HI outflow from NGC3628 is shown in the negative plate at right, from Haynes, Giovanelli & Roberts 1979 ApJ 229 83. The HI contours tell a plain story of gas which is venting out of NGC3628 and filling up a gravitational bowl around the galaxy, then overflowing the lip of the bowl towards the left and flowing viscously into the intergalactic medium (IGM), free of the gravitational influence of its parent galaxy. There is also a smaller flow to the south.
The significance of this picture is that NGC3628 has a clearly defined gravitational boundary not far from the visible disk. This contradicts a major tenet of "dark matter" astronomy that galaxies have enormous haloes of material which extend very far away from the visible galaxy.
A further significance is the commentary on the nature of gravity. Why does NGC3628's gravitational influence terminate so close to the visible disk? After all, should not the gravitational influence taper off slowly with ever greater distance, as per the inverse square law? That is does not do so shows that there is an ambient gravitation to the IGM which is not modelled by conventional cosmology, a gravitational scalar which has no clear source. Furthermore, the absence of a large halo means that gravitational rotation profiles are more anomalous than ever; to recapitulate this, the outer disks of spiral galaxies rotate about their nucleus about as fast as the inner disks, in flagrant contradiction of normal gravitational models. Very large (unseen) matter haloes have been posited to account for the discrepancy, the idea being that the additional matter generates stronger gravitational fields in the outer disk. But the HI around NGC3628 bleeding off into the IGM shows us that there is no large halo. So why does the outer disk rotate so fast?
The Hypatia model of cosmology encompasses such apparent contradictions by modelling space as the surface of a large 5D body with gravitational highlands and lowlands not dissimilar to terrestrial topography. This confers the gravitational scalar onto the IGM, and the rotational profiles of galaxies are controlled by their topology onto the surface of Hypatia, as well as controlling the large scale clustering of galaxies seen in large-scale surveys.