Evolution of the Sizes of Galaxies over 7<z<12 Revealed by the 2012 Hubble Ultra Deep Field Campaign

TL;DR

This study uses deep Hubble data to measure galaxy sizes at redshifts 7-12, confirming they are extremely small and decrease with increasing redshift, revealing how early galaxies evolved in size over cosmic time.

Contribution

It provides the first robust size measurements of galaxies at z>8 and extends the size-redshift relation beyond z~8 using improved deep imaging data.

Findings

Average galaxy sizes at z=7-12 are 0.3-0.4 kpc.

Galaxy sizes decrease with increasing redshift, following a (1+z)^s trend with s=-1.28.

A size-luminosity relation consistent with constant star formation surface density is observed.

Abstract

We analyze the redshift- and luminosity-dependent sizes of dropout galaxy candidates in the redshift range z~7-12 using deep images from the UDF12 campaign, data which offers two distinct advantages over that used in earlier work. Firstly, we utilize the increased S/N ratio offered by the UDF12 imaging to provide improved size measurements for known galaxies at z=6.5-8 in the HUDF. Specifically, we stack the new deep F140W image with the existing F125W data in order to provide improved measurements of the half-light radii of z-dropouts. Similarly we stack this image with the new deep UDF12 F160W image to obtain new size measurements for a sample of Y-dropouts. Secondly, because the UDF12 data have allowed the construction of the first robust galaxy sample in the HUDF at z>8, we have been able to extend the measurement of average galaxy size out to significantly higher redshifts. Restricting our size measurements to sources which are now detected at >15sigma, we confirm earlier indications that the average half-light radii of z~7-12 galaxies are extremely small, 0.3-0.4 kpc, comparable to the sizes of giant molecular associations in local star-forming galaxies. We also confirm that there is a clear trend of decreasing half-light radius with increasing redshift, and provide the first evidence that this trend continues beyond z~8. Modeling the evolution of the average half-light radius as a power-law (1+z)^s, we obtain a best-fit index of s=-1.28+/-0.13 over the redshift range z~4-12, mid-way between the physically expected evolution for baryons embedded in dark halos of constant mass (s=-1) and constant velocity (s=-1.5). A clear size-luminosity relation, such as that found at lower redshift, is also evident in both our z- and Y-dropout sample. This relation can be interpreted in terms of a constant surface density of star formation over a range in luminosity of 0.05-1.0L*_z=3.(abridged)