Global VLBI Observations of the High‐Velocity HiAbsorption toward NGC 1275

We present global VLBI observations of the ~8100 km s$^{-1}$ HI absorptionfeature detected toward the strong radio nucleus 3C 84 of NGC 1275 in thePerseus cluster (${V_{\rm sys}\sim}$5200 km s$^{-1}$). The observations wereobtained using the Very Long Baseline Array (VLBA), the phased Very Large Array(VLA), and three stations of the European VLBI Network (EVN). Our resultsprovide the first high dynamic range image of this feature at high spectral andspatial resolutions. We detect six distinct absorption peaks with opticaldepths ranging from 0.1 to 0.45, and multiple weak features with $\tau \leq$0.1. The compactness of the background radio source 3C 84, which has a linearextent of only about 16 pc, limits the conclusions that can be drawn as to thenature of the intervening object, which must be falling toward the center ofthe Perseus cluster at ~3000 km s$^{-1}$. However, the detected absorptionpeaks indicate the existence of several HI clouds with velocity differences andwidths similar to those seen in Galactic neutral hydrogen absorption andsimilar to some of the HI absorption seen in the Large Magellanic Cloud. Themost prominent HI clouds extend from 12 to 30 mas (milliarcseconds) on theplane of the sky. The derived HI column densities, assuming $T_{\rm s}=50$ K,range over $(0.35-3.8)\times 10^{20}$ cm$^{-2}$, and the implied volumedensities range between 1.4 and 11 cm$^{-3}$.Comment: 16 pages with 5 figures. To be published in the Astrophysical Journal (Feb. 2002