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BASIC EXTRAGALACTIC ASTRONOMY - Part 7: Galaxies - Morphological Diversity
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BASIC EXTRAGALACTIC ASTRONOMY
Rudy E. Kokich, Alexandra J. Kokich, Andrea I. Hudson
18 September, 2020
Part 7: Galaxies - Morphological Diversity
32) Morphological Characteristics of Galaxies
Elliptical galaxies present with featureless low density gradients in the visible band, with no evidence of a nuclear bulge. Their general structure is not determined by overall rotation. Constituent stars follow random, individual orbits, often at high relative speeds. At the present time ellipticals are virtually devoid of interstellar gas and dust, which results in minimal rates of new star formation. Accordingly, they are composed of mainly old, red population II stars. Of all galaxy types, ellipticals display the widest range of sizes and luminosities. Giant ellipticals, usually found in the centers of galaxy clusters, can reach diameters of 4 million light years (IC 1101), more than 30 times greater than the Milky Way, and encompass tens of trillions of stars. On the other extreme are very numerous dwarf ellipticals with only 10 million stars, extending over less than 1,000 light years. Giant and normal ellipticals are presumed to form by galaxy mergers, while dwarf ellipticals may form in the tidal tails of interacting galaxies. Giant ellipticals are probably the last stage in the morphological evolution of galaxies.
Fig.7-1: Elliptical galaxies, a random sample.
Lenticular galaxies have a nuclear bulge and prominent, but mostly featureless disks with no evidence of spiral arm structures. They are sometimes referred to as armless spirals. Within the nucleus, stars follow random individual motion. In the disks, star kinematics is dominated by average circular motion, indicating that lenticular galaxies are rotationally stabilized. Accurate distinction between the ellipticals and the lenticulars often depends on spectroscopic rotational velocity and velocity dispersion measurements. When seen edge-on they are easily identified by the shape which resembles a double convex lens. Face-on, unbarred ellipticals (S0A) are difficult to visually distinguish from the early ellipticals. Lenticular galaxies also have mainly old, population II stars, very low rates of new star formation, and a scarcity of interstellar gas which they either already consumed, or lost through gravitational interactions with other galaxies. However, unlike the ellipticals, they still contain interstellar dust. When seen obliquely, some of them manifest dust lanes around the nucleus. Barred lenticulars are further divided into three classes, S0B1 - S0B3, depending on the prominence of the central bar.
Virtually all, if not all, elliptical and lenticular galaxies are thought to contain a central supermassive black hole.
Fig. 7-2: Lenticular galaxies, S0A (images 1-3) and S0B (images 4-5).
Spiral galaxies are composed of a central or nuclear bulge, an equatorial disk, and a nearly spherical galactic halo of stars and globular clusters. Virtually all, if not all, also have a central supermassive black hole which plays an important role in the formation and evolution of the galaxy. Surveys reveal that spiral and irregular galaxies tend to form in lower density parts of the Universe, rarely in the centers of galactic clusters. Spirals display a wide range of sizes: between 16,000 and 325,000 light years in diameter, and containing from 1 billion to 1 trillion stars. The majority rotate so that the spiral arms trail the spin direction. The exceptions are thought to be results of galaxy interactions and mergers.
In Sa and Sb galaxies the nuclear bulge is a comparatively large, densely packed central group of Population II stars characterized by old age, red color, and low metallicity. In Sc and Sd galaxies the bulge is progressively reduced by regions of new star formation surrounding the nucleus, producing young, blue Population I stars of higher metallicity. Meanwhile, in Sm galaxies new star formation takes place throughout the central volume, and they present no distinguishable bulge at all. The size of the bulge depends on the quantity of matter available for new star formation, and on physical and gravitational perturbations which stimulate new star formation.
Approximately 67% of nearby spiral galaxies (seen as they appeared in recent cosmological epochs) present with a nuclear bar which may be subtle or prominent. At the distance of 2.5 billion light years that fraction is 22%. And, at 8 billion light years it is only 11%. It has been proposed that the formation of nuclear bars results from gravitational interactions between galaxies and from mergers, that the bars indicate the level of "maturity" in spiral galaxies, and that the bars tend to turn off new star formation, or only develop after most of the gas in the galaxy has been consumed..
