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Session 83 - Elliptical Galaxies.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[83.07] Age Dating Elliptical Galaxies via Near-IR AGB Light

D. R. Silva (NOAO/KPNO), G. D. Bothun (U. of Oregon)



We have obtained JHK images for 45 bright, nearby elliptical galaxies with a range of ``fine structure'' (ripples, shells, boxy isophotes, etc.) strengths. Our sample includes ellipticals argued to contain a significant ``young'' (t \lae 5 Gyr) stellar population component based on their strong nuclear H\beta strengths (Schweizer et al., 1990, ApJ, 364, L33; González 1993, Ph.D. thesis, UCSC) and blue global UBV colors (Schweizer amp; Seitzer, 1992, AJ, 104, 1039). Since intermediate-age (t \approx 1 -- 3 Gyr) star clusters have redder integrated H -- K colors than old clusters (Persson et al. 1983, 266, 105), ellipticals with a significant intermediate-age stellar component should have redder H--K colors than ellipticals which do not.

Only two of the observed ellipticals, NGC 3610 and NGC 5322, show significantly red H--K colors but only on the sub-kpc scale in their central regions. Both galaxies were classified as ``young'' by Schweizer amp; Seitzer (1992). The results of a merger driven nuclear starburst (Mihos, Richstone, amp; Bothun 1992, ApJ, 377, 72) may have been detected in these galaxies. However, these colors are also consistent with the presence of nuclear dust, as suggested by radiative transfer models of dusty ellipticals (Wise amp; Silva 1996, ApJ, in press; Silva amp; Wise 1996, ApJLet, in press). Nuclear dust has been detected in NGC 5322 by HST imagery (Lauer et al., 1995, AJ, in press). Delineating between stellar population and dust induced effects may require near-IR spectroscopy.

The outer region near-IR colors of these ``red core'' galaxies, as well as the global near-IR colors of the rest of our sample, are consistent with the presence of an old, metal-rich stellar component which dominates the K-band integrated light (cf. Frogel et al. 1978, ApJ, 220, 75). Ellipticals classified as ``young'' in the studies cited above have similar near-IR colors to ellipticals classified as ``old''. We conclude the bulk of the mass (\gae 90%) in these putatively ``young'' ellipticals must be old and metal-rich (t > 10 Gyr, [Fe/H] > --0.5), i.e. the bulk of their stars formed long before their most recent merger event.

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