The Fundamental Plane of Cluster Elliptical Galaxies at z=1.25
Rosati, P.; Rix, H. -W.; van der Wel, A.; van Dokkum, P.; Illingworth, G. D.; Blakeslee, J. P.; Franx, M.; Holden, B. P.; Postman, M.; Ford, H.; Magee, D.
United States, Netherlands, Germany
Abstract
Using deep Hubble Space Telescope Advanced Camera for Surveys imaging and Very Large Telescope FOcal Reducer/low dispersion Spectrograph 2 spectra, we determined the velocity dispersions, effective radii, and surface brightnesses for four early-type galaxies in the z=1.237 cluster RDCS 1252.9-2927. All four galaxies are massive, greater than 1011 Msolar. These four galaxies, combined with three from RDCS 0848+4453 at z=1.276, establish the fundamental plane of massive early-type cluster galaxies at z=1.25. The offset of the fundamental plane shows that the luminosity evolution in rest-frame B is Δln(M/LB)=(-0.98+/-0.06)z for galaxies with M>1011.5 Msolar. To reproduce the observed mass-to-light ratio (M/L) evolution, we determine the characteristic age of the stars in these M>1011.5 Msolar galaxies to be 3.0+0.3-0.3 Gyr; i.e., z*=3.4+0.5-0.4. Including selection effects caused by morphological bias (the ``progenitor bias''), we estimate an age of 2.1+0.2-0.2 Gyr, or z*=2.3+0.2-0.2 for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived M/L values, which is confirmed by the spread in the galaxies' colors. Two lower mass galaxies in our z=1.25 sample have much lower M/L values, implying significant star formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, M~=1011 Msolar, galaxies at z~=1.5.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Also based on observations obtained at the European Southern Observatory using the ESO Very Large Telescope on Cerro Paranal (ESO programme 169.A-0458).