Astron. Astrophys. 336, 433-444 (1998)
The central 20" (kpc) of NGC 3079 exhibits a
large range of near-infrared colours, representing a varying
combination of intrinsic stellar colours, scattered stellar light,
emission by hot dust and extinction increasing towards the nucleus. As
a consequence, proper interpretation of the observed light in terms of
nuclear structure and composition cannot be achieved by the use of
photometry in multi-arcsec apertures, but requires imaging at the
highest possible resolution.
The nucleus suffers from significant extinction, even at
near-infrared wavelengths. The peak extinction at a resolution of 1"
is . The mean extinction of the inner
disk is .
The eastern part of the NGC 3079 bulge has ()
colours too blue to be explained by stars in a typical quiescent
bulge, and provide evidence either for a 20% contribution of directly
emitted light from young stars in the bulge, or a 20-30% contribution
by scattered light from stars in the bulge or in the stellar disk.
Scattering of power-law () emission from a
nuclear source is less likely as it would require a rather unusual
The JHK colours show the presence of two or three dark lanes
obscuring stellar light west of the nucleus. These dust lanes cause
significant extinction of both bulge and disk.
The colours of the central 3" (pc) are
extremely red, peaking at and
. They can be explained by the presence of hot
dust in the central region, radiating at temperatures close to
K. The K-band luminosity of this hot
dust is at most of the far-infrared luminosity
from the central region, and of the mechanical
luminosity that appears to be available from nuclear winds in the
Molecular hydrogen S(1) emission originates
in a compact source centred on the nucleus and elongated along the
major axis, surrounded by a region of lower surface brightness. East
of the nucleus, some H2 emission appears associated with
the inner outflow seen at radio and optical wavelengths. It may
represent material swept away from the molecular disk out of the plane
by the impacting winds. A western counterpart is lacking, and the
sharp cutoff of H2 emission testifies to the significant
near-infrared extinction caused by the galaxy disk intervening in the
line of sight.
The distribution of hot dust emission is practically identical to
that of the bright molecular hydrogen emission and just covers the
central cavity observed in the CO distribution. This morphology, and
the kinematic information obtained from the H2 images, is
supporting evidence for the nuclear wind model proposed by Duric &
Seaquist (1988) and for the conclusions reached by Veilleux et al.
(1994) and Hawarden et al. (1995). The H2 and hot dust
emission appears to originate in dense material shocked by fast
nuclear winds impacting at a radius of about 120 pc on the inner edge
of a central molecular disk.
The high-density inner molecular disk extends out to a radius of
about 290 pc. Cooler dust and molecular gas extend out to a radius of
at least 1 kpc.
The combination of extinction, absorption
optical depth and CO emission places upper limits on both the
spin temperature and the CO-to-H2
conversion factor X in the central region of NGC 3079. For
gas-to-dust ratios comparable to those in the solar neighbourhood, the
spin temperature is well below 250 K, while
is less than .
Notwithstanding the concentration of CO emission in the center of
NGC 3079, the central regions contain only a small fraction of all
molecular hydrogen in the galaxy. The molecular hydrogen content of
NGC 3079 is similar to that of other late-type galaxies, but the
centrally concentrated CO appears unusually overabundant with respect
to H2, possibly related to the nuclear activity.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998