THE EFFECTS OF DUST ON BROAD-BAND COLOR GRADIENTS IN ELLIPTIC GALAXIES

In order to examine the possibility that dust effects cause color grad ients in elliptical galaxies, we have constructed a set of elliptical galaxy models spanning a wide range of dust properties, including mass , spatial distribution, and spectral properties. These models are sphe rically symmetric and include the effects of scattering. We have calcu lated the emergent broadband colors from the far-UV bands through K: T hese results were then compared with the color properties for a sample of 52 galaxies taken from the literature. In the optical, we can repr oduce the magnitude, morphology, and color gradient ratios [i.e., Delt a U-R)/Delta(B-R)] observed in many of the galaxies. The best fits are found for distributions with rho(d) similar to r(-1) and tau - 1, whi ch corresponds to dust masses on the order of 10(6) M(.) within the ce ntral 10-30 kpc. More condensed dust profiles also produce strong colo r gradients, but only in the central regions (r < 0.5 kpc). For many o f the objects in this sample, the implied dust masses are in reasonabl e agreement with the dust masses inferred from IRAS observations, alth ough a number of objects require significantly higher dust masses than their IRAS observations imply if their color properties are to be att ributed solely to reddening by dust. Our models suggest that a simple dusty elliptical galaxy scenario can reproduce the magnitude, morpholo gy, and ratios of observed broadband color gradients in many elliptica ls. We find that currently available color gradient measurements are c onsistent with our model results but that these measurements are not p recise enough in many objects to provide tight constraints on the mode ls. Extended dust distributions will greatly complicate the interpreta tion of all broadband data. Any broadband analysis hindered by age-met allicity degeneracy will also be hindered by dust effects. In effect, all broadband observations of elliptical galaxies may be age-metallici ty-dust degenerate. We also consider the effects of dust on several gl obal relationships in ellipticals, such as the color-magnitude relatio n and various distance indicators.