In many galaxies, like NGC 3992, a disturbed appearance at the junctions between the nuclear bar and the spiral arms suggests that the bar and the spiral disk are rotating at different angular velocities. According to a recent study by Hilmi et al. (2020), the nuclear bar is a dynamic structure which, over a period of 60 - 200 million years, oscillates in length by up to 100%, and in angular velocity in inverse proportion to the length. As a result, the bar intermittently connects and disconnects from the spiral arm complex, creating a disturbed interface or a ring by the difference in angular velocities.
New models suggest that spiral arms are in part composed of collections of stars and interstellar matter bound by gravity, and physically rotating together. Another part consists of stars, gas, and dust which move in and out of the arms as they orbit the galaxy, inevitably slowing down in the regions of higher density, in accord with the density wave model.
Migration of gas clouds into the gravitational fields of the spiral arms results in cloud collapse and the formation of new, very hot stars which give the spiral arms their characteristic light blue color.
Observations of spiral galaxies in optical and microwave bands reveal that each class is markedly different in the content of interstellar matter available for new star formation. In Sc and Sd spirals 15% or more of the total mass is comprised of interstellar gas and dust. Consequently, these classes manifest high rates of new star formation, long spiral arms, small nuclear bulges, flocculence (clumpiness) resulting from rapid formation of immense OB Associations, and blue color. In contrast, in Sa spirals only about 2% of the total mass is present in the form of gas and dust. Having spent or lost most of their interstellar matter, they display low rates of new star formation (still much higher than the ellipticals), yellow color of older stars, and large nuclear bulges.
It would appear that spiral galaxy evolution proceeds in the direction opposite to the classification sequence. Sm and Sd might be the "early" types, and Sa the "late", ancient galaxies. The hypothesis is supported by the tightly wound spiral arms in the Sa class. Since the spiral arms trail the direction of galaxy rotation, it is expected that older galaxies, having undergone more rotations, would present more tightly wound.
A 2019 study assisted by citizen scientists analyzed a large sample of 6,000 galaxies, and contradicted Hubble's observation that galaxies with larger nuclear bulges present with more tightly bound spiral arms. It found no correlation between bulge size and spiral arm tightness, supporting the idea that the density wave model is not the only mechanism involved in spiral arm evolution.
If the study is confirmed, it would additionally suggest that evolutionary pathways of the bulge and the spiral arms diverge early in the life of a galaxy.
Fig. 7-3: Hubble-Vaucouleurs spiral galaxy morphological classification. Subtypes a, b, c, d, m from left to right
The irregular galaxy category is assigned to any galaxy which does not fit into the Hubble-Vaucouleurs classification scheme. Irregulars have a chaotic appearance with no definable shape or internal structure. They are commonly smaller than 30,000 light years in diameter, and contain less than 10 billion solar masses of matter, making them considerably fainter than average spirals. Lack of organization is usually attributed to gravitational interactions or collisions with neighboring galaxies, or to some type of violent internal activity. They can consist of new as well as old stars, and can have a significant proportion of mass in the form of gas and dust. Irregulars are often found as satellites of larger galaxies. The best known examples are the Magellanic Clouds which originally may have been small barred spirals. Interaction with the Milky Way resulted in starburst activity and deformed internal structures, leading to their classification as irregulars. (Based on the distribution of young Wolf-Rayet stars, long-period Cepheids, and planetary nebulae, Large Magellanic Cloud has since been reclassified as SB(s)m - a barred spiral without rings or nuclear bulge - but with a confidence level of D, next to the lowest). Approximately 25% of all galaxies are categorized as irregulars.
Fig.7-4: Irregular galaxies
Dwarf galaxies generally contain less than several billion stars. Although dwarfs are the most common galaxy type in the Universe, they are especially difficult to detect at great distances due to small size and low luminosity. They are usually found in galaxy clusters, orbiting and interacting with major galaxies which appear to influence their formation and activity. For example, more than 20 are known to accompany the Milky Way. One, the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), is presently merging with the Milky Way, contributing its core - globular cluster M54 - to our system. Omega Centauri, the largest of globular clusters, is in fact the core, with a central black hole, of a dwarf galaxy accreted by the Milky Way eons ago. Since dwarfs are observed to participate in the evolution of major galaxies, there is much scientific interest in their genesis. They are presumed to form by gravitational collapse of large clouds of intergalactic gas and dust caused by external shock waves or by association with regional concentrations of dark matter.
