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The extinction curve derived from ISO mid-infrared (IR) observations of theGalactic center (GC) exhibits a surprisingly flat behavior in the ~ 3 to 8micron region, contrary to the deep minimum expected from standard interstellardust models consisting of bare silicate and graphite dust particles. We showthat this extinction is likely caused by the presence of metallic needles inthe interstellar medium (ISM) towards the Galactic center. If the needlescontribute only to the 3 - 8 micron extinction, they must have a longwavelength cutoff at ~ 8 microns, and therefore a typical length over radiusratio of about 600, smaller than the 3000 aspect ratio determined for theneedles in Cas A. Homogeneously distributed throughout the ISM, they compriseonly 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. Theirtotal ISM abundance can then be readily explained by the combined production inSNe and O-rich stellar outflows. It is currently unclear how universal the GCextinction law is. Local 2 - 5 micron extinction measurements seem to beconsistent with the standard extinction law, suggesting a non uniformdistribution of needles in the ISM. The GC observations show that metallicneedles, in spite of their low abundance or non uniform distribution, can bethe dominant source of opacity in the 3 - 8 micron wavelength region. However,expelled into the intergalactic medium, their abundance is too low to cause anydimming of cosmological sources, and their length is too short to make them asignificant source of submillimeter emission.Comment: 9 pages, 1 figure, to appear in the ApJ Letter

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