Abstract
Swift XRT observations of the H I line source VIRGOHI 21 were performed on 2008 April 22 and 26 for a total exposure time of 9.2 ks. This is the first pointed X-ray observation of VIRGOHI 21, a putative dark galaxy in the Virgo Cluster, and no photons were detected from this source. The nondetection of extended X-ray emission within the angular extent of the H I source corresponds to a 99% confidence upper limit of 2.1 × 10−14 ergs cm−2 s−1 in the 0.3-2.0 keV band. The equivalent upper limit to the amount of diffuse hot gas associated with VIRGOHI 21 is in the range 4 × 107-2 × 108 M☉ for a hot gas temperature between 0.1 and 1 keV. The nondetection also corresponds to a 99% confidence upper limit on the flux from a pointlike source of 8 × 10−15 ergs cm−2 s−1 in the 0.3-2.0 keV band. We discuss the constraints on the nature of VIRGOHI 21 imposed by these observations and the theoretical implications of these results.
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