The MASSIVE Survey - VIII. Stellar Velocity Dispersion Profiles and Environmental Dependence of Early-Type Galaxies

TL;DR

This study measures stellar velocity dispersion profiles of 90 early-type galaxies, revealing that more massive and environmentally dense galaxies tend to have rising outer dispersion profiles, indicating variations in their mass distribution.

Contribution

It provides the first extensive analysis of radial velocity dispersion profiles in a large, volume-limited sample of massive early-type galaxies, highlighting environmental influences on their kinematic structures.

Findings

36% of galaxies have rising outer dispersion profiles

Outer profile slopes correlate with galaxy mass and environment

Rising profiles linked to variations in total mass distribution

Abstract

We measure the radial profiles of the stellar velocity dispersions, $\sigma(R)$, for 90 early-type galaxies (ETGs) in the MASSIVE survey, a volume-limited integral-field spectroscopic (IFS) galaxy survey targeting all northern-sky ETGs with absolute $K$-band magnitude $M_K < -25.3$ mag, or stellar mass $M_* > 4 \times 10^{11} M_\odot$, within 108 Mpc. Our wide-field 107" $\times$ 107" IFS data cover radii as large as 40 kpc, for which we quantify separately the inner (2 kpc) and outer (20 kpc) logarithmic slopes $\gamma_{\rm inner}$ and $\gamma_{\rm outer}$ of $\sigma(R)$. While $\gamma_{\rm inner}$ is mostly negative, of the 56 galaxies with sufficient radial coverage to determine $\gamma_{\rm outer}$ we find 36% to have rising outer dispersion profiles, 30% to be flat within the uncertainties, and 34% to be falling. The fraction of galaxies with rising outer profiles increases with $M_*$ and in denser galaxy environment, with 10 of the 11 most massive galaxies in our sample having flat or rising dispersion profiles. The strongest environmental correlations are with local density and halo mass, but a weaker correlation with large-scale density also exists. The average $\gamma_{\rm outer}$ is similar for brightest group galaxies, satellites, and isolated galaxies in our sample. We find a clear positive correlation between the gradients of the outer dispersion profile and the gradients of the velocity kurtosis $h_4$. Altogether, our kinematic results suggest that the increasing fraction of rising dispersion profiles in the most massive ETGs are caused (at least in part) by variations in the total mass profiles rather than in the velocity anisotropy alone.