Based on their shape and composition dwarfs are divided into several types:
1) Dwarf elliptical galaxies (dE) have elliptical shape, little or no gas or dust, and show no evidence of new star formation. They are common in clusters of major galaxies, often as satellite companions. Although they appear as normal ellipticals on a smaller scale, they have different light distribution and much lower metallicities which suggests they are different entities, with primordial origin in the early Universe.
2) Dwarf spheroidal galaxies (dSph) are small, low luminosity galaxies with little or no interstellar matter, no new star formation, and ancient stellar populations aged up to 13 billion years, nearly as old as the Universe. With diameters smaller than 1,600 light years, and masses less than 100 million solar, they are much smaller than dwarf ellipticals. It has been proposed that they are the same class of object as globular clusters, however stellar dynamics suggests that dwarf spheroidals are directly associated with high concentrations of dark matter. Due to small size and low luminosity, they have only been observed in the Local Group. Virtually all are satellites of the Milky Way or the Andromeda Galaxy.
3) Dwarf spiral galaxies (dS) are similar to major spiral galaxies in terms of morphological, optical, and kinematic properties. They manifest nuclear bulges, rotating spiral structures, solid presence of neutral hydrogen, and evidence of new star formation. They are quite rare, and found almost exclusively "in the field", outside of galaxy clusters. It is believed that gravitational interaction with major galaxies disrupts their internal structure, eventually converting them into dwarf ellipticals. They are typically smaller than 17,000 light years in diameter, and have absolute magnitudes in the range of -16 to -17.
4) Dwarf irregular galaxies (dIrr or dI) are small versions of the large irregular galaxies. They have a chaotic morphology with no definable structures, mixed stellar populations with relatively low average metallicity, substantial gas and dust content, and strong evidence of new star formation. They are frequently found as satellites of large galaxies, suffering gravitational distortion, starburst activity, and mergers. Dwarf irregulars are thought to be a local, more recently formed version of the most common type of galaxies which populated the early Universe.
5) Magellanic type dwarf galaxies (dSBm) are dwarf spiral galaxies under gravitational influence of a large neighbor, which resemble the Large Magellanic Cloud. They may show evidence of a bar, a single spiral arm, and starburst activity, but have no nuclear bulge. They are undergoing the process of conversion from dwarf spirals into dwarf irregulars.
6) Faint blue galaxies (FBG) are dwarf irregular galaxies which were undergoing intensive starburst activity when the light we are presently observing was emitted. In optical photographs they appear intensely blue, displaying a low B-V color index. Spectroscopically, many show hydrogen-alpha emission lines consistent with the presence of numerous young, hot, blue stars shedding their outer envelopes and exciting surrounding hydrogen clouds with ultraviolet radiation. They were initially identified on sky surveys 30-40 years ago as faint, small, blue galaxies ranging in apparent bolometric magnitude above 21, at redshifts between 0.1 and 2. More recently, Hubble deep sky images have shown that blue dwarfs are the most common galaxy type in the Universe, and are becoming less numerous over the last few billion years (at lower redshifts). It is thought that FBGs evolve into conventional dwarf galaxies as they consume their gas content, and starburst activity gradually subsides. The mechanism which initiates starburst activity in field FBGs has not been determined, but may be as simple as gravitational contraction of ordinary, baryonic matter within overdensities of dark matter and shock waves emanating from the initial generation of supernovae.
Fig. 7-5: Faint blue galaxies (FBGs), the most numerous galaxy type in the Universe. (HST deep field image)
7) Blue compact dwarf galaxies (BCD) are a special type of dwarf irregular or elliptical galaxies which manifest high rates of new star formation, resulting in large clusters of young, massive, very hot, blue stars. Their mass is generally about 1 billion solar, within a diameter less than 10,000 light years. BCD spectra reveal high concentrations of primordial neutral and ionized hydrogen and helium, stars of low metallicity (younger than 10 million years), no dust content, and steep rotation curves especially around the center. Due to small size and low absolute magnitude, they can not be observed at high redshifts, but they are suspected of being local versions of the FBGs. Regional concentrations of dark matter alone are insufficient to explain rapid rotation and angular velocity gradients within BCDs. Consequently, they are thought to form by ignition of starburst activity resulting from collisions and mergers between other dwarf galaxies, or by very close encounters with large neighbors.
