Gemini K-band NIRI Adaptive Optics Observations of Massive Galaxies at 1 < z < 2

TL;DR

This study uses high-resolution adaptive optics imaging to analyze the structure of massive galaxies at redshifts 1 to 2, revealing their compact sizes, dense cores, and evidence of ongoing minor mergers contributing to their growth.

Contribution

First detailed stellar mass surface density profiles of individual massive galaxies at 1<z<2, highlighting their compactness and merger-driven growth mechanisms.

Findings

Massive galaxies are significantly more compact at z~1-2 than today.

Inner regions are over-dense, outer regions under-dense compared to local counterparts.

Approximately 38% of galaxies show signs of recent dynamical activity.

Abstract

We present deep K-band adaptive-optics observations of eight very massive (M* ~ 4 x 10^11 Msun) galaxies at 1 < z < 2 utilizing the Gemini NIRI/Altair Laser Guide System. These systems are selected from the Palomar Observatory Wide-Field Infrared (POWIR) survey, and are amongst the most massive field galaxies at these epochs. The depth and high spatial resolution of our images allow us to explore for the first time the stellar mass surface density distribution of massive distant galaxies from 1 to 15 kpc on an individual galaxy basis, rather than on stacked images. We confirm that some of these massive objects are extremely compact with measured effective radii between 0."1 - 0."2, giving sizes which are < 2 kpc, a factor of ~ 7 smaller in effective radii than similar mass galaxies today. Examining stellar mass surface densities as a function of fixed physical aperture, we find an over-density of material within the inner profiles, and an under-density in the outer profile, within these high-z galaxies compared with similar mass galaxies in the local universe. Consequently, massive galaxies should evolve in a way to decrease the stellar mass density in their inner region, and at the same time creating more extensive outer light envelopes. We furthermore show that ~ 38% +- 20% of our sample contains evidence for a disturbed outer stellar matter distribution suggesting that these galaxies are undergoing a recent dynamical episode, such as a merger or accretion event. We calculate that massive galaxies at z < 2 will undergo on the order of five of these events, a much higher rate than observed for major mergers, suggesting that these galaxies are growing in size and stellar mass in part through minor mergers during this epoch.