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Galaxies


Contents

Types of Galaxies
Active Galactic Nuclei
Black Holes
Seyfert Galaxies
Radio Galaxies
Quasars
Extremely Red Objects (ERO)
Formation and Evolution of Galaxy
Theory of Spiral Arm Formation
The Milky Way
Black Hole at the Milky Way Center
Dark Matter in the Milky Way
Footnotes
References
Index

Types of Galaxies1

Composition

Galaxies are systems of stars, gas and dust (see for example the Sombrero galaxy in Figure 05-01a). They exist in a wide variety of shapes and sizes. The simplest classification scheme, which was devised by Edwin Hubble, recognizes 4 basic types - elliptical, spiral, barred spiral, and irregular and arranges them in a sequence called the "tuning fork" diagram.

Elliptical galaxies are denotes by the letter E followed by the number from 0 to 7 to indicate the degree of flattening of the observed elliptical shape. An E0 galaxy appears spherical, where as an E7 galaxy is markedly flattened. The viewing angle adds some complications into this kind of classification, an elongated ellipsoid would appear spherical if seen "end-on".

Figure 05-01a Composition
[view large image]

Small ellipticals are "dwarf" systems denoted by "dE". The giant ellipticals are designated as "cD". This class of galaxies usually does not contain much interstellar matter.

galaxy types

Spiral galaxies, denote by S, have a central nucleus surrounded by a flattened disc with the stars, gas, and dust organized into a pattern of spiral arms. They are categorized according to the size of the nuclear bulge, the tightness of the spiral pattern, and the degree of "patchiness" in their arms. S0 is the transitional type called lenticular galaxy. An "Sa" galaxy has a large central nucleus and tightly wound, relatively smooth, arms; an "Sb" galaxy has a somewhat smaller nucleus and less tight arms that often contain conspicuous HII regions and clusters of hot young stars; and an "Sc" galaxy has a relatively small nucleus and loosely wound "knotty" arms dominated by numerous HII regions and youthful clumps of stars. In barred spirals, denoted by "SB", the arms emerge from the ends of what looks like a rigid bar of luminous matter that straddles the nucleus.

Irregular galaxies, which have no obvious nucleus or ordered structure, are denoted by "Irr" and are broadly subdivided into "Irr I" and "Irr II". Irr I galaxies display evidence of recent or ongoing star formation (e.g., OB associations (young stars) and HII regions (luminous nebulas)); Irr II galaxies have a disturbed appearance, and their shapes seem to have been distorted by violent internal activity or by collisions or close encounters with other galaxies.

Figure 05-01b Samples of Galaxy Types [view large image]

The classification for the spirals is further subdivided into five luminosity classes: from I (most luminous) to V (least luminous). Figure 05-01b shows some real

galaxy types images for the different types of galaxies; while Figure 05-01c is a schematic diagram showing the side view of the elliptical galaxies and top view of the spiral galaxies. It is believed that a galaxy's type is determined by the amount of angular momentum it contains and the rate at which star formation has proceeded. Elliptical galaxies, and the spheroidal Population II halos of spirals, show little net systematic rotation. Their individual member stars and globular clusters move around their centers in random directions.

Figure 05-01c Types of Galaxies [view large image]

galaxy merger Where the overall angular momentum was small, and star formation proceeded rapidly (thereby mopping up most of the gas early on in the evolutionary process), the end result would be an elliptical dominated by older stars and containing little, if any, gas. Where the angular momentum was greater, the result would be a more flattened system. Where star formation proceeded relatively slowly, the gaseous component would settle into a flattened disclike distribution. The first generation of stars would form within the spheroidal system and the later generations within the flattened disc as observed in the spiral and lenticular galaxies. Dwarf galaxies are much smaller than ordinary galaxies. Because of their size, they have relatively low gravity and matter can escape from them more easily. This property, combined with the fact that dwarf galaxies are the most common type of galaxy in the universe, makes them very important in understanding how the universe was seeded with various elements billions of years ago, when galaxies were forming. Recently in 2005, it is suggested that merger of gas clouds may also played a role in creating different galaxy type. Where a large galaxy was formed by the merger of many small gas clouds, it prevented the formation of disk structure and developed to a large elliptical galaxy (see Figure 05-01d).

