IRREGULAR GALAXY: FORMATION, GENERAL CHARACTERISTICS, TYPES, EXAMPLES - ARTE - 2025

An irregular galaxy is a conglomeration of stars, planets, gas, dust, and matter that, while held together by the force of gravity, is visually unorganized. It is estimated that 15% of galaxies are irregular.

Unlike galaxies like the Milky Way and Andromeda, which have a well-defined nucleus, disk, and spiral arms, or giant elliptical galaxies, irregular galaxies do not have symmetry or any of these structures. However, some with bars or incipient arms have been observed.

Figure 1. The irregular galaxies known as the Magellanic Clouds, in the constellation of the Southern Cross. Source: Wikimedia Common s. THOSE. Brunier

Formation and evolution

Lack of organization can be attributed to various causes. One of the most widely accepted is that there was some kind of colossal explosion that took place in the nucleus and fragmented and dispersed part of the content, without completely losing cohesion.

An irregular galaxy may also owe its deformation to the gravity exerted by a larger neighboring galaxy. Our Milky Way galaxy, a large spiral galaxy, has distorted the two dwarf galaxies known as Magellanic Clouds.

It has been suggested that the Magellanic Clouds are merging with the Milky Way. In the distant future, all the matter they contain may become part of it.

Another irregular galaxy that was already part of the Messier catalog of astronomical objects is the M82 galaxy, also known as the Cigar Galaxy. It is located in the constellation Ursa Major and about 12 million light-years away.

The Cigar Galaxy is very bright, about 5 times brighter than the Milky Way. It is rich in interstellar matter and within it stars are forming at an accelerated rate. When they are young, the stars are blue and bright, which explains the extraordinary brightness of this irregular galaxy.

General characteristics

To establish the astronomical dimensions the light-year, the parsec (pc) and the kiloparsec (kpc) are used. The light year is the distance that light travels in a vacuum during one year, equivalent to 9,460,730,472,580.8 kilometers.

One parsec (parallax of one arc second) is equivalent to 3.3 light years, so one kiloparsec is 3300 light-years.

As for the mass of astronomical objects such as stars and galaxies, a good idea is to express it in terms of the unit called solar mass, denoted as M☉ and which is equal to 2 x 10 ^ 30 kg. A galaxy contains an enormous number of solar masses, and its mass is conveniently expressed in powers of 10.

Another distinctive feature is the luminosity L, which comes from the energy per second that the galaxy emits at all frequencies and is proportional to the number of stars it has. It is sometimes called the bolometric magnitude.

As a reference, the luminosity of the Sun L☉ equivalent to 3.85 × 1026 W. The greater the mass of the galaxy, the greater its luminosity.

The magnitude of an astronomical object refers to the amount of emitted energy that manages to reach the Earth, but it must be taken into account that a light source is brighter when it is closer, since the energy decreases with the inverse of the square of distance.

For its part, color is a quality related to the predominant stellar population. As said at the beginning, young stars are blue, while old ones are red.

Color, mass and dimensions of an irregular galaxy

The diagram below shows three regions that correlate color and lightness. They are named red sequence, green valley and blue cloud.

Figure 2.- Color-magnitude diagram of the galaxies. Source: Joshua Schroeder

As mentioned, the color is related to the stellar population. There are two types of stellar populations: I and II.

The stars belonging to population I are generally young and in them predominate elements heavier than helium (in astronomical terminology these elements are considered metals). Population II is of low metallicity and they are considered older.

Galaxies with little or no stellar genesis appear in the red sequence. To this category belongs most of the elliptical galaxies. On the other hand, in the blue cloud are the galaxies with a high rate of star formation, to which the irregular galaxies such as the aforementioned Cigar Galaxy belong.

Finally the green valley is a transition region where galaxies that have young and old stellar populations meet. The Milky Way and Andromeda are examples of these types of galaxies.

Irregular galaxies are very interesting because they are the bluest of all, especially towards the center, which suggests that the stellar birth rate there is extremely high. They are also considered the youngest among galaxies.

Being small, they are in the range of 108-10 M☉, with sizes between 0.5-50 kpc. Of course, they have a lot of gas, up to 50-90% of the total mass is atomic gas.

