Anemic galaxy

[1] Anemic galaxies not only show spiral arms of low contrast but also a low content and density of neutral hydrogen (the raw material needed to form stars),[2][3] redder colours than a normal spiral, fewer H II regions, and thus a low star formation activity.

[3] At first it was believed their molecular hydrogen content was similar to that of a normal spiral,[4] but subsequent studies have shown that a number of them are deficient in molecular gas.

Studies of spiral galaxies in the nearby Virgo cluster have shown not only how, unlike in isolated spiral galaxies, in most cases their neutral gas and star formation are truncated within their optical disks, in some cases quite severely,[8][9] but also how star formation activity in them is lower than in spirals outside clusters;[9] this means that processes that take place in galaxy clusters, such as interactions with the intracluster medium like ram-pressure stripping and/or interactions with other neighboring galaxies, are responsible for the origin of anemic galaxies, stripping the normal spirals of their gas, increasing in some cases their star formation activity, and thus in the end quenching the latter as their gas is exhausted and not replenished.

[10] Spiral galaxies may have become anemic ones by exhausting their supply of gas via star formation activity.

[12] According to computer simulations, they are systems on the way to becoming lenticular galaxies as they have lost the hydrogen that is assumed to be present in the haloes of spiral galaxies and that replenishes them with new gas to form stars.