|Annu. Rev. Astron. Astrophys. 1999. 37:
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4.2. Structural Variations
However, the UV-bright population is not a simple extension of the well-studied, optically bright one. This was evident from the scatter in the ratio of UV to optical light first reported by Code et al (1972), Code & Welch (1982), which has been amply confirmed by later observations (see Section 5.2). In addition, large internal gradients in UV/optical colors have been revealed in almost all cases studied with sufficient S/N. Five of the galaxies shown in Figure 3 from the Ohl et al (1998) UIT sample display large internal 1500-B color gradients with net changes up to ~ 1.0 mag over the region photometered. The 1500-B colors of 7 of the 8 objects become redder outward, meaning that the far-UV light is more concentrated to the galaxy centers than the optical light. Both in amplitude and sign, these changes are dramatically unlike the very mild, bluer-outward color gradients encountered in the optical and IR (e.g. Peletier et al 1990). M32 is the only object which becomes bluer in 1500-B at larger radii.
Figure 3. Radial FUV-B color profiles for 8 early-type galaxies obtained by comparing Astro/UIT far-UV surface photometry with B-band data from the literature. The curves have been offset for clarity and arranged in order of increasing central UVX. One sigma error bars are shown. FUV-B colors redden with increasing radius in all cases except M32, which shows a strong, reversed profile. It is the only object currently known to have this behavior. There is an interesting two-component structure in most of the profiles. Offsets in order from the top down are C = -2.5, -2.0, -1.5, 0.0, 0.0, +1.0, +2.0, and +3.5 mag arcsec-2. From Ohl et al (1998).
Internal extinction by dust cannot be responsible for these gradients. Aside from the absence of dust structures in the images and the sense of the typical gradient (implying more extinction at larger radii), the gradients are so large that significant optical-band effects would be expected, since A(4400 Å) ~ 0.5 A(1500 Å), where A is the total extinction in magnitudes. HUT spectroscopy also places strict limits on the amount of internal extinction (Ferguson & Davidsen 1993, Brown et al 1997). Instead, the gradients are apparently driven by a radial change in the properties of the old star population.