Galaxies are categorised according to their visualmorphology aselliptical,[5]spiral, orirregular.[6] The Milky Way is an example of a spiral galaxy. It is estimated that there are between 200 billion[7] (2×1011) to 2 trillion[8] galaxies in theobservable universe. Most galaxies are 1,000 to 100,000parsecs in diameter (approximately 3,000 to 300,000light years) and are separated by distances in the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly)[9][a] and is separated from theAndromeda Galaxy, its nearest large neighbour, by just over 750,000 parsecs (2.5 million ly).[12]
The space between galaxies is filled with a tenuous gas (theintergalactic medium) with an average density of less than oneatom per cubic metre. Most galaxies are gravitationally organised intogroups,clusters andsuperclusters. The Milky Way is part of theLocal Group, which it dominates along with the Andromeda Galaxy. The group is part of theVirgo Supercluster. At thelargest scale, these associations are generally arranged intosheets and filaments surrounded by immensevoids.[13] Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure namedLaniakea.[14]
Etymology
The wordgalaxy was borrowed viaFrench andMedieval Latin from theGreek term for the Milky Way,galaxías (kúklos)γαλαξίας (κύκλος)[15][16] 'milky (circle)', named after its appearance as a milky band of light in the sky.[17][18]In the astronomical literature, the capitalised word "Galaxy" is often used to refer to theMilky Way galaxy, to distinguish it from the other galaxies in theuniverse.[citation needed]
Galaxies were initially discovered telescopically and were known asspiral nebulae. Most 18th- to 19th-century astronomers considered them as either unresolvedstar clusters orextragalactic nebulae,: 220 but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like theAndromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way. For this reason they were popularly calledisland universes.Harlow Shapley began to advocate for the term "galaxy" and against using "universes" and "nebula" for the objects but the very influentialEdwin Hubble stuck to nebulae. The nomenclature did not fully change in until Hubble's death in 1953.[19]
Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomerGalileo Galilei used atelescope to study it and discovered it was composed of a huge number of faint stars.[30][31] In 1750, English astronomerThomas Wright correctly speculated that it might be a rotating body of a huge number of stars held together bygravitational forces, akin to theSolar System but on a much larger scale, and that the resulting disk of stars could be seen as a band on the sky from a perspective inside it.[b][33][34] In his 1755 treatise,Immanuel Kant elaborated on Wright's idea about the Milky Way's structure.[35]
The shape of the Milky Way as estimated from star counts byWilliam Herschel in 1785; the Solar System was assumed to be near the center.
The first project to describe the shape of the Milky Way and the position of the Sun was undertaken byWilliam Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy withthe Solar System close to the center.[36][37] Using a refined approach,Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method byHarlow Shapley based on the cataloguing ofglobular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the centre.[34] Both analyses failed to take into account theabsorption of light byinterstellar dust present in thegalactic plane; but afterRobert Julius Trumpler quantified this effect in 1930 by studyingopen clusters, the present picture of the Milky Way galaxy emerged.[38]
Distinction from other nebulae
A few galaxies outside the Milky Way are visible on a dark night to theunaided eye, including theAndromeda Galaxy,Large Magellanic Cloud,Small Magellanic Cloud, and theTriangulum Galaxy. In the 10th century, Persian astronomerAbd al-Rahman al-Sufi made the earliest recorded identification of the Andromeda Galaxy, describing it as a "small cloud".[39] In 964, he apparently mentioned the Large Magellanic Cloud in hisBook of Fixed Stars, referring to "Al Bakr of the southern Arabs",[40] since at adeclination of about 70° south it was not visible where he lived. It was not well known to Europeans untilMagellan's voyage in the 16th century.[41][40] The Andromeda Galaxy was later independently noted bySimon Marius in 1612.[39]
In 1734, philosopherEmanuel Swedenborg in hisPrincipia speculated that there might be other galaxies outside that were formed into galactic clusters that were minuscule parts of the universe that extended far beyond what could be seen. Swedenborg's views "are remarkably close to the present-day views of the cosmos."[42]In 1745,Pierre Louis Maupertuis conjectured that somenebula-like objects were collections of stars with unique properties, including aglow exceeding the light its stars produced on their own, and repeatedJohannes Hevelius's view that the bright spots were massive and flattened due to their rotation.[35]In 1750,Thomas Wright correctly speculated that the Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways.[34][43]
Toward the end of the 18th century,Charles Messier compiled acatalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae.[34] In 1845,Lord Rosse examined the nebulae catalogued by Herschel and observed the spiral structure ofMessier object M51, now known as the Whirlpool Galaxy.[44][45]
In 1912,Vesto M. Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher discovered that the spiral nebulae have highDoppler shifts, indicating that they are moving at a rate exceeding the velocity of the stars he had measured. He found that the majority of these nebulae are moving away from us.[46][47]
In 1917,Heber Doust Curtis observed novaS Andromedae within the "GreatAndromeda Nebula", as the Andromeda Galaxy,Messier objectM31, was then known. Searching the photographic record, he found 11 morenovae. Curtis noticed that these novae were, on average, 10magnitudes fainter than those that occurred within this galaxy. As a result, he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies.[48]
In 1920 a debate took place betweenHarlow Shapley andHeber Curtis, theGreat Debate, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim that the Great Andromeda Nebula is an external galaxy, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[49]
In 1922, theEstonian astronomerErnst Öpik gave a distance determination that supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.[50] Using the new 100-inchMount Wilson telescope,Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified someCepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.[51] In 1926 Hubble produced a classification ofgalactic morphology that is used to this day.[52][53]
This ultraviolet image ofAndromeda shows blue regions containing young, massive stars.
