2006ApJ...643L..49K

Although some 20,000-30,000 supernova remnants (SNRs) are expected to exist in the Milky Way, only about 230 are currently known. This implies that most SNRs are ``missing.'' Recently, we proposed that small (≲1°), faint, high-velocity features seen in large-scale 21 cm line surveys of atomic hydrogen (H I) in the Galactic plane could be examples of such missing old SNRs. Here we report on high-resolution H I observations of one such candidate, FVW 190.2+1.1, which is revealed to be a rapidly expanding (∼80 km/s) shell. The parameters of this shell seem only consistent with FVW 190.2+1.1 being the remnant of a SN explosion that occurred in the outermost fringes of the Galaxy some ∼3x10<sup>5</sup> yr ago. This shell is not seen in any other wave band, suggesting that it represents the oldest type of SNR, that which is essentially invisible except via its H I line emission. FVW 190.2+1.1 is one of a hundred ``forbidden-velocity wings'' (FVWs) recently identified in the Galactic plane, and our discovery suggests that many of these are likely to be among the oldest SNRs. We discuss the possible link between FVWs and fast-moving atomic clouds in the Galaxy.