BARRED SPIRAL GALAXY: FORMATION, EVOLUTION, CHARACTERISTICS - ARTE - 2025

The barred spiral galaxy is a grouping of astronomical objects, gas, dust, and dark matter that is maintained by the force of gravity. Its shape is spiral with a central bar of stars that crosses it, and from which the spiral arms arise that start from diametrically opposite points.

In this they differ from regular spiral galaxies, in which the arms start from various points around the nucleus. The spiral also presents varying degrees of development.

Figure 1.- The barred spiral galaxy NGC 1300, 61 million light-years away, in the constellation Eridano. Source: NASA, ESA, and The Hubble Heritage Team STScI / AURA)

Bars are quite frequent in spiral galaxies. It is estimated that up to 2/3 of spiral galaxies have one. However, some irregular galaxies are barred, such as the Large Magellanic Cloud, a neighboring galaxy without spiral arms.

Formation and evolution

In the Hubble sequence, a classification system for galaxies proposed by astronomer Edwin Hubble (1889-1953) in 1936, elliptical galaxies are assigned the letter S for spiral in English, the letter B for barred, and lowercase letters for specify particular characteristics, such as the opening of the spiral arms.

In this way the main subgroups are distinguished: the galaxies SBa, SBb and SBc. In the former both arms are more closed, in the SBc group the nucleus is small with wide and well separated spiral arms, while the SBb subgroup has intermediate characteristics between the two.

Our galaxy, the Milky Way, belongs to the group of barred spiral galaxies, although the bar is believed to be rather modest. It has been classified as SBbc, a galaxy with intermediate characteristics between SBb and SBc.

General characteristics

The light profile of spiral galaxies is less concentrated than that of elliptical galaxies. Towards the center of the spiral galaxy the stars are redder and older, while in the arms they are blue and hot, because there is a high rate of star formation, very hot at the beginning of their life. Barred spiral galaxies are characterized by:

Bulb: a very luminous spheroidal component, because it contains a large number of stars. There is the galactic nucleus, where it is common to find a black hole.

Disk: it is the more or less circular structure that forms the middle plane of the galaxy, rich in gas and interstellar matter. On the disc you will find a mixed population of stars: new and old.

Bar: this structure crosses the disk and, according to more recent waves of theories, acts as a sort of nursery for stars and allows the passage of gas from the spiral arms, thus activating the formation of stars.

There are degrees of intensity in the bar, in order to distinguish galaxies with a strong bar or a weak bar.

Spiral arms: abundant in interstellar material -gas and dense dust-, which gives rise to new stars. That is why they are rich in hot, blue, young stars, with a much higher rate of formation.

Halo: it is the dim and diffuse structure that completely surrounds the galaxy, made up largely of so-called dark matter.

Figure 2. Artistic recreation of the Milky Way, showing the rod and spiral arms. Source: Wikimedia Commons. Image created by NASA.

Effects of the bar on the properties of a galaxy

The galactic bar is believed to play important transport functions, as well as in the overall dynamics of the galaxy. By means of numerical simulations it has been verified that the bar is, as it was said previously, a way for the gas transport from the external zones towards the galactic center.

The gas clouds interact at the edges of the bar, losing angular momentum and thus facilitating the creation of a flow of matter. Computer simulations also show that if the mass accumulates in sufficient quantity in the center, the rod is destroyed.

This is the reason why it is believed that many regular galaxies may have had a bar in the past. And they also consider that the presence of a bar could be recurrent, under certain conditions that favor its formation.

By facilitating the flow of matter into the interior of the galaxy, the bar affects the rate of star formation and is decisive in color and chemical composition. When it comes to galaxies, the color is determined by the predominant type of stellar population.

Stellar populations are classified into population I, young stars in which elements heavier than helium predominate –high metallicity - and population II, older and with low metallicity. Some barred galaxies are more reddish in color, so the bar's effects on color are not yet clear.

Another important point is that some researchers suggest that the rod is capable of activating the galactic nucleus to produce intense emissions of energy at high frequencies, as well as altering the structure, promoting the formation of bulbs and pseudobulbs.

Examples

Barred spiral galaxies are the most abundant of the spiral galaxies. They are generally large galaxies, whose mass ranges between 10 ⁹ -10 ¹² solar masses and a diameter of between 5-50 kpc -16,500 to 165,000 light-years-, except for Magellanic-type galaxies, such as the Large Magellanic Cloud., a small irregular galaxy with an incipient bar and arms.

NGC 1672

The 75,000 light-year diameter spiral galaxy NGC 1672 has an especially bright and highly active core, with a 20 kpc bar - about 66,000 light years - and asymmetric spiral arms. It is located in the constellation Dorado about 60 million light-years away.

It is very likely that there is a black hole in the center of its extraordinarily bright and reddish core. The spiral arms show bright blue areas, rich in newly formed stars.

Figure 3. The barred spiral galaxy NGC 1672 in the southern constellation Dorado. Source: Wikimedia Commons. NASA, ESA, and The Hubble Heritage Team (STScI / AURA) -ESA / Hubble Collaboration

Magellan spiral galaxies

Also classified as an Irr I type irregular galaxy, the Large Magellanic Cloud is the prototype of the Im Magellan spiral galaxies, which have a central bar, but barely formed spiral arms. This galaxy has an extensive region of great stellar activity.

Figure 4. Large Magellanic Cloud, although considered an irregular galaxy, has an incipient bar and arms. Source: Wikimedia Commons. Andrew Z. Colvin.

Reference

1. Matías, S. 2016. Effect of bars on the galaxy properties. Published in: Astronomy & Astrophysics.
2. Chicken, A. Galaxy properties. Recovered from: pta.edu.pl.
3. Schneider, P. 2015. Extragalactic Astronomy and Cosmology. Second edition. Springer Verlag. 54-67 and 116-126.
4. Wikipedia. Barred spiral galaxy. Recovered from: es.wikipedia.org
5. Wikipedia. Large Magellanic Cloud. Recovered from: es.wikipedia.org.