One of the qualities of all matter is its capacity to organize itself to form complex structures, and although it may seem that this contradicts the second law of thermodynamics, in reality it is nothing more than an expression of it. The four fundamental forces have the capacity to build, from the quantum world to the cosmic scale, structures that scale in levels of complexity; galaxies are one of these structures, and the last ones with a clearly defined characterization that we can identify. Beyond them, the structures that make up the universe become more diffuse and difficult to identify, because of the magnitudes that characterize them.
What are galaxies?
It can be said that galaxies are a grouping of stars, stellar remnants, planets, satellites, other minor bodies, gas, dust and dark matter, which are gravitationally bound together to form an identifiable structure.
The main and most notable components of galaxies are the stars, their number can vary from 10^7 in dwarf galaxies, such as the Small Magellanic Cloud, to 10^14 in the giants.
In the beginning, galaxies were considered gas clouds, which were part of our neighborhood, for this reason they were initially called nebulae, for example Charles Messier in his Catalogue des Nébuleuses et des amas d'Étoiles, que l'on découvre parmi les Étoiles fixes sur l'horizon de Paris (Catalog of Nebulae and Star Clusters, which are observed among the fixed stars above the Paris horizon), of 1774, classified the Andromeda Galaxy as M31 or Andromeda Nebula.
The first to propose the idea that the, by then, so-called spiral nebulae, were actually clusters of stars that were more than 150,000 Parsec (1) away, was Heber D. Curtis, in 1917, who came to this conclusion by studying the photographic record of novae observed in different spiral nebulae and noticing that they were 10 times less bright than those observed in the Milky Way. This observation led him to postulate the hypothesis of island universes, of which the Milky Way would be one.
It was Edwing Hubble, using the Mount Wilson telescope, who in 1924, managed to distinguish stars in the Andromeda Nebula, by then called Andromeda Nebula, and identify a particular type of them, the Cepheid VV, the study of the redshift of its spectrum allowed him to determine that they were between 800 and 1000 million light years away, eight times more distant than the most distant known stars of the Milky Way.
How did galaxies form?
Theoretically, the origin of galaxies goes back to the early universe, during the period of inflation, for some unknown reason, quantum fluctuations caused matter to expand unevenly in all places, causing what is reflected in the cosmic microwave background as anisotropies, the areas of greater concentration of matter were distributed in filaments or cosmic strings, formed as a result of the phase changes that the universe underwent, these accumulations of matter gave rise to gigantic clouds of gas from which originated clusters of stars that would later give rise to galaxies.
This would explain why galaxies would be distributed as clusters that are organized as networks surrounded by empty space, and are not uniformly dispersed throughout the universe.
However, recent observations indicate that galaxies could have originated some 600 million years after the Big Bang, which would not allow enough time for the small anisotropies observed in the cosmic microwave background to give rise to such large structures; which seems to question the accepted theory of their formation.
On the other hand, the theory is also insufficient to explain several features observed in our own galaxy that could be common to others, such as: why the Galactic Halo does not rotate or does it very slowly and is so dispersed, while the Galactic Disk rotates and is so dense. It is also inexplicable that the stars that form the halo, with lower density, are older and of lower metallicity than those that form the disk, as well as it does not explain why even though in general the Globular Clusters that are supposed to be formed by stars born at the same time and are formed by very old stars, in some cases almost as old as the universe, in some others they seem to be formed by younger stars and even by stars born at different times.
How are galaxies classified?
The instrument used to classify galaxies is the Hubble Sequence, this is a classification proposed by Edwing Hubble in 1936, which is based on the shape of galaxies, basically four types of galaxies can be identified, the spirals, which at the same time are divided into simple and barred spirals, ellipticals, lenticular and irregular galaxies.
Some astronomers claim that the Hubble sequence should be read backwards, which will be explained later.
Spiral galaxies: Spiral galaxies have the shape of a more or less flat disk with a bulging central region formed by older stars, which is called a bulge, from which the spiral arms emerge, giving them their name.
The spiral arms would contain most of the interstellar gas and dust so they would have the highest star formation rate, while the bulge would have little interstellar matter.
Two types of spiral galaxies can be found, the regular or simple ones which are denoted by the letter S in the classification and the Barred Spirals, which have a band of stars extending on both sides of the bulge and from which the arms arise, these are denoted by the letters SB in the classification.
