A Population of Massive and Evolved Galaxies at z$\ga$5

TL;DR

This study identifies a rare population of massive, evolved galaxies at redshifts greater than 5, revealing their properties, formation history, and implications for early galaxy evolution.

Contribution

It introduces a novel color selection technique based on the Balmer break to find high-redshift evolved galaxies and provides the first estimates of their number density and stellar mass density.

Findings

11 candidate galaxies with z=4.9-6.5 and old stellar populations.

Most stars formed at z>9, indicating early galaxy formation.

Detected galaxies are very compact, with sizes <2 kpc.

Abstract

We report results from a search for massive and evolved galaxies at z>5 in the GOODS southern field. Combining HST ACS, VLT ISAAC and Spitzer IRAC photometric data, we develop a color selection technique to identify candidates for being evolved galaxies at high redshifts. The color selection is primarily based on locating the Balmer-break using the K- and 3.6micron bands. Stellar population synthesis models are fitted to the SEDs of these galaxies to identify the final sample. We find 11 candidates with photometric redshifts in the range 4.9 < z < 6.5, dominated by an old stellar population, with ages 0.2-1.0 Gyr, and stellar masses in the range (0.5 - 5) 10^{11} Msun. The majority of the stars in these galaxies were formed at z > 9. One candidate has a spectroscopically confirmed redshift, in good agreement with our photometric redshift. The galaxies are very compact, with half-light radii in the observed K-band smaller than ~2 kpc. Seven of the 11 candidates are also detected at 24micron with the MIPS instrument on Spitzer. The 24micron emission could be interpreted as PAH emission from a dusty starburst at z~2-3, however, it is also consistent with the presence of an obscured AGN at z>5. We estimate the completeness of the Balmer break galaxy sample to be ~40%. The comoving number density of galaxies with a stellar mass >10^{11} Msun, at an average redshift z=5.2, is 3.9 10^{-5} Mpc^{-3} (no-MIPS sample: 1.4 10^{-5} Mpc^{-3}). The corresponding stellar mass density is 8 10^{6} Msun/Mpc^3 (no-MIPS sample: 6.2 10^6 Msun/Mpc^3).