X-ray emission, optical nebulosity and dust in early-type galaxies - I. The dusty nebular filaments in NGC 5846

We present new optical imagery of the X-ray bright elliptical galaxy NGC 5846, the dominant galaxy of a small group of galaxies. A filamentary dust lane with a dust mass of similar to 7 10(3) M. is detected in the central few kpc of NGC 5846. The optical extinction properties of the dust features are consistent with those of dust in our Galaxy. The morphology of the dust features are strikingly similar to that observed for the optical nebulosity and the X-ray emission. A physical connection between the different phases of the interstellar medium therefore seems likely. We discuss three different options for the origin of the dusty nebular filaments: Condensation out of a cooling flow, mass loss of late-type stars within NGC 5846, and material donated by a small neighbouring galaxy. We conclude that the dust as well as the optical nebulosity are most likely products of a recent interaction with a small, relatively gas-rich galaxy, probably of Magellanic type. Dust grains in the dust lane are destroyed by sputtering in the hot, X-ray-emitting gas in less than or similar to 10(7) yr, which is shorter than the crossing time of a (small) galaxy through the central 5 kpc of NGC 5846. This indicates that the dust must be replenished to be consistent with the observed dust mass, at a rate of similar to 10(-3) M. yr(-1). We argue that this replenishment can be achieved by evaporation of cool, dense gas cloudlets that were brought in during the interaction. The evaporation rate of cool gas in NGC 5846 is consistent with the "mass deposition rate" derived from X-ray measurements. The energy lost by the hot gas through heating of dust grains and evaporation of cool gas clouds in the central few kpc of NGC 5846 is adequately balanced by heat sources: transport of heat by electron conduction into the core of the X-ray-emitting gas and loss of kinetic energy of the in-falling galaxy. There does not seem to be a need to invoke the presence of a "cooling flow" to explain the X-ray and optical data.