Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007aas...21112303m&link_type=abstract
American Astronomical Society, AAS Meeting #211, #123.03; Bulletin of the American Astronomical Society, Vol. 39, p.953
Astronomy and Astrophysics
Astronomy
Scientific paper
We present results from a detailed study of massive (>1011 Msun), passively evolving galaxies at z 1.5 that are the progenitors to present-day giant elliptical galaxies and represent the earliest major episodes of star formation in the Universe. Comparisons of broad-band photometry with improved stellar synthesis models yield ages for the stellar populations in these galaxies that indicate star formation was complete more than 1 Gyr before the observed epoch. Rest-frame UV spectra corroborate the age estimates from the photometry and show little evidence for recent star formation. The prevalence of old stellar populations, however, does not correlate exclusively with early-type morphologies as it does in the local Universe; light profiles for some of these galaxies appear to be dominated by massive exponential disks. This evidence for massive, old disks, along with the uniformity of stellar age across the disk, suggests formation by a mechanism much more similar to monolithic collapse than hierarchical merging. These old disks could not have undergone a single major merging event since the bulk of their stars were formed. There is at least one case, however, that appears to be undergoing a "dry merger", which may be an example of the process that converts these unusual galaxies into the familiar spheroids that dominate galaxies comprising old stellar populations at the present epoch.
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