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

Abstract

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.