High-resolution study of luminous infrared galaxies. I. The composite nature of the Seyfert 1 galaxy IRAS 20044-6114 (NGC 6860)

High spatial resolution optical imaging and optical and near-infrared spectroscopy of the Seyfert 1 galaxy IRAS 20044-6114 (NGC 6860) are used to study the physical conditions in the ionized gas, the stellar population, and the kinematics of this luminous IR source. The broad-band images of the host galaxy show a wealth of morphological details: a compact nucleus, two weak spiral arms, a bar, a bulge, an inner ring, and a possible outer ring. The Hα image reveals bright emission-line regions associated with the Seyfert nucleus and an inner ring of intense star formation. This ring is located at the position of the turnover of the velocity curve (r ∼ 11″, 3.1 kpc) and also at the end of the bar, and may be a result of bar resonances. The [O III] λ5007 image shows that the high-excitation gas is elongated perpendicularly to the direction of the bar, and a bright compact object at ∼40″ NE of the nucleus which is undetectable in the broad-band images. We interpret this object as a dwarf young H II galaxy. The spectrum of NGC 6860 shows typical Seyfert 1.5 features, with relatively strong near-IR emission lines. In the nucleus we detect variability in the UV continuum and in the emission-line profile, and very high electron temperature T_e = 33,000-35,000 K, untypical for the ordinary nuclear or extranuclear regions in AGNs. At the position of the star formation inner ring, the spectrum of the underlying galaxy becomes much bluer, with clear signatures of enhanced population of young stars. The velocity curves for the stellar population and for the ionized gas are generally well fitted by a model of spherical gravitational potential and purely circular motions. This implies a mass for the galaxy of the order of 1.5 × 10¹¹ M_⊙ within 35″ (10 kpc) radius. In the innermost 5″ (1.4 kpc) radius, emission lines show systematic deviation from the smooth model. Under certain assumptions, the sense of these deviations is consistent with inflow along the bar. We speculate that this gas inflow can be fueling the Seyfert nucleus as suggested by Shlosman and collaborators. The optical, near-IR, and far-IR results show clear evidence that the nuclear and circumnuclear regions have composite and complex structure: a variable Seyfert 1 nucleus embedded in an intense and dusty star formation environment.