AAS 203rd Meeting, January 2004
Session 91 Galaxy Surveys
Poster, Wednesday, January 7, 2004, 9:20am-6:30pm, Grand Hall

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[91.07] The Magnesium-Velocity Dispersion Relation for Early-type Galaxies

G. Worthey (Washington State), M. Collobert (Oxford)

Data for magnesium absorption feature indices and velocity dispersion for about 2000 early type galaxies is analyzed. We include Mg1, Mg2, and Mg b definitions. As noted previously, the Mg residuals from a fitted line are roughly Gaussian near the median but have an asymmetric blue tail, probably from subpopulations of relatively young stars. We define robust statistics and compare the observations to models of various merging scenarios. Most models are eliminated. All models with merger rates proportional to (1+z)n are eliminated if n>0. Models that approximate early monolithic collapse followed by drizzles of star formation are eliminated. Models with too few (less than about 20) or too many (more than 100) mergers do not work. We could not find a heirarchical cold dark matter model that worked. Successful models have about 50 mergers and gas fractions per merger of about 0.25. This suggests that disk-disk mergers are not a dominant formation mechanism for elliptical or lenticular galaxies. Mg-index strength as a function of lookback time is indistinguishable from passive evolution of a monolithic collapse in our simulations, and we suggest that Mg zero-point is not a good test of galaxy evolution.

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.