the results imply environmental processes play a key role in the quenching and SMBH growth of low-mass galaxies, challenging simple monotonic SMBH–quenching relations at the low-mass end and linking TDE demographics to dense environments and E+A galaxies.

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.

Figures (4)

• Figure 1: SMBH mass as a function of host galaxy total stellar mass for the TDE sample (stars) combined with the high-mass kormendy_13 sample split into classical, ellipsoidal and pseudobulge galaxy types (squares, diamonds and triangles). Each galaxy is coloured by the logarithm of its specific star formation rate (sSFR). The colour bar is normalised to the range of the TDE sample (10$^{-12}$$\lesssim$ log sSFR $\lesssim$ 10$^{-9}$). Three lines of best fit computed by least-squares minimisation indicate fits to: high mass regime quenched galaxies (solid, red); TDE star-forming galaxies (dashed, blue); TDE quenched galaxies (dot-dashed, red).
• Figure 2: SMBH mass as a function of total stellar mass for the TDE host galaxy sample. Each galaxy is coloured by the logarithm of its specific star formation rate (sSFR). The background 2D density map shows galaxies from the EAGLE simulation, coloured using the same sSFR metric. The colour bar is normalised to the range of the TDE sample (10$^{-12}$$\lesssim \log$(sSFR/yr$^{-1}$) $\lesssim$ 10$^{-9}$). E+A and merging galaxies are highlighted by purple and black outlines respectively. The dashed line (see text for details) derived in ramsden_25 splits the sample 50:50 -- we find that galaxies below the line are dominated by quenched systems, while those above are primarily star-forming.
• Figure 3: Top: SMBH mass as a function of bulge mass for the TDE host galaxy sample. Bottom: SMBH mass as a function of velocity dispersion for the TDE host galaxy sample. Each galaxy is coloured by the logarithm of its specific star formation rate (sSFR). The colour bar is normalised to the range of the TDE sample (10$^{-12}$$\lesssim \log$(sSFR/yr$^{-1}$) $\lesssim$ 10$^{-9}$). E+A and merging galaxies are highlighted by purple and black outlines respectively.
• Figure 4: Redshift distribution of the two host galaxy sub-samples detailed in section \ref{['quenching']}. Galaxies below the dashed 50:50 line, hosting under-massive SMBHs and dominated by quenched systems, are shown in red, while those above the line, hosting over-massing SMBHs and typically star-forming, are shown in blue.