The Most Massive Early-type Galaxies Exhibit Tidal Features More Frequently in Lower-density Environments

TL;DR

Massive early-type galaxies are more likely to display tidal features in low-density environments, indicating recent mergers are more common there, especially among the most massive galaxies, due to extended formation histories or shorter tidal feature lifetimes in dense areas.

Contribution

This study reveals a clear correlation between environment density and tidal features in the most massive early-type galaxies, highlighting the role of recent mergers in their evolution.

Findings

Tidal features are more frequent in low-density environments.

The fraction of galaxies with tidal features triples from high to low-density areas.

Bluer, dust lane-rich massive ETGs are more common in low-density environments.

Abstract

The most massive early-type galaxies (ETGs) are known to form through numerous galaxy mergers. Thus, it is intriguing to study whether their formation in low-density environments, where nearby companions are almost absent, is associated with mergers, which are directly traced by tidal features. Using the 436 most massive ETGs with $M_\mathrm{star}>10^{11.2}\,M_{\odot}$ at $z<0.04$, we determine the variation in the fraction of massive ETGs with tidal features ($f_T$) across different environments and verify whether the most massive ETGs commonly have tidal features in very low density environments. Our main discovery is that the most massive ETGs exhibit tidal features more frequently in lower-density environments. In the highest-density environments, like galaxy clusters, $f_T$ is $0.21\pm0.06$, while in the lowest-density environments it triples to $0.62\pm0.06$. This trend is stronger for more extremely massive ETGs, with $f_T$ reaching $0.92\pm0.08$ in the lowest-density environments. One explanation for our finding is that the most massive ETGs in lower-density environments have genuinely experienced recent mergers more frequently than their counterparts in higher-density environments, suggesting that they possess extended formation histories that continue into the present. Another possibility is that tidal features last shorter in denser environments owing to external factors inherent in these environments. Our additional findings that massive ETGs with bluer $u-r$ colors are a more dominant driver of our main discovery and that dust lanes are more commonly observed in massive ETGs in low-density environments imply that gas-abundant mergers primarily contribute to the increased rate of recent mergers in low-density environments.