Types of Galaxies

> Types of Galaxies

Galaxies come in three main types: elliptical, spirals, and irregulars. A slightly more extensive description of galaxy types is given by the Hubble sequence which serves well for many purposes. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence it may miss the importance of certain characteristics of galaxies such as star formation rate.

Contents

1. Spiral Galaxy

1.1 Barred Spiral Galaxy

2. Elliptical Galaxy

3. Irregular Galaxy

Spiral Galaxy

A spiral galaxy is a type of galaxy in the Hubble sequence which is characterized by the following physical properties:

A considerable total angular momentum
Composed of a central bulge surrounded by a disk
The bulge resembles an elliptical galaxy, containing many old, so-called "Population II" stars, and usually a supermassive black hole at its center.
The disk is a flat, rotating assembly consisting of interstellar matter, young "Population I" stars and open star clusters.

Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge. Though sometimes difficult to discern, such as in flocculent spirals, these arms distinguish spiral galaxies from their lenticular counterparts, which exhibit a disk structure but no evident spiral.

Andromeda Galaxy (Spiral)

The Andromeda Galaxy, a spiral galaxy similar to our own, though somewhat larger. It is the farthest object that is visible with the naked eye. It can be seen in the northern-sky constellation Andromeda. The Milky Way and Andromeda galaxies are the dominant members of the Local Group of galaxies, which in turn is an outlying part of the Virgo Cluster, which comprises thousands of galaxies.

The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.

Our galaxy, the Milky Way, has long been thought to be a spiral, with a Hubble sequence classification of Sbc (possibly SBb); recent research, however, suggests that it may in fact be a barred spiral.

The early pioneer of studies on the formation of the spiral arms was Bertil Lindblad. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is not what is observed.

The first acceptable theory was devised by C. C. Lin and Frank Shu in 1964. They suggested that the spiral arms were manifestations of spiral density waves. They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. This is illustrated in the diagram. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.

Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy; the bright stars produced by the star formation die out quickly, leaving darker regions behind the waves, and hence making the waves visible.

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Barred Galaxy

A barred spiral galaxy is a spiral galaxy with a band of bright stars emerging from the center and running across the middle of the galaxy. Spiral arms appear to emerge from the ends of the "bar" in these galaxies, whereas they appear to emerge directly from the core in ordinary spiral galaxies.

Edwin Hubble classified these types of spiral galaxies as "SB" ("Spiral", "Barred"), and arranged them into three sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb type galaxies lie in between. A fourth type, SBm, was subsequently created to describe dwarf barred spirals, such as the Magellanic Cloud galaxies, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures.


NGC 1300, viewed nearly face-on. Hubble Space Telescope image.	NGC 253, a barred spiral starburst galaxy, (2MASS).

Barred spiral galaxies are relatively common, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers. The bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in Messier 83.

The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure. Another possible culprit in bar creation are tidal disruptions between galaxies.

Bars are thought to be a temporary phenomenon in the life of spiral galaxies, the bar structure decaying over time, transforming the galaxy from a barred spiral to a "regular" spiral pattern. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, stubby bars. Since so many spiral galaxies have a bar structure, it is likely that it is a reoccurring phenomenon in spiral galaxy development.

In 2005, observations by the Spitzer Space Telescope backed up previously collected evidence that suggested the Milky Way is a barred spiral galaxy. Observations by radio telescopes had for years suggested our galaxy to be barred, but Spitzer's vision in the infrared region of the spectrum has provided a more definite calculation.

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Elliptical Galaxy

An elliptical galaxy is a type of galaxy in the Hubble sequence characterized by the following physical properties:

No or at most an inconsiderable amount of angular momentum.
No or at most an inconsiderable amount of interstellar matter, no young stars, no open star clusters
Consisted of old, so-called Population II stars.
Larger elliptical galaxies typically have a system of globular clusters, indicating an old population.

NGC 4881 Elliptical Galaxy

The giant elliptical galaxy NGC 4881 (the spherical glow at upper left) lies at the edge of the Coma Cluster of Galaxies.

This traditional portrait of elliptical galaxies paints them as galaxies where the star formation was over after the initial burst, now shining only with their aging stars. No significant star formation was thought to happen. In general they appear yellow-red, which is in contrast to the distinct blue tinge of a typical spiral galaxy, a colour emanating largely from the young, hot stars in its spiral arms.

There is a wide range in size and mass for elliptical galaxies: a small as a tenth of a kiloparsec to over 100 kiloparsecs, and from 10⁷ to 10¹³ solar masses. The smallest, the Dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. The single largest known galaxy, M87 (which also goes by the NGC number 4881), is an elliptical. This range is much broader for this galaxy type than for any other.

Their shape varies from spherical to highly elongate. The Hubble classification of elliptical galaxies ranges from E0 for those that are most spherical, to E7, which are long and thin.

Some recent observations have found young, blue star clusters inside a few elliptical galaxies, along with other structures that can be explained by galaxy mergers. Current thinking is that an elliptical galaxy is the result of a long process where one or more smaller galaxies, of any type, collide and fuse in a larger one.

This fusion process may at time carry on until recent or contemporary ages, and is not limited to giant elliptical. For example, our own Milky Way is known to be "digesting" a couple of small galaxies right now.

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Irregular Galaxy

An irregular galaxy is a galaxy that does not fall into the Hubble classification for galaxies. These are galaxies that feature neither spiral nor elliptical morphology. They are often chaotic in appearance, with neither a nuclear bulge or any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies.

There are two major Hubble types of irregular galaxies:

An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. de Vaucouleurs subtypes this into galaxies that have some spiral structure Sm, and those that do not Im.
An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.

A third classification of irregular galaxies are the dwarf irregulars, labelled as dI or dIrrs. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys.

Some irregular galaxy are small spiral galaxies that are being distorted by the gravity of a larger neighbour.

The Magellanic Cloud galaxies were once classified as irregular galaxies, but have since been found to contain barred spiral structures, and have been since re-classified as "SBm", a fourth type of barred spiral galaxy.

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