Anemic galaxy

Etymology

The term was coined in 1976 by the Canadian astronomer Sidney van den Bergh to classify galaxies that are an intermediate form between the gas-rich, star-forming spiral galaxies and the gas-poor, inactive lenticular galaxies. |doi-access=free}}

Characteristics

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),

At first it was believed their molecular hydrogen content was similar to that of a normal spiral, |doi-access=free

Anemic galaxies should not be confused with galaxies with red colors due to a low star formation activity but which show a normal content of neutral gas, as is the case of the Andromeda Galaxy. | doi-access = free

Evolution

As most galaxies of this type are present in rich galaxy clusters, it has been proposed that this may be one of the reasons that normal spirals convert into anemic ones. 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, |doi-access=free }}

An anemic galaxy's most probable fate is to lose its remaining gas and star formation, becoming similar to a lenticular galaxy: it therefore is likely that most lenticular galaxies in clusters, are former spiral galaxies.

Passive spiral galaxies

Passive spiral galaxies (also known as passive spirals) are a type of spiral galaxy located in rich galaxy clusters at high redshifts that present spiral structure, but little or no star formation, | doi-access = free

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.

While they share at least some properties with anemic galaxies,

Examples

NGC 4921 in the Coma Cluster and Messier 90 in the Virgo Cluster are examples of this type of galaxy; however, most spiral galaxies of the latter are more or less deficient in gas.

References