Advances in astronomy have always been driven by technology. After centuries of success inoptical astronomy, recent decades have seen major progress in other regions of theelectromagnetic spectrum.[54]
Thedust present in the interstellar medium is opaque to visual light. It is more transparent tofar-infrared, which can be used to observe the interior regions of giant molecular clouds andgalactic cores in great detail.[55] Infrared is also used to observe distant,red-shifted galaxies that were formed much earlier. Water vapor andcarbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used forinfrared astronomy.[56]
The first non-visual study of galaxies, particularly active galaxies, was made usingradio frequencies. The Earth's atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. Theionosphere blocks signals below this range.[57] Large radiointerferometers have been used to map the active jets emitted from active nuclei.
Ultraviolet andX-ray telescopes can observe highly energetic galactic phenomena. Ultraviolet flares are sometimes observed when a star in a distant galaxy is torn apart from the tidal forces of a nearby black hole.[58] The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of supermassive black holes at the cores of galaxies was confirmed through X-ray astronomy.[59]
Modern research
Plot of the rotation rate by distance from the center of the Milky Way galaxy, compared to the curve predicted for ordinary matter
In 1944,Hendrik van de Hulst predicted thatmicrowave radiation withwavelength of 21 cm would be detectable from interstellar atomichydrogen gas;[60] and in 1951 it was observed. This radiation is not affected by dust absorption, and so its Doppler shift can be used to map the motion of the gas in this galaxy. These observations led to the hypothesis of a rotatingbar structure in the center of this galaxy.[61] With improvedradio telescopes, hydrogen gas could also be traced in other galaxies.In the 1970s,Vera Rubin uncovered a discrepancy between observed galacticrotation speed and that predicted by the visible mass of stars and gas. Today, the galaxy rotation problem is thought to be explained by the presence of large quantities of unseendark matter.[62][63]
Beginning in the 1990s, theHubble Space Telescope yielded improved observations. Among other things, its data helped establish that the missing dark matter in this galaxy could not consist solely of inherently faint and small stars.[64] TheHubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion (1.25×1011) galaxies in the observable universe.[65] Improved technology in detecting thespectra invisible to humans (radio telescopes, infrared cameras, andx-ray telescopes) allows detection of other galaxies that are not detected by Hubble. Particularly, surveys in theZone of Avoidance (the region of sky blocked at visible-light wavelengths by the Milky Way) have revealed a number of new galaxies.[66]
A 2016 study published inThe Astrophysical Journal, led byChristopher Conselice of theUniversity of Nottingham, analyzed many sources of data to estimate that the observable universe (up to z=8) contained at least two trillion (2×1012) galaxies, a factor of 10 more than are directly observed inHubble images.[67]: 12 [68] However, later observations with theNew Horizons space probe from outside thezodiacal light observed less cosmic optical light than Conselice while still suggesting that direct observations are missing galaxies.[69][70]
Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by theHubble sequence. Since the Hubble sequence is entirely based upon visual morphological type (shape), it may miss certain important characteristics of galaxies such asstar formation rate instarburst galaxies and activity in the cores ofactive galaxies.[6]
Many galaxies are thought to contain a supermassive black hole at their center. This includes the Milky Way, whose core region is called theGalactic Center.[71]
The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have anellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively littleinterstellar matter. Consequently, these galaxies also have a low portion ofopen clusters and a reduced rate of new star formation. Instead, they are dominated by generally older, moreevolved stars that are orbiting the common center of gravity in random directions. The stars contain low abundances of heavy elements because star formation ceases after the initial burst. In this sense they have some similarity to the much smallerglobular clusters.[72]
The galaxy clusterAbell 1413 is dominated by this cD elliptical galaxy designated Abell 1413 BCG. It has an isophotal diameter of over 800,000 light-years across. Note thegravitational lensing.
