First Results from the 3D-HST Survey: The Striking Diversity of Massive Galaxies at z>1

TL;DR

The 3D-HST survey reveals that massive galaxies at z>1 exhibit a wide range of properties, with many actively forming stars and diverse structures, contrasting with the uniformity seen in the local universe.

Contribution

This study provides the first spectroscopic results showing the diversity of massive galaxies at z>1, highlighting differences in star formation and structure.

Findings

59% of galaxies have high Halpha equivalent widths indicating active star formation

Quiescent galaxies are compact with high Sersic index and velocity dispersion

Star-forming galaxies are large, spiral, with low Sersic index

Abstract

We present first results from the 3D-HST program, a near-IR spectroscopic survey performed with the Wide Field Camera 3 on the Hubble Space Telescope. We have used 3D-HST spectra to measure redshifts and Halpha equivalent widths for a stellar mass-limited sample of 34 galaxies at 1<z<1.5 with M(stellar)>10^11 M(sun) in the COSMOS, GOODS, and AEGIS fields. We find that a substantial fraction of massive galaxies at this epoch are forming stars at a high rate: the fraction of galaxies with Halpha equivalent widths >10 A is 59%, compared to 10% among SDSS galaxies of similar masses at z=0.1. Galaxies with weak Halpha emission show absorption lines typical of 2-4 Gyr old stellar populations. The structural parameters of the galaxies, derived from the associated WFC3 F140W imaging data, correlate with the presence of Halpha: quiescent galaxies are compact with high Sersic index and high inferred velocity dispersion, whereas star-forming galaxies are typically large two-armed spiral galaxies, with low Sersic index. Some of these star forming galaxies might be progenitors of the most massive S0 and Sa galaxies. Our results challenge the idea that galaxies at fixed mass form a homogeneous population with small scatter in their properties. Instead we find that massive galaxies form a highly diverse population at z>1, in marked contrast to the local Universe.