Massive Early-Type Galaxies in the HSC-SSP: Flux Fraction of Tidal Features and Merger Rates

TL;DR

This study statistically analyzes tidal features around massive early-type galaxies using deep imaging data, revealing their prevalence, relation to galaxy mass, and estimating the typical lifetime of tidal features to be around 3 billion years.

Contribution

It provides the first large-scale measurement of tidal feature flux fractions in massive ETGs and links these features to galaxy merger rates and lifetimes.

Findings

28% of galaxies show prominent tidal features

Tidal flux fraction increases with galaxy stellar mass

Estimated tidal feature lifetime is approximately 3 Gyr

Abstract

Here we present a statistical study on tidal features around massive early-type galaxies (ETGs). Utilizing the imaging data of the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP), we measure the flux fraction of tidal features ($f_{\rm tidal}$) in 2649 ETGs with stellar mass $M_*>10^{11}M_{\odot}$ and redshift $0.05<z<0.15$ using automated techniques. The Wide-layer of HSC-SSP reaches a depth of $\sim 28.5$ mag arcsec$^{-2}$ in $i$-band. Under this surface brightness limit, we find that about 28 % of these galaxies harbor prominent tidal features with $f_{\rm tidal}>1\%$, among which the number of ETGs decreases exponentially with $f_{\rm tidal}$, with a logarithmic slope of $\sim100$. Within the stellar mass range we probe, we note that $f_{\rm tidal}$ increases by a factor of 2 from $M_*\approx10^{11}M_{\odot}$ to $M_*\approx10^{12}M_{\odot}$. We also perform pair-count to estimate the merger rate of these massive ETGs. Combining the merger rates with $f_{\rm tidal}$, we estimate that the typical lifetime of tidal features is $\sim$ 3 Gyr, consistent with previous studies.