Thelargest galaxies are thetype-cD galaxies.First described in 1964 by a paper by Thomas A. Matthews and others,[73] they are a subtype of the more general class of D galaxies, which are giant elliptical galaxies, except that they are much larger. They are popularly known as thesupergiant elliptical galaxies and constitute the largest and most luminous galaxies known. These galaxies feature a central elliptical nucleus with an extensive, faint halo of stars extending to megaparsec scales.[74] The profile of their surface brightnesses as a function of their radius (or distance from their cores) falls off more slowly than their smaller counterparts.[75]
The formation of these cD galaxies remains an active area of research, but the leading model is that they are the result of the mergers of smaller galaxies in the environments of dense clusters, or even those outside of clusters with random overdensities.[76] These processes are the mechanisms that drive the formation of fossil groups or fossil clusters, where a large, relatively isolated, supergiant elliptical resides in the middle of the cluster and are surrounded by an extensive cloud of X-rays as the residue of these galactic collisions. Another older model posits the phenomenon ofcooling flow, where the heated gases in clusters collapses towards their centers as they cool, forming stars in the process,[77] a phenomenon observed in clusters such asPerseus,[78] and more recently in thePhoenix Cluster.[79]
Shell galaxy
NGC 3923 Elliptical Shell Galaxy (Hubble photograph)
A shell galaxy is a type of elliptical galaxy where the stars in its halo are arranged in concentric shells. About one-tenth of elliptical galaxies have a shell-like structure, which has never been observed in spiral galaxies. These structures are thought to develop when a larger galaxy absorbs a smaller companion galaxy—that as the two galaxy centers approach, they start to oscillate around a center point, and the oscillation creates gravitational ripples forming the shells of stars, similar to ripples spreading on water. For example, galaxyNGC 3923 has over 20 shells.[80]
Spiral galaxies resemble spiralingpinwheels. Though the stars and other visible material contained in such a galaxy lie mostly on a plane, the majority of mass in spiral galaxies exists in a roughly spherical halo ofdark matter which extends beyond the visible component, as demonstrated by the universal rotation curve concept.[81]
Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from thebulge are relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as typeS, followed by a letter (a,b, orc) which indicates the degree of tightness of the spiral arms and the size of the central bulge. AnSa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, anSc galaxy has open, well-defined arms and a small core region.[82] A galaxy with poorly defined arms is sometimes referred to as aflocculent spiral galaxy; in contrast to thegrand design spiral galaxy that has prominent and well-defined spiral arms.[83] The speed in which a galaxy rotates is thought to correlate with the flatness of the disc as some spiral galaxies have thick bulges, while others are thin and dense.[84][85]
In spiral galaxies, the spiral arms do have the shape of approximatelogarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constantangular velocity. The spiral arms are thought to be areas of high-density matter, or "density waves".[86] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.) This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.[87]
A majority of spiral galaxies, including theMilky Way galaxy, have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.[88] In the Hubble classification scheme, these are designated by anSB, followed by a lower-case letter (a,b orc) which indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, or else due to atidal interaction with another galaxy.[89] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[90]
Our own galaxy, theMilky Way, is a large disk-shaped barred-spiral galaxy[91] about 30 kiloparsecs in diameter and a kiloparsec thick. It contains about two hundred billion (2×1011)[92] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[93]
Super-luminous spiral
Recently, researchers described galaxies called super-luminous spirals. They are very large with an upward diameter of 437,000 light-years (compared to the Milky Way's 87,400 light-year diameter). With a mass of 340 billion solar masses, they generate a significant amount of ultraviolet and mid-infrared light. They are thought to have an increased star formation rate around 30 times faster than the Milky Way.[94][95]
Other morphologies
Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies.
Aring galaxy has a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[96] Such an event may have affected theAndromeda Galaxy, as it displays a multi-ring-like structure when viewed ininfrared radiation.[97]
Alenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars[98] (barred lenticular galaxies receive Hubble classification SB0).
Irregular galaxies are galaxies that can not be readily classified into an elliptical or spiral morphology.
An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.
Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[99] Nearby examples of (dwarf) irregular galaxies include theMagellanic Clouds.[100]
Adark or "ultra diffuse" galaxy is an extremely-low-luminosity galaxy. It may be the same size as the Milky Way, but have a visible star count only one percent of the Milky Way's. Multiple mechanisms for producing this type of galaxy have been proposed, and it is possible that different dark galaxies formed by different means.[101] One candidate explanation for the low luminosity is that the galaxy lost its star-forming gas at an early stage, resulting in old stellar populations.[102][103]
Despite the prominence of large elliptical and spiral galaxies, most galaxies are dwarf galaxies.[104] They are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, with only a few billion stars.Blue compact dwarf galaxies contains large clusters ofyoung, hot, massive stars. Ultra-compact dwarf galaxies have been discovered that are only 100 parsecs across.[105]
Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[106]Most of the information we have about dwarf galaxies come from observations of theLocal Group, containing two spiral galaxies, the Milky Way and Andromeda, and many dwarf galaxies. These dwarf galaxies are classified as eitherirregular ordwarf elliptical/dwarf spheroidal galaxies.[104]
A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 millionsolar masses, regardless of whether it has thousands or millions of stars. This suggests that galaxies are largely formed bydark matter, and that the minimum size may indicate a form ofwarm dark matter incapable of gravitational coalescence on a smaller scale.[107]
TheAntennae Galaxies are undergoing a collision that will result in their eventual merger.
Interactions between galaxies are relatively frequent, and they can play an important role ingalactic evolution. Near misses between galaxies result in warping distortions due totidal interactions, and may cause some exchange of gas and dust.[108][109]Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars of interacting galaxies usually do not collide, but the gas and dust within the two forms interacts, sometimes triggering star formation. A collision can severely distort the galaxies' shapes, forming bars, rings or tail-like structures.[108][109]
At the extreme of interactions are galactic mergers, where the galaxies' relative momentums are insufficient to allow them to pass through each other. Instead, they gradually merge to form a single, larger galaxy. Mergers can result in significant changes to the galaxies' original morphology. If one of the galaxies is much more massive than the other, the result is known ascannibalism, where the more massive larger galaxy remains relatively undisturbed, and the smaller one is torn apart. The Milky Way galaxy is currently in the process of cannibalizing theSagittarius Dwarf Elliptical Galaxy and theCanis Major Dwarf Galaxy.[108][109]
M82, a starburst galaxy that has ten times the star formation of a "normal" galaxy[110]
Stars are created within galaxies from a reserve of cold gas that forms giantmolecular clouds. Some galaxies have been observed to form stars at an exceptional rate, which is known as astarburst. If they continue to do so, they would consume their reserve of gas in a time span less than the galaxy's lifespan. Hence starburst activity usually lasts only about ten million years, a relatively brief period in a galaxy's history. Starburst galaxies were more common during the universe's early history,[111] but still contribute an estimated 15% to total star production.[112]
Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to createH II regions.[113] These stars producesupernova explosions, creating expandingremnants that interact powerfully with the surrounding gas. These outbursts trigger a chain reaction of star-building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the activity end.[111]
Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction isM82, which experienced a close encounter with the largerM81. Irregular galaxies often exhibit spaced knots of starburst activity.[114]
Aradio galaxy is a galaxy with giant regions of radio emission extending well beyond its visible structure. These energetic radio lobes are powered by jets from itsactive galactic nucleus.[115] Radio galaxies are classified according to theirFanaroff–Riley classification. TheFR I class have lower radio luminosity and exhibit structures which are more elongated; theFR II class are higher radio luminosity. The correlation of radio luminosity and structure suggests that the sources in these two types of galaxies may differ.[116]
Radio galaxies can also be classified as giant radio galaxies (GRGs), whose radio emissions can extend to scales of megaparsecs (3.26 million light-years).Alcyoneus is an FR II class low-excitation radio galaxy which has the largest observed radio emission, with lobed structures spanning 5megaparsecs (16×106ly). For comparison, another similarly sized giant radio galaxy is3C 236, with lobes 15 million light-years across. It should however be noted that radio emissions arenot always considered part of the main galaxy itself.[117]
A giant radio galaxy is a special class of objects characterized by the presence of radio lobes generated byrelativistic jets powered by the central galaxy'ssupermassive black hole. Giant radio galaxies are different from ordinary radio galaxies in that they can extend to much larger scales, reaching upwards to several megaparsecs across, far larger than the diameters of their host galaxies.[118]
A "normal" radio galaxy do not have a source that is a supermassive black hole or monster neutron star; instead the source issynchrotron radiation from relativistic electrons accelerated by supernova. These sources are comparatively short lived, making the radio spectrum from normal radio galaxies an especially good way to study star formation.[119]
A jet of particles is being emitted from the core of the elliptical radio galaxyM87.
