Emission and absorption lines of H2 and CO in nuclei of several galaxies with strong NIR emission, including Centaurus A (NGC 5128), were studied at the La Silla observatory to obtain constraints... Show moreEmission and absorption lines of H2 and CO in nuclei of several galaxies with strong NIR emission, including Centaurus A (NGC 5128), were studied at the La Silla observatory to obtain constraints on the physical parameters of molecular material close to the nucleus of the NGC 5128 galaxy. Results show that, besides the (C-12)O, and (C-13)O emission and absorption lines toward the nucleus of NGC 5128, an unresolved source of excited H2 emission (of a size smaller than 95 pc) exists that appears to be collisionally excited. The new observations, combined with literature data, suggest that the nucleus of Cen A is surrounded by a disk of mass 2 x 10 to the 7th solar masses. It is shown that a circumnuclear disk with the outer edge at r - 160 pc, a thickness of 80 pc, a cavity with a radius of 40 pc, and a density distribution of about 1/sq r is consistent with all existing observations. Show less
The infrared emission spectrum of H2 excited by ultraviolet absorption, followed by fluorescence, was investigated using comprehensive models of interstellar clouds for computing the spectrum and... Show moreThe infrared emission spectrum of H2 excited by ultraviolet absorption, followed by fluorescence, was investigated using comprehensive models of interstellar clouds for computing the spectrum and to assess the effects on the intensity to various cloud properties, such as density, size, temperature, and the intensity of the UV radiation field. It is shown that the absolute H2 IR line intensities depend primarily on the density of the cloud and the strength of the incident UV radiation, and to a much lesser exent on the temperature of the gas, the total thickness of the cloud, and the optical properties of the grains. A variety of recent observational results are discussed with reference to theoretical models. It is shown that the rich H2 emission spectrum of the reflection nebula NGC 2023 can be reproduced by a model with density of about 10,000/cu cm, temperature of about 80 K, and UV flux approximately 300 times that of the Galactic background starlight. Show less
