The Gemini Deep Deep Survey. VIII. When Did Early‐Type Galaxies Form?

We have used the Hubble Space Telescope's Advanced Camera for Surveys (Fordet al. 2003) to measure the cumulative mass density in morphologically-selectedearly-type galaxies over the redshift range 0.8 < z < 1.7. Our imaging data setcovers four well-separated sight-lines, and is roughly intermediate (in termsof both depth and area) between the GOODS/GEMS imaging data, and the imagesobtained in the Hubble Deep Field campaigns. Our images contain 144 galaxieswith ultra-deep spectroscopy obtained as part of the Gemini Deep Deep Survey.These images have been analyzed using a new purpose-written morphologicalanalysis code which improves the reliability of morphological classificationsby adopting a 'quasi-Petrosian' image thresholding technique. We find that at z\~ 1 about 80% of the stars living in the most massive galaxies reside inearly-type systems. This fraction is similar to that seen in the localUniverse. However, we detect very rapid evolution in this fraction over therange 0.8 < z < 1.7, suggesting that over this redshift range the strongmorphology-mass relationship seen in the nearby Universe is beginning to fallinto place. By comparing our images to published spectroscopic classifications,we show that little ambiguity exists in connecting spectral classes tomorphological classes for spectroscopically quiescent systems. However, themass density function of early-type galaxies is evolving more rapidly than thatof spectroscopically quiescent systems, which we take as further evidence thatwe are witnessing the formation of massive early-type galaxies over the 0.8 < z< 1.7 redshift range.Comment: 31 pages, 18 figures (6 pages with 4 figures added to appendix), Accepted by Astrophysical Journa