ELLIPTICAL GALAXIES: FORMATION, CHARACTERISTICS, TYPES, EXAMPLES - ARTE - 2025

The elliptical galaxies are ellipsoidal astronomical objects. Inside, these galaxies are home to millions of stars, planets, some gas, dust and abundant dark matter, all linked thanks to the force of gravity.

They lack an obvious structure and their luminosity is quite uniform, since the stars are regularly distributed towards the edges, where the light diffuses smoothly in the form of a very faint halo.

Figure 1. The bright elliptical galaxy NGC 3610 in the constellation Ursa Major, seen by the Hubble telescope. Source: Wikimedia Commons.

Formation and evolution

Astrophysicists at first thought that a great collapse was what gave rise to an elliptical galaxy, which gave rise to intense star formation that eventually ceased. This hypothesis is supported by the fact that the stellar population of these galaxies is older than that of the other types.

On the other hand, in elliptical galaxies there is very little gas and dust, which is known as interstellar matter, which is precisely the raw material necessary for the formation of new stars.

But current observations confirm that despite their apparent stability, galaxies are not static. The force of gravity causes them to actively interact with each other whenever there is opportunity.

For this reason, the hypothesis currently prevails that elliptical galaxies have diverse origins and that galaxies of other shapes are likely to become elliptical over time.

The gravitational attraction can cause collisions that produce an eventual merger. Events of such magnitude are not uncommon, since gravity opens the doors to this possibility. Furthermore, elliptical galaxies are often found in the middle of galactic clusters, where there is an opportunity to trap material and merge with other galaxies.

Figure 2. These two merging galaxies are known as "The Mice." They are in the constellation Coma Berenice. Source: Wikimedia Commons.NASA, H. Ford (JHU), G. Illingworth (UCSC / LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

This is confirmed by the fact that young blue stars have been detected in the interior of some elliptical galaxies - blue dwarf galaxies - showing that they are not completely devoid of interstellar matter.

It has also been suggested that when spiral galaxies use up their raw material, they evolve into a lenticular shape, that is, a disk shape without spiral arms. Successive collisions with other galaxies would lead to the loss of the disk and the transformation into an ellipsoid.

General characteristics

To get an approximation to the dimensions in the universe, the units of distance commonly used on Earth are not appropriate. In astronomy the light-year, the parsec (pc) and the kiloparsec (kpc) are in common use:

1 kpc = 1000 pc = 3300 light-years

In the measurement of the mass of objects as huge as galaxies, the unit called solar mass is used, which is denoted as M☉ equivalent to 2 x 10 ^ 30 kg.

Regarding the general characteristics of elliptical galaxies, it is clear that the most distinctive is their shape, ranging from almost spherical to very flattened ellipsoids.

As explained at the beginning, elliptical galaxies are very unstructured. They have a fairly regular distribution of ellipsoidal shape and are surrounded by a faint luminous halo, of greater or lesser extent. They lack a disk or other structure that stands out remarkably.

They can have satellite galaxies, much smaller galaxies that are under their gravitational dominance, although this is not unique to elliptical galaxies, because our Milky Way, a barred spiral galaxy, has the Magellanic Clouds as satellites.

Some also have globular star clusters, which can be mistaken for elliptical dwarf galaxies. In terms of kinematics, the stars that make up an elliptical galaxy tend to follow complicated trajectories and the angular momentum of the galaxy is thought to be of low magnitude.

Mass and dimensions

There is a lot of variability in terms of size. Because they have little interstellar gas and dust, the mass of an elliptical galaxy is stellar mass. The number of stars can vary from a few million stars to a million million stars.

Estimates to date show diameters of 1-200 kpc and in exceptional cases 1 Megaparsec - about 3 million light years.

Normally the mass is in the range of 10 ^ 6-10 ^ 13 M☉. Small elliptical galaxies, also called dwarf galaxies, are abundant in the vicinity of our Milky Way galaxy.

At the other extreme are the giant elliptical galaxies, of extraordinary luminosity. In fact, this class has the largest known galaxies, which are generally at the center of galaxy clusters, so they very possibly owe their immense size to merging with neighboring galaxies.

Types

Astronomer Edwin Hubble classified galaxies according to their shape and established five basic patterns. Its classification includes: elliptical, lenticular, spiral, barred and irregular spirals. Most galaxies, about 90% are elliptical or spiral.

Hubble placed elliptical galaxies at the beginning of his classification scheme, referring to them as "early-type galaxies" because he believed that later they evolved into other forms.

If a is the semi-major axis and b the semi-minor axis of the ellipse, the ellipticity e is given by:

E = 1 - b / a

E is an indicative measure of how flattened the ellipse is, for example if a and b are very close values, the quotient b / a is approximately 1 and the ellipticity is zero, resulting in a spherical galaxy.

The highest accepted value for E is 3 and in the Hubble classification, the first place to the left is occupied by spherical galaxies, which are denoted as E0, followed by the intermediate types E1, E2,… until reaching EN, where N = 10 (1- b / a).