Some observable galaxies are classified as "active" if they contain an active galactic nucleus (AGN).[120] A significant portion of the galaxy's total energy output is emitted by the active nucleus instead of its stars, dust andinterstellar medium. There are multiple classification and naming schemes for AGNs, but those in the lower ranges of luminosity are calledSeyfert galaxies, while those with luminosities much greater than that of the host galaxy are known as quasi-stellar objects orquasars. Models of AGNs suggest that a significant fraction of their light is shifted to far-infrared frequencies because optical and UV emission in the nucleus is absorbed and remitted by dust and gas surrounding it.[121]
The standard model for anactive galactic nucleus is based on anaccretion disc that forms around asupermassive black hole (SMBH) at the galaxy's core region. The radiation from an active galactic nucleus results from thegravitational energy of matter as it falls toward the black hole from the disc.[122][123] The AGN's luminosity depends on the SMBH's mass and the rate at which matter falls onto it.In about 10% of these galaxies, a diametrically opposed pair ofenergetic jets ejects particles from the galaxy core at velocities close to thespeed of light. The mechanism for producing these jets is not well understood.[124]
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses; but unlike quasars, their host galaxies are clearly detectable.[125] Seen through a telescope, a Seyfert galaxy appears like an ordinary galaxy with a bright star superimposed atop the core. Seyfert galaxies are divided into two principal subtypes based on the frequencies observed in their spectra.[126]
Quasars are the most energetic and distant members of active galactic nuclei. Extremely luminous, they were first identified as high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared more similar to stars than to extended sources similar to galaxies. Their luminosity can be 100 times that of the Milky Way.[127] The nearest known quasar,Markarian 231, is about 581 million light-years from Earth,[128] while others have been discovered as far away asUHZ1, roughly 13.2 billion light-years distant.[129][130] Quasars are noteworthy for providing the first demonstration of the phenomenon thatgravity can act as a lens for light.[131]
Other AGNs
Blazars are believed to be active galaxies with arelativistic jet pointed in the direction of Earth. Aradio galaxy emits radio frequencies from relativistic jets. A unified model of these types of active galaxies explains their differences based on the observer's position.[124]
Possibly related to active galactic nuclei (as well asstarburst regions) arelow-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weaklyionized elements. The excitation sources for the weakly ionized lines include post-AGB stars, AGN, and shocks.[132] Approximately one-third of nearby galaxies are classified as containing LINER nuclei.[123][132][133]
Luminous infrared galaxies (LIRGs) are galaxies with luminosities—the measurement of electromagnetic power output—above 1011 L☉ (solar luminosities). In most cases, most of their energy comes from large numbers of young stars which heat surrounding dust, which reradiates the energy in the infrared. Luminosity high enough to be a LIRG requires a star formation rate of at least 18 M☉ yr−1. Ultra-luminous infrared galaxies (ULIRGs) are at least ten times more luminous still and form stars at rates >180 M☉ yr−1. Many LIRGs also emit radiation from an AGN.[134][135] Infrared galaxies emit more energy in the infrared than all other wavelengths combined, with peak emission typically at wavelengths of 60 to 100 microns. LIRGs are believed to be created from the strong interaction and merger of spiral galaxies.[136] While uncommon in the local universe, LIRGs and ULIRGS were more prevalent when the universe was younger.[135]
Physical diameters
Galaxies do not have a definite boundary by their nature, and are characterized by a gradually decreasing stellar density as a function of increasing distance from their center, making measurements of their true extents difficult. Nevertheless, astronomers over the past few decades have made several criteria in defining the sizes of galaxies.
