Reconciling optical and X-ray mass estimates: the case of the elliptical galaxy NGC3379

摘要

NGC3379 is a well studied nearby elliptical for which optical investigationshave claimed a little dark matter content, or even no dark matter. Recently,its total mass profile M(r) has been derived by exploiting Chandra observationsof its extended and X-ray emitting interstellar medium, based on the hypothesisof hydrostatic equilibrium for the hot gas. The resulting total mass within theeffective radius Re has been claimed to be a few times larger than found byoptical studies. Here we show that part of the discrepancy can be due to anunderestimate of the optically derived mass, and the remaining discrepancy of afactor of ~2 can be explained by deviations from hydrostatic equilibrium of thehot gas. By using hydrodynamical simulations tailored to reproduce the observedhot gas properties of NGC3379, and by assuming as input for the simulations thetotal mass profile derived optically, we show that i) the hot gas at thepresent time has X-ray properties consistent with those observed only if it isoutflowing over most of the galactic body, and ii) an overestimate of M of thesame size found in the recent X-ray analysis is recovered when assuminghydrostatic equilibrium. We also show that the hot gas is outflowing even for adark matter fraction within Re as large as derived with the standard X-rayprocedure based on the hydrostatic equilibrium assumption, which shows theunapplicability of the method for this galaxy. Finally, we find that the wholerange of dark mass amount and distribution allowed for by optical studies iscompatible with a hot gas flow with the observed X-ray properties.