It's a really great image that does show just how much hydrogen gas there is just floating out there. Like a few others, I'd agree that some of the Ha emission surrounding the Cent A galaxy (most specifically the Ha jets) are indeed a part of Cent A.
The vast majority of the Ha emission in this image would have to be hydrogen gas within our galaxy. Where the idea does start to get interesting is that a good portion of the baryonic mass (possibly 70%) within some galaxy clusters is hydrogen gas not inside galaxies at all. This is typically towards the centres of galaxy clusters which are long "dead" elliptical galaxies which certainly doesn't describe Cent A.
The two papers
here and
here go into a bit of detail around the surrounding mechanisms.
In short, Ha directly surround a galaxy like Ha may not be detectable due to its temperature (too hot). It's estimated that some 40-50% of the baryonic matter in the universe is in the hot intergalactic medium but it's difficult to confirm as it's so difficult to detect given it's temperature and lack of emission. This further points to most of the Ha emission in the image being within our galaxy