Figure 05-01d Development Pathways [view large image]

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Active Galactic Nuclei2

The types of galaxies in Figure 05-01b and c seems to be a good classification scheme for the nearby galaxies. However, there are other kinds of galaxies, which do not fit into such category. They seem to represent the galaxies in another phase of evolution. These special objects include Seyfert galaxies, radio galaxies, quasars and extremely red objects (ERO) in a rough order of ascending redshift (distance). Thanks to systematic surveys, the latest (2007) catalogs contain more than 13,000 quasars - a number that could eventually reach 100,000.

Black Hole in Galaxy By 2007, it is recognized that most galaxies other than dwarfs have central black holes. The idea is that black hole "seeds" either attracted matter into forming galaxies or formed within young galaxies in the early universe. This action produced quasars and explains why most quasars are extremely distant. As the black hole acquired more and more matter from galaxies' centers, the fuel became exhausted, so they slowly quieted down. Most galaxies in the recent universe have slumbering giants in their centers. They can be re-activated when interact with other galaxies, starbursts, or gas clouds falling into the central region. This scenario explains active galactic nucleus (AGN) in the nearby universe. Study of such galaxies reveals that the more massive a galaxy's central bulge, the more massive its black hole. Figure 05-01e shows two deep field galaxies in both optical and infrared. The black holes are displayed prominently in the infrared images.

Figure 05-01e Black Hole in Galaxy [view large image]

Views of AGN An Unified View of AGNs Figure 05-01f presents a unified view of the AGN. The basic ingredient consists of a central black hole surrounded by a dusty disk. The kind sporting a jet or two usually associates with strong radio emission; it includes quasar, and radio galaxy. While QSO and Seyfert galaxy belong to the radio quiet kind. Energy consumed by the jet and various forms of radiations is extracted from the in-falling gas cloud and the dusty torus - ultimately it is converted from the gravitational potential and angular momentum. The same kind of object presents a different aspect depending on the viewing angle of the observer. Figure 05-01g is an updated version of the AGN. It adds an accretion disk in between

Figure 05-01f An Unified View of AGN

Figure 05-01g Another Unified View of AGN
[view large image]

the dust torus and the black hole and classifies the Seyfert galaxies into two types (see Figure 05-01g). While the EROs (Extremely Red Objects) are these kinds of objects further away in the early universe.

A 2010 report reveals that the Fermi gamma-ray observatory has detected in 3C279 (a blazar as shown in Figure 05-01h) a giant
Jet from 3C279 gamma-ray flare lasting for about 20 days. What is unique about this observation is that the degree and direction of visible-light polarization also changed drastically during the same period. It indicates that the gamma-ray and visible-light regions are connected probably at the same location. The changing polarization could be related to a blob of gas flowing around a bend in the jet. Combining the speed of the jet with the duration of the flare provides an estimate of the size of the emitting region in the jet and thus an estimate of the distance of the emission region from the black hole. It is

Figure 05-01h Blob in Jet from 3C279

found to be probably more than 105 the radius of the black hole. This is much more than the tens of radius predicted by some models, and suggests that the region is not intimately associated with the innermost regions of the accretion disk.

X-ray AGN An international team of astronomers using NASA's Swift satellite and the Japanese/United States Suzaku X-ray observatory has discovered a new class of active galactic nuclei (AGN) in 2007. These objects are so heavily shrouded in gas and dust that virtually no light gets out. Only the high-energy X-rays can punch through such thick layer. These objects comprise about 20 percent of point sources in the X-ray background, a glow of X-ray radiation that pervades our Universe. It implies that there must be a large number of yet unrecognized obscured AGNs in the local universe. By missing this new class, previous AGN surveys were heavily biased, and thus gave an incomplete picture of how supermassive black holes and their host galaxies have evolved over cosmic history. Figure 05-01i is an artist's illustration of the X-ray AGN.

Figure 05-01i X-ray AGN [view large image]

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Black Holes4

Black Hole Worm Hole The concept of black hole has its origin in a solution of Einstein's General Relativity for a spherical object with mass M and radius R. If the mass collapses to a radius less than R = 2GM/c2, where G is the gravitational constant and c is the speed of light, then nothing (including light) can escape from inside this radius. It is called the event horizon or the Schwarzschild radius (named after the astrophysicist who solved the equation). Figure 05-02a shows a schematic diagram of the Schwarzschild geometry.

