Merger Driven or Internal Evolution? A New Morphological Study of Tidal Disruption Event Host Galaxies

TL;DR

This study addresses whether TDE host galaxies are predominantly merger-driven or shaped by secular processes. It analyzes 14 TDE hosts using high-depth r-band imaging from SDSS, DECaLS, and UNIONS, combined with a machine-learning merger classifier (MUMMI) and blinded visual identifications of bars/rings, benchmarked against carefully matched non-TDE controls. The key findings are that TDE hosts are ~16% more centrally concentrated than controls, but show no evidence for recent mergers; instead, bars and rings are significantly more common, especially in green valley hosts, pointing to bar-driven secular evolution as a primary pathway to enhanced central densities and TDE rates. These results suggest internal dynamical processes, rather than external mergers, play a crucial role in setting the nuclear conditions that favor TDE occurrence, with implications for interpreting TDE demographics in upcoming deep surveys.

Abstract

Host galaxies of tidal disruption events (TDEs) show enhanced central stellar concentration and are preferentially found in post-starburst and green valley populations. This connection has led to the proposal that TDE host galaxies likely have gone through recent mergers. We conduct a new morphological study of 14 TDE host galaxies, using the r-band images from Sloan Digital Sky Survey (SDSS), Dark Energy Camera Legacy Survey (DECaLS), and Ultraviolet Near-Infrared Optical Northern Survey (UNIONS), with the images from the latter two surveys having much higher depth and resolution than SDSS. We examine galaxy structures using conventional methods and also apply diagnostics of merger activity from machine learning models. Consistent with previous studies, our results show that TDE host galaxies are ~16% more centrally concentrated when compared to non-TDE-host controls. However, surprisingly, TDE hosts lack any indication of recent merger activity from both morphological analysis and our machine learning merger classifier. Instead, our results reveal that TDE host galaxies are approximately 1.5 to 2.5 times more likely to have bar-like or ring-like structures than their controls. This enhancement is even more prominent for TDEs in the green valley, with the factor reaching almost 3. Based on these results, we propose that bar-driven secular evolution, instead of merger, likely dominates the recent evolution of TDE hosts found in the green valley, which can simultaneously explain their distinctive nuclear properties and enhanced TDE rates.

Figures (9)

• Figure 1: Schematic diagram illustrating the overlap of image catalogs used for various analyses. The central blue region represents the initial SDSS spectroscopic catalog (with a sample of 14 TDE host galaxies). Overlaid are the UNIONS (orange) and DECaLS (red) galaxies, showing their respective overlaps: 7 of the SDSS TDE hosts are also present in UNIONS, and 5 are also found in DECaLS. Color-coded arrows indicate which image catalog was utilized for each specific analysis discussed in Sec. \ref{['sec:result']}.
• Figure 2: Representative $r$-band images of the 14 TDE host galaxies in Table \ref{['tab:TDEHostGalaxies']}. Images are sourced hierarchically: from UNIONS if available, otherwise from DECaLS, and finally from SDSS. Each panel indicates the TDE index, TDE name, its originating survey, and the structural classification ("R" for ring, and "B" for bar) in the top-left corner, with an angular size reference provided in the bottom-right corner.
• Figure 3: Comparison of $r$-band images for TDE host galaxies ASASSN-19dj (left) and PTF09ge (right) from different surveys. The top row display images from SDSS DR7, while the bottom row shows images from DECaLS (for ASASSN-19dj, bottom left) and UNIONS (for PTF09ge, bottom-right). The side-by-side comparison clearly demonstrates the significantly enhanced resolution of DECaLS and UNIONS imaging relative to SDSS. An angular size reference is provided in the bottom-right corner of each panel.
• Figure 4: Corner plots comparing parameter distributions of the general SDSS galaxy population (contours) and our TDE host galaxy sample (red circles). Each red circle marker is annotated with its corresponding index from Table \ref{['tab:TDEHostGalaxies']}. The parameters displayed (from left to right) are: black hole mass $M_{\rm BH}$, stellar mass $M_*$, star-formation rate $\rm SFR$, redshift $z$, bulge color $B_g-B_r$, Sersic index $n$, and bulge-to-total ratio $(B/T)_g$. The contours represent the $1\sigma$, $2\sigma$, and $3\sigma$ confidence levels of the SDSS galaxy distribution. These plots reveal that TDE hosts tend to have lower black hole and stellar masses, reside at low redshifts, exhibit bluer bulge colors indicative of younger stellar populations, and possess intermediate structural properties such as Sersic index and bulge dominance.
• Figure 5: Concentration parameter analysis for TDE host galaxies and their control samples. The top panel presents the statmorph concentration parameter $C_{\rm stat}$ for TDE host galaxies (star symbol) and their control sample (cross symbol) as a function of $M_{\rm BH}$, $M_*$, and z. Measurements are based on UNIONS images (orange) when available, and SDSS images (blue) otherwise. Importantly, the control sample measurements were weighted-averaged over 500 samples and consistently derived from the same survey as their TDE host galaxy. The bottom panel displays the difference in concentration, $\Delta C_{\rm stat} = C_{\rm stat,TDE} - \bar{C}_{\rm stat, control}$, with $1\sigma$ error bar. The shaded region indicate the mean $\Delta C_{\rm stat}$ for the UNIONS (orange), SDSS (blue), and combined (green) subsets. A dashed line marks $\Delta C_{\rm stat} = 0$. The rightmost panel provides a histogram of $\Delta C$ for these different samples. Overall, these results demonstrate that TDE host galaxies exhibit a signifiantly higher concentration parameter compared to their control samples, with an average $\Delta C_{\rm stat} = 0.32 \pm 0.03$, implying TDE host galaxies are $\sim 16\%$ more concentrated.