8) Ultra-compact dwarf galaxies (UCG, UCD) are among the most crowded stellar systems known. Within diameters ranging between 40 and 650 light years, they can have masses up to one billion solar. They are found in the central regions of galaxy clusters (on average 2.7 per galaxy cluster) in the local Universe because they are too small and faint to be identified at redshifts above 0.6. Containing old, less luminous stellar populations of low metallicity, without gas, dust, or new star formation, they are larger than the biggest Milky Way globular clusters, but much more compact than the smallest dwarf galaxies of similar luminosity. A number of hypotheses exist on the genesis of UCGs. The most convincing one is that UCGs are nuclear remnants of dwarf galaxies whose outer layers have been stripped by the gravitational fields of the surrounding galaxy cluster.
This scenario is supported by Seth et al., (2014), who modelled kinematic data of the ultra-compact dwarf galaxy M60-UCD1, and revealed the presence of a central supermassive black hole (SMBH) of 20 million solar masses, representing 15% of the object's total mass. M60-UCD1 mass/luminosity relation is consistent with other UCGs, implying that as yet undiscovered SMBH might be found in the rest of the population.
33) Extraordinary Galaxy Types
Dark Galaxies are hypothetical galaxy models composed of rotating concentrations of dark matter associated with primordial neutral hydrogen and helium clouds, containing either very few or no observable stars. Theoretical models predict mass estimates between 1 billion and 1 trillion solar. Unlike "ordinary" intergalactic gas clouds, dark galaxies contain dark matter which imparts high mass, structural cohesion, and specific rotational properties to the baryonic gas cloud. Search for dark galaxies involves comparison of optical surveys to radio telescope surveys at 21 cm neutral hydrogen wavelengths. Another approach is to look for lower redshift hydrogen absorption lines in the spectra of background quasars. So far, the existence of dark galaxies remains controversial. One viable candidate is VirgoHI21, an extended cloud of neutral hydrogen, of 100 billion solar masses, whose internal motion and strong gravitational effect in the form of a drawn spiral arm on nearby galaxy NGC 4254 (M99) imply the presence of dark matter.
Another candidate is Smith's Cloud, discovered in 1963 by astronomy student Gail Smith. The 2 million solar mass cloud of mostly neutral hydrogen, 9,800 ly in maximum diameter, about 40,000 ly from Earth, is approaching the Milky Way at 73 km/s, and is expected to collide with the Perseus spiral arm in 30 million years. If it were visible in the optical band, it would subtend 11 degrees on the night sky, about as wide as the constellation of Orion. Its trajectory indicates that it has already passed through the Milky Way approximately 70 million years ago, but without losing cohesion. This suggests that its ordinary matter is embedded in a protective halo of dark matter, and that its total mass is much higher than the estimates based on baryonic matter.
"Almost Dark" Galaxies are systems with exceptionally high neutral hydrogen (HI) content detected in the radio band, and exceptionally low surface brightness in the optical band due to minimal stellar populations. As such, they have the highest mass to luminosity ratios of all galaxy types. The cause of suppressed star formation rates remains unknown. Some of these galaxies (e.g. AGC 227982 and AGC 26833) were discovered in a fruitless search for dark galaxies.
Ultra Diffuse Galaxies (UDG) are extremely low luminosity systems devoid of star-forming gas, containing relatively small numbers of ancient population II stars. A study of UDGs in the Coma Cluster found them to have a similar distribution to the ordinary, luminous galaxies. It is presumed they are formed from ordinary galaxies which have lost their gas, dust, and peripheral content in gravitational interactions with larger neighbors. Studies of velocity dispersion of UDG globular clusters indicate that some are embedded in thick halos of dark matter, while others are completely free of dark matter. For example, Coma Cluster UDG Dragonfly 44 was found to have a mass of 1 trillion solar, but only 1% of the luminous star content of the Milky Way. It appears to be composed almost entirely of dark matter. Meanwhile, very low globular cluster velocity dispersion in ultra diffuse galaxies NGC 1052-DF2 and NGC 1052-DF4 indicates that their total masses are consistent with their observed stellar masses alone, with no dark matter contribution. Such galaxies with standard Newtonian orbital mechanics, are important in disproving alternate gravity hypotheses developed to dispense with the concept of dark matter.
