The Hubble Sequence beyond z=2 for Massive Galaxies: Contrasting Large Star-Forming and Compact Quiescent Galaxies

TL;DR

This study uses Hubble Space Telescope imaging to analyze the morphologies of massive galaxies at z~2.3, revealing two distinct classes—large star-forming and compact quiescent galaxies—and suggesting an early Hubble sequence at high redshift.

Contribution

It extends previous work by comparing star-forming and quiescent massive galaxies at z>2, highlighting their structural differences and potential evolutionary links.

Findings

Large star-forming galaxies have median re = 2.8 kpc.

Compact quiescent and AGN host galaxies are similarly small (~1.1 kpc).

Two galaxy classes are identified in the color-mass diagram.

Abstract

We present Hubble Space Telescope NIC2 morphologies of a spectroscopic sample of massive galaxies at z~2.3, by extending our sample of 9 compact quiescent galaxies (r_e~0.9 kpc) with 10 massive emission-line galaxies. The emission-line galaxies are classified by the nature of their ionized emission; there are six star-forming galaxies and four galaxies hosting an active galactic nucleus (AGN). The star-forming galaxies are the largest among the emission-line galaxies, with a median size of re = 2.8 kpc. The three galaxies with the highest star formation rates (> 100M \odot/yr) have irregular and clumpy morphologies. The AGN host galaxies are more similar to the compact quiescent galaxies in terms of their structures (re~1.1 kpc) and spectral energy distributions. The total sample clearly separates into two classes in a color-mass diagram: the large star-forming galaxies that form the blue cloud, and the compact quiescent galaxies on the red sequence. However, it is unclear how or even if the two classes are evolutionary related. Three out of six massive star-forming galaxies have dense cores and thus may passively evolve into compact galaxies due to fading of outer star-forming regions. For these galaxies a reverse scenario, in which compact galaxies grow inside-out by star formation is also plausible. We do caution though that the sample is small. Nonetheless, it is evident that a Hubble sequence of massive galaxies with strongly correlated galaxy properties is already in place at z > 2.