The Effects of Dust on Broad--Band Color Gradients \\ in Elliptical Galaxies
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Session 42 -- Ellipticals
Display presentation, Thursday, January 13, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[42.08] The Effects of Dust on Broad--Band Color Gradients \\ in Elliptical Galaxies

D. Silva and M. Wise (NOAO/KPNO)

Although the presence of broad--band color gradients has been well established in elliptical galaxies, the physical processes which produce these gradients are still a source of considerable controversy. If these color gradients reflect underlying stellar population gradients then they can be used to place constraints on various dissipative or merger formation scenarios. Witt, Thronson, \& Caputo (1992), however, have recently argued that observed color gradients in many elliptical galaxies are consistent with the presence of dust.

To further explore this possibility, we have generated a grid of dusty elliptical galaxy models. These models are spherically symmetric and include a range of dust masses as well as dust distributions ranging from highly centrally condensed to diffuse and very extended. 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 the UIT far--UV bands through K. In the optical, we can reproduce the magnitude, morphology, and color gradient ratios (i.e. $\Delta$(U--R)/$\Delta$(B--R)) observed in many elliptical galaxies. The best fits are found for distributions with $\rho_{d} \sim r^{-1}$ and $\tau \sim 1$ which corresponds to dust masses on the order of $10^6$ M$_\odot$ within the central 10--30 kpc. More condensed dust profiles also produce strong color gradients but only in the central regions ($r < 0.5$ kpc). The model near-IR gradients are also generally consistent with observed J--K gradients. A small fraction of observed ellipticals show color gradients which cannot be fit by our models for any reasonable dust mass or configuration. In short, our models suggest that the broad--band color gradients observed in many, if not most, elliptical galaxies may arise from the effects of dust, and not from strong stellar population gradients.

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