Structure and Formation of Massive Galaxies with Old Stellar Populations at z=1.5

Details Structure and Formation of Massive Galaxies with Old Stellar Populations at z=1.5 Structure and Formation of Massive Galaxies with Old Stellar Populations at z=1.5

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006aas...20911203m&link_type=abstract

2007 AAS/AAPT Joint Meeting, American Astronomical Society Meeting 209, #112.03; Bulletin of the American Astronomical Society,

Astronomy and Astrophysics

Astronomy

Scientific paper

Observational evidence has been mounting over the past decade that at least some luminous ( 2 L*) galaxies at high redshift have formed nearly all of their stars within the first billion years after the big bang. These are examples of the first major episodes of star formation in the universe and provide insights into the formation of the earliest massive galaxies. We have examined in detail the morphologies and stellar populations of seven z=1.5 passively evolving galaxies using high resolution HST NICMOS and ACS imaging data as well as medium resolution Keck spectroscopy. Almost all of these galaxies appear to be relaxed systems, with smooth morphologies at both rest-frame UV and visible wavelengths. Furthermore, spectral synthesis modeling favors a single burst of star formation more than 2 Gyr before the observed epoch. We note, however, that the prevalence of old stellar populations does not necessarily correlate with early-type morphologies, as the light profiles for several 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 better described as a form of monolithic collapse than as a hierarchical merger. 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.
We acknowledge our collaborators in the HST observations, Gabriela Canalizo, Masanori Iye, and Toshinori Maihara. This research was supported by NSF grant AST03-07335 and HST grant GO-10418.01-A.

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