The Low Metallicity ISM of X-ray Faint Elliptical Galaxies
Jimmy Irwin (University of Michigan)
Since the hot interstellar medium (ISM) of early-type galaxies is believed to originate primarily from stellar mass loss and supernovae, the metallicity of the ISM should reflect that of the stars contributing the mass to the ISM. This has been confirmed for gas-rich X-ray bright ellipticals, as both the X-ray-determined ISM abundances and optically-determined stellar abundances are approximately solar. However, this does not seem to be the case for gas-poor, X-ray faint ellipticals, for which very sub-solar abundances have been reported, although poor statistics and significant X-ray binary contamination have been listed as the cause of these (erroneously) low abundance measurements. We present Chandra + XMM-Newton spectra of one of the best-observed X-ray faint ellipticals, NGC4697, and confirm that O, Ne, Mg, Si, Fe, and Ni are all significantly subsolar, and are not the result of poor spectral fitting or poor statistics. We speculate that the low metal abundances are the result of dilution from the accretion over time of pristine metal-free gas that surrounds the galaxy. The implications of this accretion in the context of galaxy formation will be discussed.