Tm. Heckman et al., A POWERFUL NUCLEAR STARBURST IN THE SEYFERT-GALAXY MARKARIAN-477 - IMPLICATIONS FOR THE STARBURST-ACTIVE GALACTIC NUCLEUS CONNECTION, The Astrophysical journal, 482(1), 1997, pp. 114-132
There have been many suggestions that starbursts may play an important
role in Seyfert galaxies. Recent analyses of the UV, optical, and nea
r-IR ''featureless continuum'' (FC) in type 2 Seyfert nuclei have prov
ided indirect evidence that the FC may be produced by a compact starbu
rst and that this starburst may provide a power source for the pronoun
ced far-IR excess in type 2 Seyfert galaxies. In the present paper, we
present and discuss Hubble Space Telescope (HST) images and a UV spec
trum 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 n
ucleus in the local universe and thus offers a unique opportunity for
detailed investigation of the nature of the FC. The data provide direc
t evidence that the observed UV through near-IR continuum in the nucle
us of Mrk 477 is dominated by light from a dusty starburst. An HST ima
ge of the space-UV continuum shows that it arises in a compact (few hu
ndred parsec) region, much smaller than the sizes of previously studie
d circumnuclear starbursts in Seyfert galaxies like NGC 1068. In the H
ST space-UV spectrum of this region, we detect strong N V lambda 1240
and Si IV lambda 1400 stellar wind lines and other weak photospheric l
ines from hot stars. In the optical spectrum we detect a broad emissio
n complex around the He II lambda 4686 line that may be produced by an
ensemble of about 30,000 Wolf-Rayet stars (WN subtype). In the near-I
R spectrum we detect a strong Ca II triplet feature produced by red su
pergiants. Detailed comparison of the data with models implies that th
e nuclear starburst in Mrk 477 has an age of about 6 Myr, was of short
duration (an ''instantaneous'' burst), and has solar or higher metall
icity and a bolometric luminosity of similar to(3 x 10(10))-10(11) L..
This nuclear starburst must therefore make a significant contribution
to the overall energetics of Mrk 477, even though optical spectropola
rimetry 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 deter
mine whether Mrk 477 is typical of Seyfert 2 nuclei and, thus, whether
nuclear starbursts are an important part of the Seyfert phenomenon.