THE NATURE OF THE OPTICAL LIGHT IN SEYFERT-2 GALAXIES WITH POLARIZED CONTINUA

We investigate the nature of the optical continuum and stellar populat ion in the central kiloparsec of the Seyfert 2 galaxies Mrk 348, Mrk 5 73, NGC 1358, and Mrk 1210 using high signal-to-noise-ratio, long-slit spectra obtained along the radio axis or along the extended high-exci tation emission. These four galaxies are known to have polarized conti nua, including polarized broad lines in Mrk 348 and Mrk 1210, and prev ious studies indicate featureless continua contributions in the 20%-50 % range at lambda approximate to 5500 Angstrom. Nevertheless, our meas urements of the equivalent widths of absorption lines and continuum ra tios as a function of distance from the nuclei show no decrease of the equivalent widths (i.e., no dilution) nor blueing of the spectrum tow ard the nucleus, as expected if a blue, featureless continuum was pres ent at the nucleus in the above proportions. We investigate one possib ility to account for the lack of dilution: that the stellar population at the nucleus is the same as that from the surrounding bulge and dom inates the nuclear light. By comparing the nuclear and the extranuclea r spectra of each galaxy, we conclude that this hypothesis works for M rk 348, NGC 1358, and Mrk 1210, for which we find stellar population c ontributions at the nucleus larger than 90% at all wavelengths. Our ap proach differs from that adopted in previous studies, where an ellipti cal galaxy template is used to represent the stellar population of the nucleus. Although the latter may be valid for some galaxies-as, for e xample, Mrk 573-in several cases the stellar population may be differe nt from that of an elliptical galaxy. We find that a larger stellar po pulation contribution to the nuclear spectra can play the role of the ''second featureless continuum'' source inferred from previous studies . In particular, stellar population synthesis shows that the nuclear r egions of Mrk 348 and Mrk 1210 have important contributions of young t o intermediate-age stars (0-10(8) yr) not present in templates of elli ptical galaxies. In the case of Mrk 1210, this is further confirmed by the detection of a ''Wolf-Rayet feature'' in the nuclear emission-lin e spectrum.