Imaging observations of gas and dust in NGC 4696 and implications for cooling flow models

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Abstract

Continuum and emission-line direct CCD observations of the gas and dust in NGC 4696 are presented and discussed. We derive the wavelength-dependence of extinction and show that it is normal. The line luminosities are derived and evidence that the gas and dust are well mixed is presented. The data are used to constrain physical conditions within the dust lane. An external (infall) origin for the dust-lane material is favored from the observations. We discuss the interaction between cool infalling gas and the hot X-ray gas. Heat transfer from the X-ray gas to the dust lane can be energetically significant, and indeed is calculated to be of the same order of magnitude as the total emitted radiation of the dust lane. We therefore suggest heat transfer might provide the excitation for both the optical emission lines and infrared radiation. If infall is generally responsible for optical line-emission systems, then (i) the correlation between optical emission and X-rays may not be due to the presence of unstable, cooling filaments; (ii) cooling of the hot halo by the dust lane may produce cooler X-ray gas; (iii) the structure of some thermal conduction models would be altered by the imposition of an inner cold boundary condition.

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