The Effects of Dust on Broad-Band Color Gradients in Elliptical Galaxies: Centrally Concentrated Distributions

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Session 52 -- Elliptical Galaxies
Display presentation, Tuesday, 10, 1995, 9:20am - 6:30pm

[52.13] The Effects of Dust on Broad-Band Color Gradients in Elliptical Galaxies: Centrally Concentrated Distributions

David R. Silva (NOAO/KPNO), Michael W. Wise (MIT)

Space and ground--based observations have shown that many elliptical galaxies contain 10$^4$ -- 10$^6$ M$_\odot$ of dust. If the dust is very extended ($\rho_d \sim r^{-1}$), such quantities of dust will have an important impact on the extended radial broad--band color gradients observed in ellipticals. Broad--band color gradients are usually presumed to be due to underlying stellar population gradients; however, recent work has shown that, in many ellipticals, dust provides a viable alternative explanation (Witt, Thronson, \& Caputo 1992; Wise \& Silva 1994). Consequently, interpreting elliptical broad--band color gradients may require considering the competing effects of age, metallicity, and dust.

Previous theoretical studies of the effects of dust on the color properties of ellipticals have concentrated on extended distributions of dust, since these distributions are most likely to affect large--scale ($r >$ 100 pc) color gradients. However, the true dust spatial distribution in elliptical galaxies remains largely undetermined. Thus, centrally concentrated dust distributions are also possible. To explore this possibility, we have generated a grid of dusty elliptical galaxy models with dust distributions which are centrally concentrated relative to the assumed stellar distribution. The models are spherically symmetric and include a range of dust masses. Only models with dust core radii $\leq$ 100 pc are considered. The effects of dust scattering are included. The input elliptical galaxy SED was taken from Bruzual \& Charlot (1993) and spans the range 1000\AA\ to 30,000\AA.

We have calculated the emergent broad-band colors from U to K. As expected, these models produce very strong color gradients within the central 100 pc. Such color gradients, while virtually undetectable from the ground, should be easily detectable with HST. If strong central color gradients are not detected by HST, then the dust must be extended increasing the probability that the extended color gradients observed in elliptical galaxies are caused at least in part by the presence of dust.

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