Structure and Kinematics of Early-Type Galaxies from Integral-Field Spectroscopy

TL;DR

This study uses integral-field spectroscopy to classify early-type galaxies into fast and slow rotators, revealing their distinct structures, dynamics, and evolutionary pathways, and establishing links between their properties, mass, and environment.

Contribution

It demonstrates the effectiveness of integral-field spectroscopy in distinguishing galaxy classes and uncovers their different formation and evolution channels based on kinematic and structural analysis.

Findings

Fast and slow rotators are separable by stellar kinematics.

Slow rotators are weakly triaxial and dominate above a critical mass.

Fast rotators resemble spiral galaxies below the critical mass.

Abstract

Observations of galaxy isophotes, longs-slit kinematics and high-resolution photometry suggested a possible dichotomy between two distinct classes of E galaxies. But these methods are expensive for large galaxy samples. Instead, integral-field spectroscopic can efficiently recognize the shape, dynamics and stellar population of complete samples of early-type galaxies (ETGs). These studies showed that the two main classes, the fast and slow rotators, can be separated using stellar kinematics. We showed there is a dichotomy in the dynamics of the two classes. The slow rotators are weakly triaxial and dominate above $M_{\rm crit}\approx2\times10^{11} M_\odot$. Below $M_{\rm crit}$, the structure of fast rotators parallels that of spiral galaxies. There is a smooth sequence along which, the metals content, the enhancement in $\alpha$-elements, and the "weight" of the stellar initial mass function, all increase with the CENTRAL mass density slope, or bulge mass fraction, while the molecular gas fraction correspondingly decreases. The properties of ETGs on galaxy scaling relations, and in particular the $(M_{\ast}, R_{\rm e})$ diagram, and their dependence on environment, indicate two main independent channels for galaxy evolution. Fast rotators ETGs start as star forming disks and evolve trough a channel dominated by gas accretion, bulge growth and quenching. While slow rotators assemble near the center of massive halos via intense star formation at high redshift, and remain as such for the rest of their evolution via a channel dominated by gas poor mergers. This is consistent with independent studies of the galaxies redshift evolution.