Thick Disks in the Hubble Space Telescope Frontier Fields

TL;DR

This study analyzes the structure of thick disks in high-redshift galaxies using Hubble Space Telescope data, revealing scale heights proportional to galaxy mass and evidence for a two-component disk structure.

Contribution

It provides the first detailed measurements of thick disk scale heights in distant galaxies, highlighting their dependence on galaxy mass and evidence for thin and thick disk components.

Findings

Scale heights are proportional to galaxy mass.

Evidence for a two-component disk structure.

Color differential suggests a mixture of thin and thick disks.

Abstract

Thick disk evolution is studied using edge-on galaxies in two Hubble Space Telescope Frontier Field Parallels. The galaxies were separated into 72 clumpy types and 35 spiral types with bulges. Perpendicular light profiles in F435W, F606W and F814W (B, V and I) passbands were measured at 1 pixel intervals along the major axes and fitted to sech^2 functions convolved with the instrument line spread function (LSF). The LSF was determined from the average point spread function (PSF) of ~20 stars in each passband and field, convolved with a line of uniform brightness to simulate disk blurring. A spread function for a clumpy disk was also used for comparison. The resulting scale heights were found to be proportional to galactic mass, with the average height for a 10^9.5-10^10.5 Msun galaxy at z=1.5-2.5 equal to 0.63+-0.24 kpc. This value is probably the result of a blend between thin and thick disk components that cannot be resolved. Evidence for such two-component structure is present in an inverse correlation between height and midplane surface brightness. Models suggest that the thick disk is observed best between the clumps, and there the average scale height is 1.06+-0.43 kpc for the same mass and redshift. A 0.63+-0.68 mag V-I color differential with height is also evidence for a mixture of thin and thick components.