Low Surface Brightness Galaxies (LSB) belong to a large and diverse group of systems whose central surface brightness is lower than the background sky-glow. While no convention exists for defining the group, discussions generally involve galaxies fainter than blue apparent magnitude of 23 arcsec^-2. Due to low luminosity and contrast, it is believed the numbers of LSBs in galaxy surveys are significantly underestimated in the local Universe, but especially at higher redshifts. These galaxies are characterized by very abundant gas content, blue color, low star counts, low metallicity, and no known supernova activity. They usually occur isolated in the field, outside of gravitational influence of other galaxies, which might explain very low rates of new star formation. Kinematic studies reveal them to have extremely high ratios of mass / (star + gas) luminosity, suggesting that up to 95% of their mass consists of non-baryonic dark matter. LSBs range in size from dwarfs to Giant low surface brightness galaxies(GLSB) which can span over 160,000 light years in diameter, and compare in mass to the largest spiral galaxies in the Universe. Dwarf LSBs are the most numerous subgroup in the local space, but are as yet not detectable at large distances. Some studies suggest that they may have been present among the primordial galaxies, while others suggest they are a relatively recent feature in the evolution of galaxy structure.
Luminous Infrared Galaxies (LIRG) are galaxies which emit more energy in the infrared band than all other bands combined. Inconspicuous at optical wavelengths, they were first detected in 1983 by the Infrared Astronomical Satellite (IRAS), the first space telescope to perform an all-sky survey at infrared wavelengths. Their numbers were greatly increased in subsequent infrared surveys by the Spitzer Space Telescope and the Herschel Space Observatory. In the optical band, virtually all of these objects were found to be interacting or merging galaxies with central supermassive black holes (SMBH). Their high intrinsic luminosity, above 100 billion solar, is due to active galactic nuclei (AGN) and extremely high rates of new star formation, between 100 and nearly 3,000 times the rate observed in quiescent spiral galaxies. Total energy output of these galaxies is comparable to quasars, previously thought to be the most energetic objects in the Universe. In fact, the source of energy in LIRG active galactic nuclei are supermassive black holes, just as in quasars. The reason they are not as prominent in the optical band is that LIRG AGNs are surrounded by dense clouds of gas and dust which absorb intense ultraviolet radiation released by SMBH accretion disks, and re-emit the energy in the form of heat in the infrared band. Depending on luminosity, LIRGs are sorted under different subclasses. ULIRGs are ultraluminous infrared galaxies (> 1 trillion solar), HyLIRGs are hyperluminous LIRGs (> 10 trillion solar), and ELIRGs are extremely luminous LIRGs (>100 trillion solar). The most luminous ELIRG so far discovered is WISE J224607.57−052635.0, at 350 trillion solar. It is formed by a merger of three galaxies, one containing an AGN with a 10 billion solar mass SMBH. Another one, WISE J101326.25+611220.1, was found to have a star formation rate of 2,810 per year, about 3,000 times higher than in the local quiescent galaxies. There is convincing evidence that LIRG luminosity increases as the distance between interacting galaxies decreases, resulting in exponentially stronger gravitational forces and higher star formation rates. The most energetic systems were found to be in the most advanced stages of merging. Although LIRGs are present in the local Universe, they are increasingly more abundant at higher redshifts. For this reason they are thought to be a crucial intermediate stage in the evolutionary pathway from gas and dust rich spiral galaxies to the giant ellipticals.