Figure 05-02a Black Hole
[view large image]

Figure 05-02b Worm Hole [view large image]

It is known as the embedding diagram. The two dimensional circles are slices of three dimensional spheres (of the same radius) - the hyperspace. The verticle axis denotes the "stretch" of space in the radial direction. The slope of the curve can be considered as representing the curvature of the space. It is flat (or zero) at the outer edge and becomes infinity at the Schwarzschild radius. This pictorial representation is very similar to a rubber sheet stretched by a rock. The shape of the region inside the horizon is somewhat arbitrary. It is only known that everything plunges inevitably to the central singularity once passing over the horizon. In a more realistic drawing the event horizon would be placed far below the diagram at infinity. The complete Schwarzschild geometry consists of a black hole, a white hole, and two singularities connected at their horizons by a worm hole as shown in Figure 05-02b. A white hole is a black hole running backwards in time. Just as black holes swallow things irretrievably, so do white holes spit them out. White holes cannot exist, since they violate the second law of thermodynamics by allowing some time reversal events such as reassembling a broken glass back to its original whole. The white hole geometry outside the horizon represents another Universe. The worm hole joining the two separate singularities is known as the Einstein-Rosen bridge, but even if it can somehow be generated, it would be unstable and pinch off immediately. Therefore, only the black hole geometry is applicable to the physical world.

Black Hole It is believed that every quasar, active galactic nucleus, and even normal galactic nucleus contains a black hole with a mass of between ten million to several billion solar masses at its core. The difference in appearance is related to the intensity of the activity. Since galaxies rotate, matter falling toward the central black hole will form a rapidly spinning disk of gas - an accretion disk - rather than falling directly into the hole. Kinetic energy released by in-falling matter, and frictional effects within the accretion disk, raise the temperature of the nner parts of the disk to enormous values and provide plenty of energy to power AGN's on all scales from Seyferts to quasars. By a process that is still not fully understood but seems to be related to rotating black hole, the central engine accelerates streams of charged particles to very high speeds. The inner rim of the accretion disk, together with surrounding gas and magnetic fields, forms a nozzle that confines the outward flow of energetic particles into narrow streams that shoot out perpendicularly to the plane of the disk. Figure 05-02c shows a model of the black hole.

Figure 05-02c AGN Model [view large image]

Magnetic Field and Jet Magnetic Field and Spin Magnetohydrodynamics is the branch of physics dealing with the behavior of the combined system of magnet field and ionized gas at low density. A popular theoretical scenario applies such formulation to explain the relativistic jet(s) powered by the black hole. The study found that magnetic field lines are twisted into a helix (as shown in Figure 05-02d) by the differential rotation of matter in the accretion disk. Such field provide a force to expel the ionized particles in a collimated jet from the inner circumference of the accretion disk. The energy to maintain the process is derived from the braking action on the rotation of the disk. In this way, matter can slow down and eventually flow inward into the black hole. Figure 05-02e illustrates a mechanism to extract the spinning energy from a rotating black hole, wherein the surrounding ionized gas and the magnetic field act together to set up a voltage

Figure 05-02d Magnetic Field and Jet [view large image]

Figure 05-02e Rotating BH

difference between the poles. This is similar to a battery driving electric current around to provide another source of energy for the outflowing jet(s).

Figure 05-02f is a HST (Hubble Space Telescope) image of NGC4261, which is a radio galaxy. The image strongly suggests that it is a black hole fitting the description of the theoretical model. Infrared observation of
NGC1068 in 2004 was able to resolve the inner region down to a few parsec. Figure 05-02g penetrates the dusty central region and shows the structures on arcsec scales. The picture on the right is a model for the nucleus of NGC1068. It contains a central hot component (dust temperature > 800K, yellow) marginally resolved along the source axis. Its finite width and the dashed circle indicate the currently undetermined extent. The
Black Hole NGC4261 Black Hole NGC1068 much larger warm component (T=320K, red) is well resolved. The arrows indicate the projected orientation of the two interferometer baselines and the angular resolution L/2B, where L is the wavelength and B is the projected baseline. The image shows that the active galactic nuclei are arranged like a thick doughnut. This model requires a continuous injection of kinetic energy to maintain such cloud system. The mechanism is currently unknown; thus a better understanding of the physics of these spectacular objects is needed.