Angular diameter
As early as the time ofEdwin Hubble in 1936, there have been attempts to characterize the diameters of galaxies. The earliest efforts were based on the observed angle subtended by the galaxy and its estimated distance, leading to anangular diameter (also called "metric diameter").[137]
Isophotal diameter
Theisophotal diameter is introduced as a conventional way of measuring a galaxy's size based on its apparent surface brightness.[138]Isophotes are curves in a diagram - such as a picture of a galaxy - that adjoins points of equal brightnesses, and are useful in defining the extent of the galaxy. The apparent brightness flux of a galaxy is measured in units ofmagnitudes per squarearcsecond (mag/arcsec2; sometimes expressed asmag arcsec−2), which defines the brightness depth of the isophote. To illustrate how this unit works, a typical galaxy has a brightness flux of 18 mag/arcsec2 at its central region. This brightness is equivalent to the light of an 18th magnitude hypothetical point object (like a star) being spread out evenly in a one square arcsecond area of the sky.[139] The isophotal diameter is typically defined as the region enclosing all the light down to 25 mag/arcsec2 in the blueB-band,[140] which is then referred to as the D25 standard.[141]
Examples of isophotal diameters (25.0 B-mag/arcsec2 isophote)
Thehalf-light radius (also known aseffective radius; Re) is a measure that is based on the galaxy's overall brightness flux. This is the radius upon which half, or 50%, of the total brightness flux of the galaxy was emitted. This was first proposed byGérard de Vaucouleurs in 1948.[145] The choice of using 50% was arbitrary, but proved to be useful in further works by R. A. Fish in 1963,[146] where he established a luminosity concentration law that relates the brightnesses of elliptical galaxies and their respective Re, and byJosé Luis Sérsic in 1968[147] that defined a mass-radius relation in galaxies.[138]
In defining Re, it is necessary that the overall brightness flux galaxy should be captured, with a method employed by Bershady in 2000 suggesting to measure twice the size where the brightness flux of an arbitrarily chosen radius, defined as the local flux, divided by the overall average flux equals to 0.2.[148] Using half-light radius allows a rough estimate of a galaxy's size, but is not particularly helpful in determining its morphology.[149]
Variations of this method exist. In particular, in the ESO-Uppsala Catalogue of Galaxies values of 50%, 70%, and 90% of the total blue light (the light detected through a B-band specific filter) had been used to calculate a galaxy's diameter.[150]
Petrosian magnitude
First described by Vahe Petrosian in 1976,[151] a modified version of this method has been used by theSloan Digital Sky Survey (SDSS). This method employs a mathematical model on a galaxy whose radius is determined by the azimuthally (horizontal) averaged profile of its brightness flux. In particular, the SDSS employed the Petrosian magnitude in the R-band (658 nm, in the red part of the visible spectrum) to ensure that the brightness flux of a galaxy would be captured as much as possible while counteracting the effects of background noise. For a galaxy whose brightness profile is exponential, it is expected to capture all of its brightness flux, and 80% for galaxies that follow a profile that followsde Vaucouleurs's law.[152]
Petrosian magnitudes have the advantage of being redshift and distance independent, allowing the measurement of the galaxy's apparent size since the Petrosian radius is defined in terms of the galaxy's overall luminous flux.[153]
A critique of an earlier version of this method has been issued by theInfrared Processing and Analysis Center,[154] with the method causing a magnitude of error (upwards to 10%) of the values than using isophotal diameter. The use of Petrosian magnitudes also have the disadvantage of missing most of the light outside the Petrosian aperture, which is defined relative to the galaxy's overall brightness profile, especially for elliptical galaxies, with higher signal-to-noise ratios on higher distances and redshifts.[155] A correction for this method has been issued by Grahamet al. in 2005, based on the assumption that galaxies followSérsic's law.[153]
Near-infrared method
This method has been used by2MASS as an adaptation from the previously used methods of isophotal measurement. Since 2MASS operates in the near infrared, which has the advantage of being able to recognize dimmer, cooler, and older stars, it has a different form of approach compared to other methods that normally use B-filter. The detail of the method used by 2MASS has been described thoroughly in a document by Jarrettet al., with the survey measuring several parameters.[156]
The standard aperture ellipse (area of detection) is defined by the infrared isophote at theKs band (roughly 2.2 μm wavelength) of 20 mag/arcsec2. Gathering the overall luminous flux of the galaxy has been employed by at least four methods: the first being a circular aperture extending 7 arcseconds from the center, an isophote at 20 mag/arcsec2, a "total" aperture defined by the radial light distribution that covers the supposed extent of the galaxy, and the Kron aperture (defined as 2.5 times the first-moment radius, an integration of the flux of the "total" aperture).[156]
Deep-sky surveys show that galaxies are often found in groups andclusters. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past few billion years are relatively scarce.[157] Only about 5% of the galaxies surveyed are isolated in this sense.[158][159] However, they may have interacted and even merged with other galaxies in the past,[160] and may still be orbited by smaller satellite galaxies.[161]
On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (seeHubble's law). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process, as well as an influx of infalling gas, heats the intergalactic gas in a cluster to very high temperatures of 30–100megakelvins.[162] About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.[163]
Most galaxies are gravitationally bound to a number of other galaxies. These form afractal-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of thebaryonic mass) in the universe.[164][165] To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (seeVirial theorem). If there is insufficientkinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.[166]
Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.[167] Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as thebrightest cluster galaxy, which, over time,tidally destroys its satellite galaxies and adds their mass to its own.[168]
Southern plane of the Milky Way from submillimeter wavelengths[169]
Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At thesupercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[170] Above this scale, the universe appears to be the same in all directions (isotropic andhomogeneous),[171] though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. TheHercules–Corona Borealis Great Wall, currently thelargest structure in the universe found so far, is 10 billionlight-years (three gigaparsecs) in length.[172][173][174]
The Milky Way galaxy is a member of an association named theLocal Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two.[175] The Local Group itself is a part of a cloud-like structure within theVirgo Supercluster, a large, extended structure of groups and clusters of galaxies centered on theVirgo Cluster.[176] In turn, the Virgo Supercluster is a portion of theLaniakea Supercluster.[177]
Magnetic fields
Galaxies havemagnetic fields of their own. A galaxy's magnetic field influences its dynamics in multiple ways, including affecting the formation of spiral arms and transporting angular momentum in gas clouds. The latter effect is particularly important, as it is a necessary factor for the gravitational collapse of those clouds, and thus for star formation.[178]
The typical averageequipartition strength forspiral galaxies is about 10 μG (microgauss) or 1nT (nanotesla). By comparison, the Earth's magnetic field has an average strength of about 0.3 G (Gauss) or 30 μT (microtesla). Radio-faint galaxies likeM 31 andM33, theMilky Way's neighbors, have weaker fields (about 5μG), while gas-rich galaxies with high star-formation rates, like M 51, M 83 and NGC 6946, have 15 μG on average. In prominent spiral arms, the field strength can be up to 25 μG, in regions where cold gas and dust are also concentrated. The strongest total equipartition fields (50–100 μG) were found instarburst galaxies—for example, in M 82 and theAntennae; and in nuclear starburst regions, such as the centers of NGC 1097 and otherbarred galaxies.[178]
Artist's impression of a protocluster forming in the early universe[179]
Current models of the formation of galaxies in the early universe are based on theΛCDM model. About 300,000 years after theBig Bang, atoms ofhydrogen andhelium began to form, in an event calledrecombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. As a result, this period has been called the "dark ages". It was from density fluctuations (oranisotropic irregularities) in this primordial matter thatlarger structures began to appear. As a result, masses ofbaryonic matter started to condense withincold dark matter halos.[180][181] These primordial structures allowed gasses to condense in toprotogalaxies, large scale gas clouds that were precursors to the first galaxies.[182]: 6
As gas falls in to the gravity of the dark matter halos, its pressure and temperature rise. To condense further, the gas must radiate energy. This process was slow in the early universe dominated by hydrogen atoms and molecules which are inefficient radiators compared to heavier elements. As clumps of gas aggregate forming rotating disks, temperatures and pressures continue to increase. Some places within the disk reach high enough density to form stars.
Artist impression of a young galaxy accreting material
Once protogalaxies began to form and contract, the firsthalo stars, calledPopulation III stars, appeared within them.[183] These were composed of primordial gas, almost entirely of hydrogen and helium.Emission from the first stars heats the remaining gas helping to trigger additional star formation; the ultraviolet light emission from the first generation of stars re-ionized the surrounding neutral hydrogen in expanding spheres eventually reaching the entire universe, an event calledreionization.[184] The most massive stars collapse in violentsupernova explosions releasing heavy elements ("metals") into theinterstellar medium.[185][182]: 14 This metal content is incorporated intopopulation II stars.
Theoretical models for early galaxy formation have been verified and informed by a large number and variety of sophisticated astronomical observations.[182]: 43 The photometric observations generally need spectroscopic confirmation due the large number mechanisms that can introduce systematic errors. For example, a high redshift (z ~ 16) photometric observation byJames Webb Space Telescope (JWST) was later corrected to be closer to z ~ 5.[186]Nevertheless, confirmed observations from the JWST and other observatories are accumulating, allowing systematic comparison of early galaxies to predictions of theory.[187]
Evidence for individual Population III stars in early galaxies is even more challenging. Even seemingly confirmed spectroscopic evidence may turn out to have other origins. For example, astronomers reported HeII emission evidence forPopulation III stars in theCosmos Redshift 7 galaxy, with a redshift value of 6.60.[188] Subsequent observations[189] found metallic emission lines, OIII, inconsistent with an early-galaxy star.[183]: 108
Different components of near-infrared background light detected by theHubble Space Telescope in deep-sky surveys[190]
Evolution
Once stars begin to form, emit radiation, and in some cases explode, the process of galaxy formation becomes very complex, involving interactions between the forces of gravity, radiation, and thermal energy. Many details are still poorly understood.[191]
Within a billion years of a galaxy's formation, key structures begin to appear.[192]Globular clusters, the centralsupermassive black hole, and agalactic bulge of metal-poorPopulation II stars form. The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[193] During this early epoch, galaxies undergo a major burst of star formation.[194]
During the following two billion years, the accumulated matter settles into agalactic disc.[195] A galaxy will continue to absorb infalling material fromhigh-velocity clouds anddwarf galaxies throughout its life.[196] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing theformation ofplanets.[197]
XDF view field compared to theangular size of theMoon. Several thousand galaxies, each consisting of billions ofstars, are in this small view.
