The Majority of Compact Massive Galaxies at z~2 are Disk Dominated (1101.2423v3)

Abstract: We investigate the stellar structure of massive, quiescent galaxies at z~2, based on Hubble Space Telescope/WFC3 imaging from the Early Release Science program. Our sample of 14 galaxies has stellar masses of M* > 10^{10.8} Msol and photometric redshifts of 1.5 < z < 2.5. In agreement with previous work, their half-light radii are <2 kpc, much smaller than equally massive galaxies in the present-day universe. A significant subset of the sample appears highly flattened in projection, which implies, considering viewing angle statistics, that a significant fraction of the galaxies in our sample have pronounced disks. This is corroborated by two-dimensional surface brightness profile fits. We estimate that 65% +/- 15% of the population of massive, quiescent z~2 galaxies are disk-dominated. The median disk scale length is 1.5 kpc, substantially smaller than the disks of equally massive galaxies in the present-day universe. Our results provide strong observational evidence that the much-discussed ultra-dense high-redshift galaxies should generally be thought of as disk-like stellar systems with the majority of stars formed from gas that had time to settle into a disk.

Compact Massive Galaxies at z∼2: Disk Dominance Confirmed

The paper by van der Wel et al., submitted to The Astrophysical Journal, presents a detailed examination of the internal stellar structure of massive, quiescent galaxies at a redshift of approximately z∼2. Utilizing imaging data from the Hubble Space Telescope's WFC3 Early Release Science program, the authors analyze a sample of 14 galaxies that are predominantly disk-dominated, revealing insights into their structural evolution compared to present-day massive galaxies.

Study Overview

The focus of the paper is on the compactness and morphological characteristics of galaxies that possess stellar masses greater than $10^{10.8} M_\odot$ and reside within the redshift range of 1.5 < z < 2.5. These galaxies demonstrate half-light radii smaller than 2 kpc, indicating a marked size difference compared to similarly massive galaxies observed in the local universe. The authors establish that a significant portion, approximately 65% ± 15%, of these galaxies are disk-dominated, characterized by a median disk scale length of 1.5 kpc. This is notably smaller than disks in contemporary massive galaxies, which typically have scale lengths around 4 kpc.

Implications and Conclusions

The observations undertaken by van der Wel et al. are crucial in understanding the formation mechanisms and evolutionary paths of high-redshift massive galaxies. The disk-dominated nature of these galaxies suggests that during their early stages, stellar formation processes allowed the settling of gas into disk-like structures. This contrasts with some proposed highly dissipative formation scenarios that may not allow sufficient time for disk formation. Consequently, the paper provides observational support for the proposal that high-redshift compact galaxies should be regarded as disk-like systems.

The findings have significant implications for theoretical models describing galaxy formation and evolution. Specifically, they suggest that subsequent evolutionary processes—possibly involving merger events—may account for both the growth in size and the transition from disk-dominated structures to the more bulge-dominated profiles characteristic of massive galaxies in the present universe. The transformation is corroborated by the comparable stellar densities between these z∼2 galaxies and the cores of current elliptical galaxies.

Future Research Directions

Further investigation should aim to refine our understanding of the evolutionary trajectory from these compact disk-like galaxies to the massive elliptical galaxies prevalent today. Addressing questions about the nature of the merging processes and their frequency will be essential for elucidating how disk-like structures transition to bulge-dominated systems. Additionally, increasing sample sizes and employing more advanced imaging techniques will enhance the robustness of these findings. There is also a potential for developing more sophisticated simulations to replicate the observed structures and their evolution, aiding in bridging theoretical models with observational data.

Overall, van der Wel et al.'s research contributes valuable insight into the structural dynamics and evolutionary fate of massive, quiescent galaxies in the high-redshift universe, presenting essential considerations for future investigations into galaxy evolution.