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 metallicity. Yet, this conclusion remains uncertain due to the age/metallicity degeneracy inherent in most optical data. Furthermore, optical broad-band colors are susceptible to reddening in the presence of dust. Near-infrared colors, on the other hand, are significantly less age sensitive than optical colors in old stellar populations and are much less affected by dust. In principle, the combination of optical and near-IR data should provide less ambivalent age and metallicity discrimination than using optical or near-IR data alone. To investigate this possibility, near-IR images of early-type galaxies with significant U-R gradients have been obtained. Using standard surface photometry techniques, near-IR color profiles and gradients have been measured. Comparison of the optical and near-IR results leads to the primary conclusion that broad-band optical and near-IR gradients are not tracing metallicity in concert but are affected by different astrophysical parameters. Three general possibilities are discussed: reddening, radial age gradients, and differing metallicity sensitivities. Proving the absence or presence of significant reddening is difficult from broad-band colors alone. In the absence of reddening, the optical color gradients would suggest that age decreases with radius, leading to somewhat contrived evolution scenarios. Alternatively, it is proposed that the optical color gradients may be tracing light element (e.g., CNO) abundances while the near-IR gradients are tracing Fe-peak element abundances. This scenario leads to the conclusion that many of these galaxies have enhanced nuclear [light/Fe] ratios, consistent with the recently published studies of nuclear line indices in these galaxies. Given the quality of the currently available data, these hypotheses remain somewhat unconstrained. Nevertheless, this study reinforces the necessity of obtaining data over a long spectral baseline to properly interpret the ensemble radial properties in early-type galaxies. Independent of other data, the near-IR data reveals several interesting features in individual 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 the mean than the rest of the galaxy. NGC 5813 also shows this effect albeit at a lower level of significance. NGC 1600 has a significant H-K gradient suggesting a significant age gradient in the sense that the nuclear region is younger. The possible origins of these features are discussed.