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Session 41 - Elliptical Galaxies.
Display session, Tuesday, January 14
High-resolution images of the candidate dynamically young elliptical galaxy NGC 3610 have been obtained with the WFPC2 of the Hubble Space Telescope. Our observations show that NGC 3610 contains a remarkably twisted stellar disk within 2.7'' (0.4 kpc) of the center, but no apparent dust features. The smooth, symmetric appearance of this disk and the lack of correlation with the color image suggest that it is a relaxed stellar component, possibly a disk twisted by differential precession. A search for intermediate-age globular clusters formed during merger or accretion events reveals a population of roughly 70 candidates in NGC 3610 which are 0.2 mag redder in V-I and 1.0 mag brighter in V than a typical old, metal-poor population. Both the color and magnitude differences are consistent with an age of 3 \pm 1 Gyr for these clusters, assuming solar metallicity, a Salpeter IMF, the same mass function as the old clusters, and Bruzual-Charlot stellar evolution models. Combining these results with HST observations of five other merger remnants shows generally good agreement between previous age estimates based on photometric, spectroscopic, and kinematic observations of the remnants, and age estimates based on the colors and luminosities of young and intermediate-age star clusters. Hence, the star clusters can be used to help establish a link between ongoing mergers (e.g., NGC 4038/4039), recent merger remnants (e.g., NGC 3921, NGC 7252), intermediate-age merger remnants classified as ellipticals (e.g., NGC 3610), and even old ellipticals (NGC 4486 = M87). A comparison of the candidate young/intermediate-age globular clusters with the old globular clusters in NGC 7252, NGC 3610, and M87 suggests that the typical increase in the number of clusters during a merger event is 70 %.
Program listing for Tuesday