Both types of spirals are subdivided according to the tightness of their spiral arms, for which a lower case letter 'a' to 'c' is added to denote the tightest to the loosest, respectively.
Additionally it has been proposed to add an additional type of spiral galaxy that would be an intermediate step between the simple spirals and the barred ones called Semi Barred, and would be denoted by the letters SAB.
Lenticular galaxies: they are formed by a defined disk surrounding a large central cloud of elliptical shape, they are intermediate states between spirals and ellipticals.
The reason why it is stated that the Hubble sequence should be read backwards is because lenticular galaxies would be the result of the collision of two galaxies, either spirals or a spiral and one of another type.
Like spirals, lenticular galaxies can be regular and barred, the former being denoted by the letters SO and the latter by SOB, additionally, according to the density of the disk and the central bar are added numbers from 1 to 3 from the least to the densest.
Additionally, other classifications have been proposed, such as SOAB for those with a rudimentary central bar, or E/SO to denote elliptical galaxies that resemble lenticular galaxies, among others.
Elliptical galaxies: Elliptical galaxies are usually formed by old stars and show little star formation, have little interstellar matter and consequently few open clusters.
They are denoted with the letter S and numbered from 0 to 7 according to their eccentricity, being the S0 spherical and the S7 plate-shaped.
These galaxies would be the result of the stabilization of lenticular galaxies and the exhaustion of their interstellar matter, consequently, they would also be the product of the collision of spiral or spiral galaxies and others.
These galaxies are usually the largest and are generally found in the nuclei of large galaxy clusters.
Irregular galaxies: Irregular galaxies lack an identifiable shape that allows them to be classified in the Hubble sequence, they are usually small in size and many of them are small spiral galaxies that are deformed by the presence of larger neighbors.
These galaxies are classified as Irr I, which denotes those that present some structure that could resemble a spiral, and Irr II, which do not present any identifiable shape. In addition, the name dI is used to refer to irregular dwarf galaxies.
There are other types of galaxies, which differ from the others not in their shape, since they can be characterized within the Hubble sequence, but in their ability to emit electromagnetic radiation in different spectra in a particularly remarkable way; they are called Active Galaxies.
Active galaxies are those in which a good part of their emission in the electromagnetic spectrum is not due to the components that are normally sources of emission, such as stars or clouds of ionized gas. Very often these intense emission sources are associated with the supermassive black hole that all galaxies have in their interior, which in most galaxies is inactive.
Seyfert galaxies: they owe their name to Carl K. Seyfert, who, in 1943, noticed that a group of galaxies, mainly spirals, had a nucleus that seemed to emit a considerable amount of electromagnetic radiation in the infrared and visible range. Later, with the discovery of supermassive black holes in galactic nuclei, it was concluded that these emissions were due to the accretion of matter in them.
In these galaxies the nucleus would have a considerable amount of interstellar matter that when trapped by the gravity of the black hole would begin to rotate around it, forming a disk of ionized gas that would be the source of light and infrared emissions.
Radiogalaxies: these are galaxies that present an intense emission in the frequency of radio waves, this emission is due to Synchrotron Radiation (2). In this type of galaxies, there are large jets of matter in the form of plasma that are ejected at relativistic velocities, which interact with the electromagnetically charged interstellar medium generating by synchrotron effect the emission of very intense radio waves.
This type of galaxies are usually elliptical galaxies, in which the supermassive black hole in their nucleus is active and generates plasma jets by ejecting part of the matter that is accelerated in the accretion disk. It is thought that this phenomenon does not occur in spiral galaxies, because the abundance of interstellar matter at low temperature prevents these plasma jets from being projected.
Quasars: unlike Seyfer galaxies that emit infrared radiation and visible light, and radio galaxies that emit radio waves, quasars have the peculiarity of emitting large amounts of electromagnetic radiation in both the visible spectrum and in the infrared, ultraviolet, X-rays and radio waves, the intensity of its brightness is such that led astronomers to think that they were stars that were in our galaxy, and called them quasi-stellar objects because of their resemblance to a star and their intensity in radio wave emissions that differentiated them from common stars (Quasar is the acronym for quasi-stellar radio source), however, the redshift of their spectrum showed that they were the most distant objects observed to date.
