Star-Forming, Recently Star-Forming, and “Red and Dead” Galaxies at 1 < Z < 2

Abstract

We summarize some of the key results on red galaxies presented in the first five papers from the Gemini Deep Deep Survey (GDDS). The GDDS is an ultra-deep (K < 20.6 mag, I < 24.5 mag) spectroscopic redshift survey, which preferentially targeted galaxies in the redshift range 0.8 < z < 2. The survey was completed in 2004, and the data is publicly available. The primary goal of the GDDS was to make the first direct measurement of the evolving stellar mass function over 0.8 < z < 2. We find that ∼ 15% of the local stellar mass density is already in place by z = 1.8, rising to 40-50% by z = 1. Nearly half the stellar mass density at z = 1.8 is in massive “red-and-dead” galaxies. Almost all of these red-and-dead galaxies exhibit early-type morphologies, although around 20% of these show tidal distortions consistent with recent (dry?) merger activity. A decomposition of the star-formation history of the Universe into the sum of individual histories for mass-segregated galaxy populations reveals strong evidence for the “down-sizing” paradigm espoused by Cowie et al. (1997). We find that the most massive galaxies form early in the history of the Universe and galaxy formation proceeds from larger to smaller mass scales.

Gemini Observatory