Differential Morphology Between Rest-frame Optical and UV Emission from 1.5 < z < 3 Star-forming Galaxies

TL;DR

This study compares the rest-frame optical and UV morphologies of star-forming galaxies at redshifts 1.5 to 3, revealing small but significant differences linked to stellar populations and galaxy evolution processes.

Contribution

It introduces the use of internal color dispersion (ICD) as a diagnostic tool to quantify morphological differences between UV and optical images of high-redshift galaxies, highlighting its effectiveness.

Findings

Most star-forming galaxies show small but non-zero ICD values.

Larger ICDs are associated with complex, merging, or forming galaxies.

ICD correlates with galaxy size but not with UV-optical color.

Abstract

We present the results of a comparative study of the rest-frame optical and rest-frame ultraviolet morphological properties of 117 star-forming galaxies (SFGs), including BX, BzK, and Lyman break galaxies with B<24.5, and 15 passive galaxies in the region covered by the Wide Field Camera 3 Early Release Science program. Using the internal color dispersion (ICD) diagnostic, we find that the morphological differences between the rest-frame optical and rest-frame UV light distributions in 1.4<z<2.9 SFGs are typically small (ICD~0.02). However, the majority are non-zero (56% at >3 sigma) and larger than we find in passive galaxies at 1.4<z<2, for which the weighted mean ICD is 0.013. The lack of morphological variation between individual rest-frame ultraviolet bandpasses in z~3.2 galaxies argues against large ICDs being caused by non-uniform dust distributions. Furthermore, the absence of a correlation between ICD and galaxy UV-optical color suggests that the non-zero ICDs in SFGs are produced by spatially distinct stellar populations with different ages. The SFGs with the largest ICDs (>~0.05) generally have complex morphologies that are both extended and asymmetric, suggesting that they are mergers-in-progress or very large galaxies in the act of formation. We also find a correlation between half-light radius and internal color dispersion, a fact that is not reflected by the difference in half-light radii between bandpasses. In general, we find that it is better to use diagnostics like the ICD to measure the morphological properties of the difference image than it is to measure the difference in morphological properties between bandpasses.