Morphological evolution of galaxies from ultradeep HST WFC3 imaging: the Hubble sequence at z~2

TL;DR

This study investigates the diverse morphologies and color gradients of galaxies at z~2 using ultradeep HST imaging, revealing inside-out growth patterns and early Hubble sequence relations.

Contribution

It provides new insights into galaxy morphology, color gradients, and structural correlations at z~2, supporting early establishment of the Hubble sequence.

Findings

Most z~2 galaxies have red cores, indicating older stellar populations.

Galaxy morphology correlates with specific star formation rate at z~2.

Color gradients are consistent across redshifts, supporting inside-out growth.

Abstract

We use ultradeep HST WFC3/IR imaging of the HUDF to investigate the rest-frame optical morphologies of a mass-selected sample of galaxies at z~2. We find a large variety of galaxy morphologies, ranging from large, blue, disk-like galaxies to compact, red, early-type galaxies. We derive rest-frame u-g color profiles for these galaxies and show that most z~2 galaxies in our sample have negative color gradients such that their cores are red. Although these color gradients may partly be caused by radial variations in dust content, they point to the existence of older stellar populations in the centers of z~2 galaxies. This result is consistent with an "inside-out" scenario of galaxy growth. We find that the median color gradient is fairly constant with redshift: [Delta(u-g_{rest})/Delta(log r)]_{median} = -0.46, -0.44 and -0.49 for z~2, z~1 and z=0, respectively. Using structural parameters derived from surface brightness profiles we confirm that at z~2 galaxy morphology correlates well with specific star formation rate. At the same mass, star forming galaxies have larger effective radii, bluer rest-frame u-g colors and lower Sersic indices than quiescent galaxies. These correlations are very similar to those at lower redshift, suggesting that the relations that give rise to the Hubble sequence at z=0 are already in place for massive galaxies at this early epoch.