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

Abstract

The extinction curve derived from Infrared Space Observatory mid-infrared observations of the Galactic center (GC) exhibits a surprisingly flat behavior in the ~3-8 μm 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) toward the GC. If the needles contribute only to the 3-8 μm extinction, they must have a long-wavelength cutoff at ~8 μm and therefore a typical length over radius ratio of ~600, smaller than the ~3 × 103 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 ~5 × 10-6, which is equivalent to 0.14% of the silicate dust mass. Their total ISM abundance then can be readily explained by the combined production in supernovae and O-rich stellar outflows. The GC observations show that metallic needles, in spite of their low abundance or nonuniform distribution, can be the dominant source of opacity in the 3-8 μm 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.