Fig. 7-6: HST optical images of luminous infrared galaxies in the sequence of increasing intrinsic IR luminosity
Ultrared Dusty Star-Forming Galaxies (DSFG) are large starburst systems which are immensely luminous in the infrared band, but may be completely obscured at ultraviolet and optical wavelengths that are readily absorbed by the natal clouds of gas and dust. The precise difference between DSFGs and LIRGs remains debatable. Some authors define DSFGs as those galaxies which were originally discovered at infrared wavelengths, even if they were later found to have optical complements. Since the 1980s space-based infrared observatory surveys (IRAS, COBE, Spitzer, Herschel) have demonstrated that the infrared radiation field has the same energy density as starlight emission from all galaxies visible in the ultraviolet and optical bands. Thus, traditional observations in these bands miss approximately half of the total star formation activity in the Universe. Although nearly a million DSFGs have so far been identified, their abundance is insufficient to proclaim them as progenitors of all the local, quiescent galaxies. However, they remain of great interest in the study of mechanisms by which extreme star formation and the assembly of stellar populations in the Universe operate. DSFGs are believed to be a relatively early morphological type which appeared in the young and intermediate Universe. Several studies performed at different wavelengths found their median redshifts to be ~1.95, ~2.2, ~2.6, and ~3.1. It is likely these values will increase with future advances in optics and photodetectors. The most distant DSFG was observed at redshift 6.02, indicating that multiple stages of nucleosynthesis required for dust formation had already occurred by the time the Universe was only 1 billion years old. DSFGs range from small to the largest and most luminous galaxies known. Their star formation rates reach up to several thousand solar masses per year, which implies a relatively short lifespan before they consume most of their natal gas and dust. A subsample of DSFGs is known to contain dust-shrouded SMBH. Infrared luminosity in that group may be dominated by AGN heating mechanisms rather than starburst activity.
Fig. 7-7: Infrared galaxies in the GOODS-N region devoid of foreground objects, 15x60 arcmin. (Herschel-SPIRE)
Extremely UV-Luminous Galaxies (EUVLG) is a new galaxy type recently identified by Marques-Chaves et al. (2020). The authors discovered the first representative of the group, now named BOSS-EUVLG1, to be a dwarf galaxy of extreme luminosity caused by starburst activity of very hot massive stars. Previously it had been classified as a quasar (SDSS J122040.72+084238.1) presumad to be powered by a central supermassive black hole. The estimated star formation rate is 1,000 solar masses/yr, about 1,000 times higher than in the Milky Way, although the galaxy is 30 times smaller. EUVLGs have similar energy generation mechanisms and interstellar medium properties to the Faint Blue Galaxies, however their energy output is markedly higher, and equivalent to quasars. Based on its apparent magnitude of 20.98 and a redshift of 2.469, the galaxy is about 500 times brighter than the Milky Way in the visible band. The authors of the study estimate its absolute magnitude in the ultraviolet band at -24.40, which is 83 times brighter in the UV band than the giant Andromeda Galaxy. Spectroscopic studies of BOSS-EUVLG1 reveal it is composed of young stars of extremely low metallicity born from primordial gas in a very early phase of starburst activity. The rate of star formation is comparable only to the brightest Luminous Infrared Galaxies, but the absence of metals and dust in the interstellar medium allows the ultraviolet and visible radiation to reach us without significant attenuation. The authors believe this galaxy will enter a dusty phase after a number of supernova generations, within only a few hundred million years. At that time its ultravioled and visible light will become absorbed and obscured, and the galaxy will evolve into a luminous infrared type (DSFG or LIRG).
Fig 7-8: Extremely UV-Luminous Galaxy BOSS-EUVLG1, previously classified as a quasar.
Tadpole galaxies are a rare galaxy type characterized by bright, compact heads and long tails composed of stars and gas. While only 0.2% of the local galaxies are tadpoles, they are much more common in the early Universe, comprising about 10% of the galaxies in the HST Deep Field image. When discovered in the 1990s, it was presumed they represent galaxy mergers. More recent work by Elmegreen et al. (2016) suggests that they form when a dwarf galaxy passes through a filament of primordial gas composed primarily of hydrogen and some helium. Rapid accretion and perturbation of gas in the leading edge of the galaxy head trigger rapid formation of massive, hot, O-type stars of very low metallicity. As these stars go through very brief lifetimes, their supernovas enrich the interstellar medium with heavier elements, so that the stars forming further down the tail have progressively higher metallicities.
Fig. 7-9: Tadpole galaxies: A) in the HST Deep Field image, and B) in the local Universe (LEDA 36252 / HST)
Tadpole galaxies illustrate that, In addition to galaxy mergers, another common mechanism of small galaxy growth is primordial gas accretion resulting from kinematic motion through a primordial gas filament.
Tadpole galaxies should not be confused with the Tadpole Galaxy, UGC 10214, which is a spiral galaxy with a long tidal trail of stars and gas drawn out during a merger about 100 million years ago.
