SPECTRAL-SYNTHESIS OF METALLICITY GRADIENTS IN BRIGHT EARLY-TYPE GALAXIES

The main question posed in this investigation is whether spectral synthesis methods confirm the metallicity gradient suggested by the observed Mg2 gradients. To this end we apply the method of optimized spectral population synthesis to spatially resolved spectroscopic data for a sample of six bright early-type galaxies. The data were obtained at Mont Mégantic with a panoramic photon-counting system (LIPS) and cover the 4800-5500 Å spectral interval and a spatial interval of more than one effective radius. We find an overall trend of decreasing metallicity with radial distance which can be expressed as Δ[Fe/H]/Δ(r/re )1/4 = -0.76 ± 0.29 (s.e.) for the mean of our sample. The statistical significance of this mean metallicity gradient is lower than that of the mean Mg2 gradient obtained by Couture and Hardy (1988), but consistent with the adopted [Fe/H] vs Mg2 calibration employed there. Our results lend confidence to the conclusion that Mg2 gradients are controlled mainly by metallicity. The requirements, limitations, and possible improvements of the method employed here are discussed.