Line strengths and gradients in S0 galaxies

We infer bulge and disk gradients for the nine most edge-on galaxies for which we have both major- and minor-axis profiles. The metal line strengths decrease with radius along the major and minor axes in the bulge-dominated central regions. At larger radii, however, the major-axis metal line strength profiles flatten while the minor-axis bulge profiles fall to lower values.

Representative color maps in B-R are presented that display a separation between bulge and disk colors corresponding to the metal line strength profiles. Based on our Mg_2 profiles, the average metal gradient found in the disks of our sub-sample is Delta[Fe/H]/Delta(r/h)=-0.08+/-0.06, which corresponds to a reduction in the mean metallicity of the disk stellar population by <~15% per disk scale length (h). These shallow metallicity gradients are approximately a factor of 2-3 smaller than those derived for the disks of late-type spiral galaxies from H II regions and are consistent with previous investigations that showed a trend for disk metallicity gradients to decrease toward earlier Hubble types.

As inferred from our Mg and Fe line strengths, the mean size of the bulge metallicity gradients is Delta[Fe/H]/Delta(log r)=-0.7+/-0.4, which is steeper than typical elliptical galaxy gradients. Our findings do not support formation scenarios in which bulges formed either from heated disk material at late times after disk formation or through dissipationless stellar merging, as neither process includes mechanisms for producing the observed metallicity gradients.

Our observations are better explained in terms of formation via dissipative collapse (or merging) at early times.