Quasars would be the most intense sources of visible light emission that have been detected, most are so distant that they can only be observed with large telescopes. The brightest observable is 3C 273 located 2,200 light years away in the constellation Virgo, its apparent magnitude is 12.8, but its absolute magnitude at 10 parsec (33 light years) is -26.7, which means that, if it were at that distance from us, its brightness would exceed that of the Sun, this is the only quasar that can be seen with an amateur telescope.
The intense electromagnetic radiation emitted by these objects is supposed to come from matter orbiting at high speed in the accretion disk around the supermassive black hole of a galaxy, however it is not really known what processes make possible the emission of such amount of energy, even it is not known for sure what kind of galaxy houses this type of objects, some observations of the closest ones allow us to assume that they are elliptical galaxies, but this is not yet fully confirmed.
Within this context, there is another type of especially luminous objects called Blazars. Basically a Blazar is a Quasar, whose jet of particles points directly in our direction, which is why they are particularly bright.
What is the relationship between dark matter and galaxies?
I have previously spoken in this blog about dark matter, although never with the rigorousness of the case, in astrophysics and astronomy this term is used to refer to a theoretical type of matter that would correspond to approximately 80% of the total matter that makes up the universe. It is called dark matter because it does not emit any type of electromagnetic radiation, nor does it interact with it, making it imperceptible by the means currently available.
Its nature is completely unknown and its existence has been deduced and confirmed by different theoretical means and indirect evidence. One of these evidences is the fact that, in galaxies, its components can be kept together thanks to gravity and have not ended up dispersing.
The concept of dark matter is due to the Swiss Caltech astrophysicist Fritz Zwicky, who in 1933, estimated the mass of the Coma cluster of galaxies, based on the motion of galaxies at its edges, this estimate turned out to be 400 times larger than what could be calculated by the observational evidence of the number of galaxies and their brightness, then proposed that there must be some kind of "non-visible matter" that was the origin of the additional mass.
Later between the 1960s and 1970s, Vera Rubin, an astronomer at the Carnegie Institution of Washington, determined by spectrography, that in spiral galaxies the angular velocity of rotation of many stars in different orbits was almost equal, which would imply that the galaxies as a whole would have a more or less uniform spin speed, this included the bulge much denser than the spiral arms, from this I can deduce that either Newtonian gravity does not apply on a galactic scale or more than 50% of the mass of the galaxy is in the galactic halo and is not visible. This result, questioned at the time, was later confirmed by observations of other astronomers, which led to the confirmation that the mass of galaxies would be composed mostly of dark matter.
At present the origin of dark matter is still unknown, it is speculated that it could be some kind of heavy Neutrino (3) or other hypothetical particles such as Axions (4) or WIMPs (5), it has also been proposed the possibility that it is dark objects such as planets or dwarf stars, however the latter have been associated with another type of matter called Baryonic (6) dark matter, which estimates indicate that it could be 50% of the total baryonic matter.
Recently, theoretical physicists Juan García-Bellido and Sébastien Clessese, among others, have hypothesized that dark matter could be formed by clusters of primordial black holes, which would originate during the process of cosmic inflation. The recent LIGO observations of gravitational waves from collisions of black holes with masses up to 50 solar masses raise the possibility of finding evidence for the existence of hypothetical intermediate-mass black holes, which would exceed 100 solar masses and could not be attributed to stellar collapse, since there could not be stars so massive as to have cores exceeding 100 solar masses.
It has been speculated that these black holes may be more frequent than thought and would exist in clusters in the galactic halo, which would explain the missing mass that is associated with dark matter.
What are galaxy clusters?
Galaxies are grouped into clusters and these into superclusters. In galaxy clusters, galaxies tend to move towards each other as an effect of gravity and together towards the center of gravity of the cluster. While in superclusters, the clusters tend to move together in the direction of the center of gravity of the supercluster.
The clusters of galaxies would be located in a network, a sort of spider web, formed by filaments of baryonic matter of low density, mainly gas and dust, in whose nodes would be the clusters of galaxies, this network would be the product of irregularities in the distribution of matter during the process of cosmic inflation, these at the same time caused by quantum fluctuations. The structure of filaments would be produced during the phase changes that occurred in the early universe, which would give rise to cosmic strings which would give this peculiar form of network to the distribution of baryonic matter.
What does our galaxy look like?
The galaxy in which the solar system is located is the Milky Way, its name originating in ancient Greece, where its appearance as a faint, misty white band across the night sky was associated with the milk spilled from the breast of the goddess Hera.