Protogalaxies, or primeval galaxies, is a general term for several types (FBG, UCD, UCG, EUVLG) of high redshift progenitors of local, evolved galaxies in the earliest stages of development. By strict definition, they are regarded to be systems of first generation stars forming by gravitational collapse of dense primordial hydrogen and helium clouds, contained within dark matter halos. Since they have a large fraction of massive and very hot spectral type O and B stars, they radiate most of their energy at ionizing ultraviolet wavelengths around 1,000 A, and 10% of the total energy in the lines of the Lyman series of neutral hydrogen. The term Lyman alpha emitter (LAE) was added to these objects when it was understood that it is possible to search for them in the far infrared band with narrow-band filters at the expected redshifted wavelengths. Rauch et al. (2007), Zheng et al. (2017) and Hashimoto et al. (2018) reported dozens of protogalaxies at redshifts above 6, forming during the first billion years after the Big Bang. They are characterized by small size, poorly organized structure, high rates of new star formation, between 250 and several thousand per year, low metallicity, low luminous mass of only several billion stars, but disproportionally high luminosity due to a relatively large population of energetic O and B stars. In theory, they should also have high rates of supernova events.
At the time of writing, the most ancient galaxy known is GN-z11 with a redshift of 11.1. Its light travel time is 13.4 billion years, and cosmological time (since the Big Bang) around 400 million years. HST and Spitzer imaging reveals that GN-z11 is 25 times smaller than the Milky Way, and has only several billion stars with new star formation rate of 20-40 per year.
A less constrained definition of protogalaxies includes all morphologies which are found in the very early Universe, from gravitationally bound concentrations of dark matter to immediate progenitors of evolved galaxies. This definition would combine into one broad class all unevolved galaxy types listed in this section.
While most protogalaxies are much smaller than the Milky Way, several have been discovered which are colossal in size. Martin et al. (2015) reported a protogalaxy about 400,000 light years in diameter at redshift 2.27, corresponding to light travel time distance of 10.8 billion light years. It is in gravitational interaction with quasar UM 287 (PHL 868) which triggered starburst activity in a cosmic web filament.
Marrone et al. (2017) discovered a pair of giant protogalaxies, SPT0311-58, with redshift ~7, forming in the Universe only 780 million years after the Big Bang. Starburst activity in both companions, less than 25,000 ly apart, appears to be caused by their gravitational interaction. The larger member contains 270 billion solar masses of stars and gas, and 3 billion solar masses of dust, indicating multiple prior generations of stellar nucleosynthesis. Its star formation rate (SFR) is 2,900 solar masses per year. The smaller member is about 8 times less massive, with SFR of 540. The pair is encompassed by a dark matter halo with a mass of several trillion solar, as massive as theoretically permissible at that cosmological epoch.
Working with the integral field spectrograph at the ESO’s VLT telescope, Borisova et al. (2016) documented a large nebulous halo displaying starburst activity around each one of the 19 quasars studied at redshifts between 3 and 4. In some cases, these quasar halos extended more than a million light years from the central SMBH – about 20 times the radius of the Milky Way.
Fig. 7-10: Nebulous halos around every quasar studied which formed in the early Universe
The starburst activity model was confirmed by Kurk et al. who studied five quasars with redshifts around 6, and masses ranging between 0.3 and 5.2 billion Suns. Using the infrared spectrograph at the VLT, they measured halo metallicities consistent with multiple generations of stellar nucleosynthesis. They conclude that starburst activity and stellar nucleosynthesis around primordial supermassive black holes commenced very early in the history of the Universe. It is not yet clear which type of stellar systems evolve from these objects.
While it is modelled that protogalaxies grow by gradual merger of small precursors, and by the accretion of primordial gas, there is convincing evidence that immensely large systems were also forming in the very early Universe under the influence of strong gravitational fields around primordial SMBHs and within overdensities of dark matter.
Although there exist many reasonable hypotheses regarding the genesis of individual classes of galaxies, there is no general theory of galaxy formation and evolution which explains the observed variety of structure and composition. The wide diversity of galactic morphology suggests that the evolution of galaxy-type objects does not follow a single path, but consists of a series of parallel and intersecting lineages.
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