The nature of the optical light in Seyfert 2 galaxies with polarized continua

Abstract

We investigate the nature of the optical continuum and stellar population in the central kiloparsec of the Seyfert 2 galaxies Mrk 348, Mrk 573, NGC 1358, and Mrk 1210 using high signal-to-noise−ratio, long-slit spectra obtained along the radio axis or along the extended high-excitation emission. These four galaxies are known to have polarized continua, including polarized broad lines in Mrk 348 and Mrk 1210, and previous studies indicate featureless continua contributions in the 20-50% range at λ ≈ 5500 Å . Nevertheless, our measurements of the equivalent widths of absorption lines and continuum ratios as a function of distance from the nuclei show no decrease of the equivalent widths (i.e., no dilution) nor blueing of the spectrum toward the nucleus, as expected if a blue, featureless continuum was present at the nucleus in the above proportions. We investigate one possibility to account for the lack of dilution : that the stellar population at the nucleus is the same as that from the surrounding bulge and dominates the nuclear light. By comparing the nuclear and the extranuclear spectra of each galaxy, we conclude that this hypothesis works for Mrk 348, NGC 1358, and Mrk 1210, for which we Ðnd stellar population contributions at the nucleus larger than 90% at all wavelengths. Our approach di†ers from that adopted in previous studies, where an elliptical galaxy template is used to represent the stellar population of the nucleus. Although the latter may be valid for some galaxies−as, for example, Mrk 573−in several cases the stellar population may be different from that of an elliptical galaxy. We find that a larger stellar population 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 regions of Mrk 348 and Mrk 1210 have important contributions of young to intermediate-age stars (0−10 8 yr) not present in templates of elliptical galaxies. In the case of Mrk 1210, this is further con- formed by the detection of a “Wolf-Rayet feature" in the nuclear emission-line spectrum.