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<quote>how can you be sure that the hydrogen is associated with Centaurus A and not foreground hydrogen in our galaxy?<quote>
That was my first question, but then I noticed a filament of ionized hydrogen extending from Cen a itself, then noticed that the immediate area around CentA appears to be clear, in fact it looks like CentA is surrounded by a bubble of Halpha, and the image is showing the densest parts visible through the sides of the bubble, whilst the foreground and background of the bubble is too faint to image.
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