Dependence of the Fundamental Plane of Early-type Galaxies on Age and Internal Structure

TL;DR

This study examines how the scatter in the fundamental plane of early-type galaxies varies with age and internal structure, revealing that younger, more compact galaxies exhibit larger scatter linked to recent merger activity.

Contribution

It provides new insights into the dependence of fundamental plane scatter on galaxy age and structure, highlighting the role of recent gas-rich mergers in young, compact ETGs.

Findings

Older ETGs have smaller FP scatter (~0.06 dex) compared to younger ones (~0.075 dex).

Less compact young ETGs show smaller scatter (~0.065 dex) than more compact ones (~0.10 dex).

Recent gas-rich mergers explain the properties of compact young ETGs with low mass-to-light ratios and blue centers.

Abstract

We investigate the scatter in the fundamental plane (FP) of early-type galaxies (ETGs) and its dependence on age and internal structure of ETGs, using $16,283$ ETGs with $M_r\le-19.5$ and $0.025\le z<0.055$ in Sloan Digital Sky Survey data. We use the relation between the age of ETGs and photometric parameters such as color, absolute magnitude, and central velocity dispersion of ETGs and find that the scatter in the FP depends on age. The FP of old ETGs with age $\gtrsim9$ Gyrs has a smaller scatter of $\sim0.06$ dex ($\sim14\%$) while that of young ETGs with age $\lesssim6$ Gyrs has a larger scatter of $\sim0.075$ dex ($\sim17\%$). In the case of young ETGs, less compact ETGs have a smaller scatter in the FP ($\sim0.065$ dex; $\sim15\%$) than more compact ones ($\sim0.10$ dex; $\sim23\%$). On the other hand, the scatter in the FP of old ETGs does not depend on the compactness of galaxy structure. Thus, among the subpopulations of ETGs, compact young ETGs have the largest scatter in the FP. This large scatter in compact young ETGs is caused by ETGs that have low dynamical mass-to-light ratio ($M_\mathrm{dyn}/L$) and blue color in the central regions. By comparing with a simple model of the galaxy that has experienced a gas-rich major merger, we find that the scenario of recent gas-rich major merger can reasonably explain the properties of the compact young ETGs with excessive light for a given mass (low $M_\mathrm{dyn}/L$) and blue central color.