A high stellar velocity dispersion for a compact massive galaxy at z=2.2

TL;DR

This study measures the stellar velocity dispersion of a compact galaxy at z=2.2, providing evidence for its extreme dynamical state and supporting theories of significant size evolution of massive galaxies over cosmic time.

Contribution

It presents the first stellar velocity dispersion measurement for a high-redshift compact galaxy, confirming its high velocity and supporting galaxy evolution models.

Findings

Velocity dispersion of 510 km/s supports compactness and mass estimates.

High velocity dispersion indicates significant dynamical evolution.

Results align with theories of galaxy size growth over 10 billion years.

Abstract

Recent studies have found that the oldest and most luminous galaxies in the early Universe are surprisingly compact, having stellar masses similar to present-day elliptical galaxies but much smaller sizes. This finding has attracted considerable attention as it suggests that massive galaxies have grown by a factor of ~five in size over the past ten billion years. A key test of these results is a determination of the stellar kinematics of one of the compact galaxies: if the sizes of these objects are as extreme as has been claimed, their stars are expected to have much higher velocities than those in present-day galaxies of the same mass. Here we report a measurement of the stellar velocity dispersion of a massive compact galaxy at redshift z=2.186, corresponding to a look-back time of 10.7 billion years. The velocity dispersion is very high at 510 (+165, -95) km/s, consistent with the mass and compactness of the galaxy inferred from photometric data and indicating significant recent structural and dynamical evolution of massive galaxies. The uncertainty in the dispersion was determined from simulations which include the effects of noise and template mismatch. However, we caution that some subtle systematic effect may influence the analysis given the low signal-to-noise ratio of our spectrum.