Types

Astronomer Edwin Hubble classified galaxies according to their apparent shape, known in astrophysics as visual morphology. After analyzing countless photographic plates, he established five basic patterns: elliptical, lenticular, spiral, barred spiral, and irregular.

The vast majority of galaxies are elliptical or spiral, which Hubble coded with the capital letters E and S respectively. However, some galaxies lack sufficient symmetry to fall into one of these categories.

Hubble called them "irregular" or Irr. As more was learned about galaxies, the classification expanded to accommodate new categories, both by Hubble itself and by other astronomers. Thus, Gerard de Vaucouleurs makes the distinction between type I and II irregular galaxies.

Although with certain limitations, because the only view of a galaxy is the one from Earth, the Hubble scheme continues to be of great help today in establishing the characteristics and properties of galaxies.

Irregular type I galaxies

Irr I type irregular galaxies appear in the original Hubble sequence as Magellanic Clouds type galaxies, which are the most representative example. They are also named as Sd-m

They can be considered as a type of spiral galaxy consecutive to the Sc galaxies, one that did not develop the structure, or has it in a very rudimentary way. That is why they are sometimes named as Sd-m, where S indicates a spiral shape and the letter m is for Magellan.

In fact the Large Magellanic Cloud has a bar. They are the most frequent irregular galaxies and are abundant in very blue stars, as they have a high stellar birth rate.

Type II irregular galaxies

In these galaxies the stars are generally older, redder, and dimmer. These are galaxies whose matter diffuse and are totally amorphous.

Examples

The Magellanic Clouds

The Magellanic Clouds are two irregular galaxies named in honor of the explorer Fernando de Magallanes, who left Spain in 1519 on a journey around the world that lasted 3 years.

Magellan and his crew were the first Europeans to observe them, since they are visible from the southern hemisphere, in the constellation of the Southern Cross, although there are astronomical records of the Arabs claiming to have seen them from Bab el Mandeb, at 12º 15 'latitude north.

The Large Magellanic Cloud is 180,000 light-years away, while the Small Cloud is about 210,000 light-years away. Along with the Andromeda galaxy, they are one of the few that can be seen with the naked eye. Some astronomers believe that both galaxies came to our vicinity as a result of a collision between Andromeda and another galaxy, which occurred a long time ago.

For a long time the closest galaxies were considered, but since 2003 the position has been occupied by the Dwarf Galaxy of the Big Dog at 42,000 light years, followed by the Elliptical Dwarf of Sagittarius, discovered in 1994 and distant 50,000 light-years.

The Magellanic Clouds have, like most Irr I irregular galaxies, a young population of hot, blue stars. In the Large Magellanic Cloud is the Tarantula nebula, NGC 2070, of great luminosity and considered the most active region in terms of star formation, of the Local Group of galaxies, to which the Milky Way also belongs.

Cigar Galaxy

As previously stated, it is a very bright galaxy visible in Ursa Major. In the Messier catalog it has the code M82.

At its center there is high star formation activity, believed to be due to past interaction with another larger galaxy, the Bode spiral galaxy.

The Cigar Galaxy produces stars 10 times faster than the Milky Way, which is why it is said to be a boiling galaxy (starburst).

Figure 3. Cigar Galaxy M82 in the constellation Ursa Major, seen from the Hubble telescope. Source: NASA, ESA, and The Hubble Heritage Team (STScI / AURA).

So many hot stars emit radiation and charged particles that ionize hydrogen, causing the plumes and emissions that appear around the galaxy's core as red filaments.

NGC 1427A

It is a small irregular galaxy in the southern constellation Fornax, about 62 million light-years away, in which blue star clusters abound. It belongs to the Fornax cluster of galaxies and is currently traveling at about 600 km / s, through interstellar gas towards the center of the cluster.

Figure 4.- The irregular galaxy NGC 1427A seen from the Hubble telescope. Above left a spiral galaxy viewed head-on in the Fornax cluster. Source: Wikimedia Commons.

It is being attracted there by the force of gravity exerted by the other galaxies in the cluster, which in addition to deforming it, causes a high stellar birth rate in its interior. In a billion years the little galaxy will have completely dispersed

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