THE ASTROPHYSICAL JOURNAL, 459:110-132, 1996 March 1

           LINE STRENGTHS AND LINE STRENGTH GRADIENTS IN S0 GALAXIES

         DAVID FISHER,[1,2] MARIJN FRANX,[1,3] AND GARTH ILLINGWORTH[2]


                                   ABSTRACT

        Line strengths and their gradients in Mg, Fe, and HBeta have been
     determined for a sample of 20 S0 galaxies in order to study the
     stellar populations of their bulges and disks and to investigate their
     relationship to elliptical galaxies.  Data are also presented for the
     elliptical galaxy NGC 1700 and the E/S0 NGC 3585.  We find that S0
     galaxies generally follow a similar, though possibly steeper,
     relationship between central Mg_2 line strength and central velocity
     dispersion as found for elliptical galaxies.  The S0 galaxies show no
     dependence between Fe line strengths and central velocity dispersion
     -- similar to the behavior observed in elliptical galaxies.  The
     central Mg/Fe ratios in the luminous S0 galaxies show an overabundance
     of Mg to Fe with respect to solar element ratios.  The magnitudes of
     the Mg and Fe line-strength gradients within the galaxies are found to
     be correlated, i.e., objects with steep Mg gradients have
     correspondingly large Fe gradients.
        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.

     Subject headings: galaxies: abundances -- galaxies: elliptical and
                       lenticular, cD -- galaxies: stellar content

[1] Kapteyn Astronomical Institute, University of Groningen, Postbus 800,
    9700 AV Groningen, The Netherlands; fish,franx@astro.rug.nl.
[2] UCO/Lick Observatory, Board of Studies in Astronomy and Astrophysics,
    University of California, Santa Cruz, CA 95064; gdi@ucolick.org.
[3] Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
    Cambridge, MA 02138