The Milky Way is a barred spiral galaxy, this can be deduced by the shape of a flat band that can be observed in the sky, which gives away its flat disk shape, it is estimated to have a mass of 10^12 solar masses, is formed by about 200,000 to 400,000 stars and has a diameter of between 100,000 and 200,000 light years.
In our galaxy three structures can be distinguished, which are common to all spiral galaxies. The nucleus or bulge, the disk and the halo.
The bulge would be formed mainly by old stars and would have less interstellar matter, so there would be almost no star formation processes, in the Milky Way there would not be a point bulge, but a pseudo bulge, which would have an elongated bar shape. In the center of the bulge would be a super massive black hole that has been named Sagittarius A, which would have a mass of 2.6 million solar masses. Sagittarius A would be an inactive black hole, because it would have no interstellar matter in its vicinity to be absorbed.
The disk corresponds to the arms that arise from the central bar or pseudo bulb, it would be formed by 4 main arms; the Perseus, Orion, where the solar system is located, the Shield-Centaur and Sagittarius. Other arms of lesser density have also been identified, such as the Norma and the outer arms.
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The disk is the region of the galaxy where the highest rate of star formation occurs, due to the abundance of interstellar matter, is formed mainly by young stars.
The halo is a region around the galactic disk with sparse stellar population, formed mainly by globular clusters, the halo would have a predominance of dark matter, and would originate even before the galactic disk, when the galaxy was still a gas cloud that was collapsing to a flat shape.
There would be little interstellar matter in the halo, so there would be no star formation and its stars would be the oldest. The motion of the halo would be transverse to the rotation of the galactic disk, so that some of its components at some point cross the disk or bulge, a star that is part of the halo, can be identified by transverse motion to the galactic plane and its low metallicity.
What is our galactic neighborhood like?
The Milky Way is part of the so-called local group, a grouping or cluster of galaxies, which consists of about forty galaxies, of which the main ones are Andromeda, the Milky Way and the Triangulum Galaxy, the rest are dwarf galaxies, some of which are satellites of one of the main ones.
The local group is part of the Virgo supercluster, a grouping of about 100 galaxy clusters, which is dominated by the Virgo Cluster, and whose center of gravity is the so-called Great Attractor. In whose direction the members of the supercluster move.
Going a little further, the Virgo Supercluster, together with three other superclusters, is part of a filament called the Liniakea Hypercluster, which is formed by more than 100 thousand galaxies and has a diameter of 520 million light years.
The Milky Way, together with the local group, move in the direction of the Virgo Cluster, in its trajectory the Milky Way and Andromeda will collide, giving rise to a larger galaxy, which has been named Lactomeda and which will most likely be an elliptical galaxy, In the process, part of the matter that forms the disk of both galaxies will be expelled to the extra galactic space, this collision will begin in about 3870 million years and will conclude 2000 million years later, when both galaxies have merged completely. In this process the supermassive black holes of both galaxies will also merge, giving rise to an even larger one and possibly could be activated giving rise to the emission of jets of charged particles that would turn Lactomeda into a radio galaxy.
This brief description of our galactic neighborhood and its future concludes this publication, I hope it has been to your liking and I am attentive to your comments and suggestions. Thank you for your attention.
Notes
Parsec: is a unit of astronomical length that corresponds to one arc second of parallax between the Sun and the Earth, equivalent to 3.2616 light years.
Synchrotron radiation: a type of electromagnetic radiation produced when an electrically charged particle moves in a curved path at relativistic velocities in the midst of a magnetic field.
Neutrino: a type of subatomic particle of neutral electric charge and very small mass, its name comes from Italian and means small neutron.
Axion: a type of hypothetical sub atomic particle of neutral electric charge, proposed as part of a solution to the CP symmetry problem, in theory high energy photons could temporarily become axions, which explains why high energy photons manage to cross great distances without being absorbed by the interstellar medium.
WIMP: stands for weakly interacting massive particles (weakly interacting massive particles) would be a type of hypothetical particles that would not interact through gravity or the strong nuclear force, and since they do not interact or emit electromagnetic radiation, they could not be detected.
Baryonic matter: baryonic matter is that which would be formed by baryons which are particles formed by three quarks, such as neutrons and protons, all matter formed by atoms, for example, is baryonic matter.
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