The flattest ones that are known reach up to E7, since above this value the structure of the galaxy is lost.

Hubble himself modified his original classification as more information arrived. So did other astrophysicists to include new features aside from the mere ellipsoidal shape. For this, other letters were used, as well as lowercase letters.

Elliptical square (boxy) and discoidal (disky) galaxies

Outside the Hubble sequence, Ralf Bender and his collaborators proposed in 1988 two new terms to classify elliptical galaxies, which not only take into account shape, but also other very important characteristics.

In this way they were grouped into "boxy" and "disky", which are translated respectively into square and discoidal. This classification was made taking into account the isophotic lines, which join points with the same luminosity on the galactic surface.

Interestingly, these lines do not follow elliptical shape. In some galaxies they tend to be rather rectangular and in others they take the shape of a disk, hence the name.

The square ones have a greater luminosity, are larger and more active, in the sense that they have radio sources, as well as X-rays. The discoidal ones are calmer in this aspect and their luminosity is lower.

So even with the same classification in the Hubble sequence, two elliptical galaxies can have different characteristics if one of them is boxy or square and the other is disky or discoidal. These would tend to have higher rotation, while the boxy could be the result of many mergers and galactic interactions.

Elliptical cD-type galaxies

They are elliptical galaxies so colossal that it is impossible to miss them when it comes to the subject. They can be 1 Mega-parsec wide and are found in the middle of galactic clusters.

Their size is probably due to the fact that they are the result of the merger of several galaxies: between 10 ¹³ and 10 ¹⁴ M☉. They have a very bright central nucleus and are home to hundreds of thousands of globular clusters. In addition, they are presumed to contain a large amount of dark matter, necessary to explain that it remains cohesive.

Figure 3. Comparison of galaxies in which the colossal elliptical galaxy IC 1101 stands out. Source: Wikimedia Commons.

The largest of all so far is IC 1101 in the Abell 2029 cluster, in the constellation Virgo. It was discovered by William Herschel in 1790 and a maximum diameter of 6 million light-years is estimated.

Since its core is extremely active, it does not seem likely that it hosts life forms, or at least not as we know it on Earth.

Examples

Elliptical galaxies are usually found in the middle of galaxy clusters, which are associations of more or less large galaxies. In the constellation Virgo and in Coma Berenice there are notable clusters.

Since most galaxies are so far away, it is quite difficult for the eye to identify them, but using telescopes or even good quality binoculars, it is possible to distinguish galaxies of all types.

On the net there are many maps, as well as applications to discover astronomical objects. The galaxies do not usually have proper names, with few exceptions such as the Milky Way, Andromeda, the Whirlpool or Whirlpool galaxy and the Sombrero galaxy.

Most are denoted by a catalog code: the Messier (M) catalog, the NGC or New General Catalog, and the IC Index Catalog, for its acronym in English.

Galaxy M87

The stellar object known as M87 (or NGC 4486) belongs to the cluster of galaxies in the constellation Virgo. It is among the closest elliptical galaxies to Earth, some 53 million light-years away, and is of the boxy type described in the previous section. It has a very active nucleus in terms of radio frequency and plasma emission.

It is about twice the mass of our Milky Way, not including dark matter. If this could be detected, M87 would turn out to be about 200 times more massive than the Milky Way. Some 12,000 globular clusters have been identified in M87.

Figure 4. elliptical galaxy M87 seen with the Hubble telescope. Source: Wikimedia Commons.

M87 emits a jet of matter about 5,000 light-years long, believed to come from a colossal black hole surrounded by hot material that is not exactly in the center.

Galaxy M32

This is a dwarf elliptical galaxy that accompanies Andromeda, in the constellation of the same name. Because it is very compact and rotates around a very massive object, some experts suggest that it is the core of an ancient galaxy dismembered by some gravitational collapse.

Figure 5. The figure shows the Andromeda spiral galaxy, and the small elliptical galaxy M32 is the small dot to the left of the center. Source: Wikimedia Commons. Torben hansen

It is possible that in ancient times it collided with Andromeda itself, and in the images you can see how the outer stars of M32 are inexorably drawn towards their larger neighbor.

References

1. Carroll, B. An Introduction to Modern Astrophysics. 2nd. Edition. Pearson. 874-1037.
2. Galaxy. Recovered from: es.wikipedia.org
3. How it works. 2016. Book of Space. 8th. Ed. Imagine Publishing Ltd. 134-150.
4. The galaxies. Recovered from: astrofisica.cl/astronomiaparatodos.
5. Mutlaq, J. Elliptical Galaxies. Retrieved from: docs.kde.org.
6. Oster, L. 1984. Modern Astronomy. Editorial Reverté. 315-394.
7. Pasachoff, J. 1992. Stars and Planets. Peterson Field Guides. 148-154.
8. Wikipedia. Elliptical galaxy M87. Recovered from: es.wikipedia.org.