An anemic galaxy is a type of

spiral In mathematics, a spiral is a curve which emanates from a point, moving further away as it revolves around the point. It is a subtype of whorled patterns, a broad group that also includes concentric objects. Two-dimensional A two-dimension ...

galaxy A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...

characterized by a low contrast between its

spiral arm Spiral arms are a defining feature of spiral galaxies. They manifest as spiral-shaped regions of enhanced brightness within the galactic disc. Typically, spiral galaxies exhibit two or more spiral arms. The collective configuration of these arms i ...

s and its disk.

Etymology

The term was coined in 1976 by the

Canadian Canadians () are people identified with the country of Canada. This connection may be residential, legal, historical or cultural. For most Canadians, many (or all) of these connections exist and are collectively the source of their being ''C ...

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

Sidney van den Bergh Sidney Van den Bergh (born 20 May 1929) is a retired Dutch-Canadian astronomer. Van den Bergh showed an interest in science from an early age, learning to read with books on astronomy. In addition to being interested in astronomy, he also like ...

to classify galaxies that are an intermediate form between the gas-rich, star-forming spiral galaxies and the gas-poor, inactive

lenticular galaxies A lenticular galaxy (denoted S0) is a Galaxy morphological classification, type of galaxy intermediate between an elliptical galaxy, elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-s ...

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), redder colours than a normal spiral, fewer

H II region An H II region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light year ...

s, and thus a low star formation activity. At first it was believed their

molecular hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

content was similar to that of a normal spiral, but subsequent studies have shown that a number of them are deficient in molecular gas. 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 The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

Evolution

As most galaxies of this type are present in rich

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. Clusters consist of galax ...

s, 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 The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the Virgo constellation. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the larger ...

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, but also how star formation activity in them is lower than in spirals outside clusters; this means that processes that take place in galaxy clusters, such as interactions with the

intracluster medium In astronomy, the intracluster medium (ICM) is the superheated plasma (physics), plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ...

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

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and e ...

s that present spiral structure, but little or no star formation, in some cases hidden by

dust Dust is made of particle size, fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian processes, aeolian process), Types of volcan ...

and concentrated within their innermost regions. Often, they seem to have few or no massive (>20

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es) stars. According to computer simulations, they are systems on the way to becoming

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, their relationship with them is unclear: they may be a more advanced stage in the evolution of a spiral galaxy to become a lenticular than the anemic galaxies, or passive spirals and anemic galaxies may be the same type of object, the difference being that the former are much farther away than the latter.

Examples

NGC 4921 in the

Coma Cluster The Coma Cluster (Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster (Abell 1367), it is one of the two major clusters comprising the Coma Supercluster. It is located in and tak ...

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

{{Galaxy * Galaxy morphological types 1976 in science