Ma. Malkan et al., A HUBBLE-SPACE-TELESCOPE IMAGING SURVEY OF NEARBY ACTIVE GALACTIC NUCLEI, The Astrophysical journal. Supplement series, 117(1), 1998, pp. 25-88
We have obtained WFPC2 images of 256 of the nearest (z less than or eq
ual to 0.035) Seyfert 1, Seyfert 2, and starburst galaxies. Our 500 s
broadband (F606W) exposures reveal much fine-scale structure in the ce
nters of these galaxies, including dust lanes and patches, bars, rings
, wisps, and filaments, and tidal features such as warps and tails. Mo
st of this fine structure cannot be detected in ground-based images. W
e have assigned qualitative classifications for these morphological fe
atures and a Hubble type for the inner region of each galaxy, and we h
ave also measured quantitative information such as 0.'' 18 and 0.'' 92
aperture magnitudes, position angles, and ellipticities, where possib
le. There is little direct evidence for unusually high rates of intera
ction in the Seyfert galaxies. Slightly less than 10% of all the galax
ies show tidal features or multiple nuclei. The incidence of inner sta
rburst rings is about 10% in both classes of Seyfert galaxies. In cont
rast, galaxies with H II region emission-line spectra appear substanti
ally more irregular and clumpy because of their much higher rates of c
urrent star formation per unit of galactic mass. The presence of an un
resolved central continuum source in our Hubble Space Telescope images
is a virtually perfect indicator of a Seyfert 1 nucleus as seen by gr
ound-based spectroscopy. Fifty-two percent of these Seyfert 1 point so
urces are saturated in our images; we use their wings to estimate magn
itudes ranging from 15.8 to 18.5. The converse is not universally true
, however, as over one-third of Seyferts with direct spectroscopic evi
dence for broad Balmer wings show no nuclear point source. These 34 re
solved Seyfert 1's have fainter nonstellar nuclei, which appear to be
more extinguished by dust absorption. Like the Seyfert 2's, they have
central surface brightnesses consistent with those expected for the bu
lges of normal galaxies. The rates for the occurrences of bars in Seyf
ert 1's and 2's and non-Seyferts are the same. We found one significan
t morphological difference between the host galaxies of Seyfert 1 and
Seyfert 2 nuclei. The Seyfert 2 galaxies are significantly more likely
to show nuclear dust absorption, especially in lanes and patches that
are irregular or reach close to the nucleus. A few simple tests show
that the difference cannot be explained by different average redshifts
or selection techniques. It is confirmed by our galaxy morphology cla
ssifications, which show that Seyfert 1 nuclei reside in earlier type
galaxies than Seyfert 2 nuclei. If, as we believe, this is an intrinsi
c difference in host galaxy properties, it undermines one of the postu
lates of the strong unification hypothesis for Seyfert galaxies, that
they merely appear different because of the orientation of their centr
al engine. The excess galactic dust we see in Seyfert 2's may cause su
bstantial absorption that obscures their hypothesized broad emission l
ine regions and central nonstellar continua. This galactic dust could
produce much of the absorption in Seyfert 2 nuclei that had instead be
en attributed to a thick dusty accretion torus forming the outer part
of the central engine.