PROBING RADIAL AGE METALLICITY DEGENERACY IN EARLY-TYPE GALAXIES

It has been generally concluded that the optical broad band color and line index gradients observed in early-type galaxies are driven by met allicity. Yet, this conclusion remains uncertain due to the age/metall icity degeneracy inherent in most optical data. Furthermore, optical b road-band colors are susceptible to reddening in the presence of dust. Near-infrared colors, on the other hand, are significantly less age s ensitive than optical colors in old stellar populations and are much l ess affected by dust. In principle, the combination of optical and nea r-IR data should provide less ambivalent age and metallicity discrimin ation than using optical or near-IR data alone. To investigate this po ssibility, near-IR images of early-type galaxies with significant U-R gradients have been obtained. Using standard surface photometry techni ques, near-IR color profiles and gradients have been measured. Compari son of the optical and near-IR results leads to the primary conclusion that broad-band optical and near-IR gradients are not tracing metalli city in concert but are affected by different astrophysical parameters . Three general possibilities are discussed: reddening, radial age gra dients, and differing metallicity sensitivities. Proving the absence o r presence of significant reddening is difficult from broad-band color s alone. In the absence of reddening, the optical color gradients woul d suggest that age decreases with radius, leading to somewhat contrive d evolution scenarios. Alternatively, it is proposed that the optical color gradients may be tracing light element (e.g., CNO) abundances wh ile the near-IR gradients are tracing Fe-peak element abundances. This scenario leads to the conclusion that many of these galaxies have enh anced nuclear [light/Fe] ratios, consistent with the recently publishe d studies of nuclear line indices in these galaxies. Given the quality of the currently available data, these hypotheses remain somewhat unc onstrained. Nevertheless, this study reinforces the necessity of obtai ning data over a long spectral baseline to properly interpret the ense mble radial properties in early-type galaxies. Independent of other da ta, the near-IR data reveals several interesting features in individua l galaxies. The J-K color gradient in NGC 3377 becomes bluer near the center of the galaxy suggesting that this region is more Fe poor in th e mean than the rest of the galaxy. NGC 5813 also shows this effect al beit at a lower level of significance. NGC 1600 has a significant H-K gradient suggesting a significant age gradient in the sense that the n uclear region is younger. The possible origins of these features are d iscussed.