Constraints on the Assembly and Dynamics of Galaxies: I. Detailed Rest-frame Optical Morphologies on Kiloparsec-scale of z ~ 2 Star-forming Galaxies

TL;DR

This study combines high-resolution optical imaging and spectroscopic data to analyze the detailed morphologies and structures of star-forming galaxies at z~2, revealing their irregular, clumpy nature and insights into mass assembly.

Contribution

It provides the first detailed analysis of rest-frame optical morphologies of z~2 star-forming galaxies using combined imaging and spectroscopic data, revealing their clumpy, irregular structures and mass distribution.

Findings

Galaxies have shallow profiles with median effective radii of ~5kpc.

Rest-frame optical emission is dominated by stellar mass distribution, not line-emitting gas.

Galaxies exhibit low Sersic indices, low Gini, and high Psi and M20 coefficients.

Abstract

We present deep and high-resolution HST/NIC2 F160W imaging at 1.6micron of six z~2 star-forming galaxies with existing near-IR integral field spectroscopy from SINFONI at the VLT. The unique combination of rest-frame optical imaging and nebular emission-line maps provides simultaneous insight into morphologies and dynamical properties. The overall rest-frame optical emission of the galaxies is characterized by shallow profiles in general (Sersic index n<1), with median effective radii of ~5kpc. The morphologies are significantly clumpy and irregular, which we quantify through a non-parametric morphological approach, estimating the Gini (G), Multiplicity (Psi), and M_20 coefficients. The strength of the rest-frame optical emission lines in the F160W bandpass indicates that the observed structure is not dominated by the morphology of line-emitting gas, and must reflect the underlying stellar mass distribution of the galaxies. The sizes and structural parameters in the rest-frame optical continuum and Halpha emission reveal no significant differences, suggesting similar global distributions of the on-going star formation and more evolved stellar population. While no strong correlations are observed between stellar population parameters and morphology within the NIC2/SINFONI sample itself, a consideration of the sample in the context of a broader range of z~2 galaxy types indicates that these galaxies probe the high specific star formation rate and low stellar mass surface density part of the massive z~2 galaxy population, with correspondingly large effective radii, low Sersic indices, low G, and high Psi and M_20. The combined NIC2 and SINFONI dataset yields insights of unprecedented detail into the nature of mass accretion at high redshift. [Abridged]