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Session 49 - Elliptical Galaxies & Bulges.
Display session, Thursday, January 08
Exhibit Hall,

[49.02] Mid-UV Determination of Ages and Metallicites of Elliptical Galaxies

J. Lotz (STScI, Johns Hopkins University), H. C. Ferguson (STScI)

Using simple population synthesis models, we have examined the mid-UV features of 11 nearby ellipticals and spiral bulges. The mid-UV (1800-3200 Å\ ) for evolved stellar populations is dominated by light from the turn-off of the main sequence, unlike the optical region of the spectrum which is governed by the effective temperature of the red giant branch. By looking in the mid-UV and at the main sequence stars, we may break ``age-metallicity degeneracy'' (Worthey 1994) that plaques optical spectral synthesis models of evolved populations. In order to examine the spectral properties of only the main sequence stars, we have excluded P-AGB and EHB stars from our models' isochrones and have attempted to subtract out the hot component from the galaxies' spectra due to these far-UV emitting stars. Specifically, we measure the strength of the blend of FeI, SiI and SiIII at 2538 Å\ (Bl2538) and introduce a new tool for examining the mid-UV continua, the slope of the continuum between 2600 and 3100 Å\ (S2850). We find that populations older that 3 Gyr separate according to metallicity in the S2850 v. Bl2538 plane. Although globular cluster spectra agree with our single metallicity, single age populations, the spectral features of NGC 1399 require a multi-metallicity model including a small ( \leq 10 % by mass) but mid-UV bright sub-component of low metallicity (Z \leq 0.001) main sequence stars.

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Program listing for Thursday