A Powerful Nuclear Starburst in the Seyfert Galaxy Markarian 477: Implications for the Starburst-Active Galactic Nucleus Connection^*

There have been many suggestions that starbursts may play an important role in Seyfert galaxies. Recent analyses of the UV, optical, and near-IR "featureless continuum" (FC) in type 2 Seyfert nuclei have provided indirect evidence that the FC may be produced by a compact starburst and that this starburst may provide a power source for the pronounced far-IR excess in type 2 Seyfert galaxies. In the present paper, we present and discuss Hubble Space Telescope (HST) images and a UV spectrum plus new ground-based near-UV through near-IR spectra of Mrk 477 (=I Zw 92). This object is arguably the most powerful type 2 Seyfert nucleus in the local universe and thus offers a unique opportunity for detailed investigation of the nature of the FC.

The data provide direct evidence that the observed UV through near-IR continuum in the nucleus of Mrk 477 is dominated by light from a dusty starburst. An HST image of the space-UV continuum shows that it arises in a compact (few hundred parsec) region, much smaller than the sizes of previously studied circumnuclear starbursts in Seyfert galaxies like NGC 1068. In the HST space-UV spectrum of this region, we detect strong N V λ1240 and Si IV λ1400 stellar wind lines and other weak photospheric lines from hot stars. In the optical spectrum we detect a broad emission complex around the He II λ4686 line that may be produced by an ensemble of about 30,000 Wolf-Rayet stars (WN subtype). In the near-IR spectrum we detect a strong Ca II triplet feature produced by red supergiants. Detailed comparison of the data with models implies that the nuclear starburst in Mrk 477 has an age of about 6 Myr, was of short duration (an "instantaneous" burst), and has solar or higher metallicity and a bolometric luminosity of ~(3 × 10¹⁰)-10¹¹ L_☉.

This nuclear starburst must therefore make a significant contribution to the overall energetics of Mrk 477, even though optical spectropolarimetry demonstrates that a powerful "hidden" type 1 Seyfert nucleus is also present. We speculate that the compact starburst is occurring in the outer reaches of the "obscuring torus" that hides the type 1 Seyfert nucleus. An observational program that is underway will determine whether Mrk 477 is typical of Seyfert 2 nuclei and, thus, whether nuclear starbursts are an important part of the Seyfert phenomenon.