por The barred spiral galaxy It 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 runs through it, and from which spiral arms emerge from diametrically opposite points.
In this they differ from regular spiral galaxies, in which the arms start at various points around the nucleus. The spiral also presents various degrees of development.
Bars are quite common 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.
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Formation and evolution
In the Hubble sequence, a classifying 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 lower case letters to specify particular characteristics, such as the opening of the arms in a spiral.
In this way the main subgroups are distinguished: the galaxies SBa, SBb and SBc. In the first, 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 thought to be rather modest. It has been classified as SBbc, a galaxy with characteristics intermediate 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 reddish and older, while in the arms they are blue and hot, because there is a high rate of star formation there, very hot at the beginning of its life. Barred spiral galaxies are characterized by presenting:
Bulb: a very bright 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. In the disk is a mixed population of stars: new and old.
Bar: This structure crosses the disk and according to more recent theories, it acts as a kind 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 formation rate.
Halo: is the tenuous and diffuse structure that completely surrounds the galaxy, composed largely of the so-called dark matter.
Effects of the bar on the properties of a galaxy
The galactic bar is believed to play important roles in transport, as well as in the overall dynamics of the galaxy. By means of numerical simulations it has been verified that the bar is, as previously stated, a way for the transport of gas 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 enough mass accumulates in the center, the bar is destroyed.
This is the reason why it is believed that many regular galaxies could, in the past, have been barred. 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 within the galaxy, the bar affects the rate of star formation and is determinant of color and chemical composition. As far as galaxies are concerned, the color is determined by the type of predominant stellar population.
Stellar populations are classified in 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 effects of the bar on color are not yet clear.
Another important point is that some researchers suggest that the rod is capable of activating the galactic nucleus so that it produces intense energy emissions at high frequencies, as well as altering the structure, promoting the formation of bulges and pseudobulbs.
examples
Barred spiral galaxies are the most abundant of the spiral galaxies. In general, they are large galaxies, whose mass ranges between 109 -1012 solar masses and diameter 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 bar and fledgling arms.
NGC 1672
The spiral galaxy NGC 1672, 75,000 light-years in diameter, has a particularly bright and highly active nucleus, with a bar of 20 kpc -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 at the center of its extraordinarily bright and reddish core. Bright blue areas, rich in newly formed stars, can be seen in the spiral arms.
Magellanic spiral galaxies
Also classified as an Irr I irregular galaxy, the Large Magellanic Cloud is the prototype of Im Magellanic spiral galaxies, which have a central bar, but barely formed spiral arms. This galaxy has an extensive region of great stellar activity.
References
Matías, S. 2016. Effect of bars on the galaxy properties. Posted in: Astronomy & Astrophysics.
Chicken, A. Galaxy properties. Retrieved from: pta.edu.pl.
Schneider, P. 2015. Extragalactic Astronomy and Cosmology. Second edition. Springer Verlag. 54-67 and 116-126.
Wikipedia. Barred spiral galaxy. Retrieved from: es.wikipedia.org
Wikipedia. Large Magellanic Cloud. Retrieved from: es.wikipedia.org.