Galactic Center Extinction: Evidence for Metallic Needles in the General Interstellar Medium

The extinction curve derived from ISO mid-infrared (IR) observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~ 3 to 8 micron region, contrary to the deep minimum expected from standard interstellar dust models consisting of bare silicate and graphite dust particles. We show that this extinction is likely caused by the presence of metallic needles in the interstellar medium (ISM) towards the Galactic center. If the needles contribute only to the 3 - 8 micron extinction, they must have a long wavelength cutoff at ~ 8 microns, and therefore a typical length over radius ratio of about 600, smaller than the 3000 aspect ratio determined for the needles in Cas A. Homogeneously distributed throughout the ISM, they comprise only a minor mass fraction of the ISM, with a needle-to-H mass ratio of \~5x10^{-6}, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance can then be readily explained by the combined production in SNe and O-rich stellar outflows. It is currently unclear how universal the GC extinction law is. Local 2 - 5 micron extinction measurements seem to be consistent with the standard extinction law, suggesting a non uniform distribution of needles in the ISM. The GC observations show that metallic needles, in spite of their low abundance or non uniform distribution, can be the dominant source of opacity in the 3 - 8 micron wavelength region. However, expelled into the intergalactic medium, their abundance is too low to cause any dimming of cosmological sources, and their length is too short to make them a significant source of submillimeter emission.