Magnesium Gradients in Elliptical Galaxies. II. NGC 4881

In Paper I the authors defined a magnesium index MgH based on the broad absorption feature near 5175 Å, described a procedure for determining its distribution in a galaxy out to very faint levels, and presented initial results for the cD galaxy NGC 4874 in the heart of the Coma Cluster. In the present paper the authors deal with a procedure to correct for an important observational side effect, namely, point-spread function (PSF) variability. They also define a dimensionless magnesium index gradient α(Mg₂) = δMg₂/δμ, where μ is surface brightness in a passband centered on the magnesium feature, and where both Mg₂ and μ are on magnitude scales. The authors present results for another Coma Cluster elliptical, NGC 4881. The luminosity profile of NGC 4881 was recorded out to 120arcsec from the nucleus, where the V surface brightness is 28th mag arcsec^-2 or fainter. The dimensionless magnesium index gradient in NGC 4881 is α(Mg₂) = -0.023±0.006, based on measurement over a 5 mag range of surface brightness. The observed gradient implies a threefold drop in metal abundance within that 5 mag drop in surface brightness.