XDF (2012) view: Each light speck is a galaxy, some of which are as old as 13.2 billion years[198] – theobservable universe is estimated to contain 200 billion to two trillion galaxies.
XDF image shows (from left) fully mature galaxies, nearly mature galaxies (from five to nine billion years ago), andprotogalaxies, blazing withyoung stars (beyond nine billion years).
Star formation rates in galaxies depend upon their local environment. Isolated 'void' galaxies have highest rate per stellar mass, with 'field' galaxies associated with spiral galaxies having lower rates and galaxies in dense cluster having the lowest rates.[199]
The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.[200] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen inNGC 4676[201] or theAntennae Galaxies.[202]
The Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two might collide in about five to six billion years. Although the Milky Way has never collided with a galaxy as large as Andromeda before, it has collided and merged with other galaxies in the past.[203] Cosmological simulations indicate that, 11 billion years ago, it merged with a particularly large galaxy that has been labeled theKraken.[204][205]
Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common. Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked about ten billion years ago.[206]
Spiral galaxies, like theMilky Way, produce new generations of stars as long as they have densemolecular clouds of interstellar hydrogen in their spiral arms.[207] Elliptical galaxies are largely devoid of this gas, and so form few new stars.[208] The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.[209][210]
The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), as the smallest, longest-lived stars in the visible universe, tinyred dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed ofcompact objects:brown dwarfs,white dwarfs that are cooling or cold ("black dwarfs"),neutron stars, andblack holes. Eventually, as a result ofgravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[209][211]
19 face-on spiral galaxies from theJames Webb Space Telescope in near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that haven't yet fully formed and are encased in gas and dust appear bright red.[212]
^This is the diameter measured using theD25 standard. A 2018 study suggested that there is a presence of disk stars beyond this diameter, although it is not clear how much of this influences the surface brightness profile.[10][11]
^Wright called the Milky Way theVortex Magnus (Great Whirlpool) and estimated its diameter to be 8.64×1012 miles (13.9×1012 km).[32]
^Hubble, Edwin P. (1926). "No. 324. Extra-galactic nebulae".Contributions from the Mount Wilson Observatory.324. Carnegie Institution of Washington:1–49.Bibcode:1926CMWCI.324....1H.
^Mattson, Barbara (November 27, 2002). Gibb, Meredith (ed.)."How many galaxies are there?".Imagine the Universe!.NASA. Archived fromthe original on July 28, 2012. RetrievedJanuary 8, 2007.
^Kennicutt, Robert C. Jr.; et al. (2005). "Demographics and Host Galaxies of Starbursts". In De Grijs, Richard; González Delgado, Rosa M. (eds.).Starbursts: From 30 Doradus to Lyman Break Galaxies. Astrophysics and Space Science Library. Vol. 329. Dordrecht:Springer. pp. 187–194.Bibcode:2005ASSL..329..187K.doi:10.1007/1-4020-3539-X_33.ISBN978-1-4020-3538-8.
Wright, Thomas (1750).An Original Theory or New Hypothesis of the Universe. London: Chapelle. Archived fromthe original on April 30, 2021. RetrievedApril 21, 2024....the stars are not infinitely dispersed and distributed in a promiscuous manner throughout all the mundane space, without order or design,... this phænomenon [is] no other than a certain effect arising from the observer's situation,... To a spectator placed in an indefinite space,... it [i.e., the Milky Way (Via Lactea)] [is] a vast ring of stars...
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![19 face-on spiral galaxies from the James Webb Space Telescope in near- and mid-infrared light. Older stars appear blue here, and are clustered at the galaxies’ cores. Glowing dust, showing where it exists around and between stars – appearing in shades of red and orange. Stars that haven't yet fully formed and are encased in gas and dust appear bright red.[212]](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aPHANGS_image_mosaic.jpg)