Figure 05-02f NGC4261
[view large image]

Figure 05-02g NGC1068
[view large image]

The quasar 3C273 is a 2-billion-solar-mass black hole encircled by a doughnut of gas (accretion disk) and with two gigantic jets shooting out along the spinning axis. The Schwrzschild radius for this object is about 6x109 km. Such supermassive black hole can be created while matter is still at quite low density (~ 10-3 gm/cm3). Since the tidal force at the event horizon of a black hole is inversely proportional to the square of its mass, its effect on a space visitor would be un-noticeable, although he would soon be in dire trouble as he plunges irrevocably toward the central singularity. However for a stationary observer, it takes an infinitely long time for the asronaut to approach the event horizon (due to gravitational time dilation) and the view of the asronaut would gradually disappear (due to gravitational red shift of light). The effect on the astronaut visiting a stellar black hole (mini-quasar) would be more violent due to the drastic increase of the tidal force.
Black Hole 2 In a November, 2004 announcement by NASA, a black hole catalogued as SDSSp J1306 appears to be about one billion times as massive as the sun. It is 12.7 billion light-years away. A similarly massive and distant black hole was studied in the same year with the European Space Agency's XMM-Newton X-ray satellite. The object, SDSSp J1030, is 12.8 billion light-years away. These two results seem to indicate that the way supermassive black holes produce X-rays has remained essentially the same from a very early date in the universe. How such massive and energetic structures formed so quickly (only after one billion years of the big bang) remains a major puzzle for scientists. Mergers of smaller galaxies and their black holes may have played a role. Researchers suspect that black hole formation and galaxy development go largely hand-in-hand, but they cannot say which comes first. Figure 05-02h is an artist's conception of a supermassive black hole with matter swirling into it.

Figure 05-02h Supermassive Black Hole [view large image]

In 2009, radio observations of 4 early galaxies (1 to 2 billion years after the Big Bang) including J1148+5251 shows that the mass ratio of central bulge to black hole is about 10 times smaller than the more recent data indicating the black hole probably came first.
Black Hole 5 A 2012 update on the supermassive black hole J1148+5251 reveals that while matter swirls inward from the accretion disk toward the black hole, intense radiation creates outflow of gas at the same time. The process is shown schematically in Figure 05-02i. The orange wavy lines represent photons emitted during the accretion process. Most of them escape the galaxy, but some imping on clouds of gas (blue) and the radiation pressure drives the gas out. The stars and dark are too heavy to be affected by the radiation. It is suggested the elliptical galaxies may lost their gas component in this way. The Milky Way has a mild imitation of such process in the form of the 30-kpc arms.

Figure 05-02i BH Gas Outflow

Black Hole Cannibalism On 28 March 2011, the Swift satellite designed to look for gamma-ray bursts (GRBs) detected a burst with unusually long duration of more than a month. The new source, called Swift J164449.3+573451, is now believed to be generated by the tidal disruption of a star moving too close to a black hole. The left portion in Figure 05-02j shows the level of X-ray flux recoded by various detectors over the past 20 years with observational duration indicated by horizontal bars. The data indicate the gradual buildup of intensity until it suddenly flared up more than 200 times in few hours. The right portion in the same diagram is the follow-up to show the flux variation for the first 7 weeks of the most recent observation. Time interval of the variation (in luminosity) can be used to calculate the size of the source since each part of

Figure 05-02j Black Hole Cannibalism [view large image]

the source would have to be in contact with others on such a time scale t to coordinate the variations; thus the size r c t. For the most rapid variability of t 100 s, r 3x1012 cm, which is about 50 time bigger than the Sun and about 1/2 the size of Mercury's orbit.
Under the assumption that the central object, i.e., the black hole, dominates the variability, r can be taken as the Schwarzschild radius of the black hole, then its mass M rc2/2G 107 Msun, which is in close agreement with the estimate from the relationship between the black hole mass and galactic bulge luminosity. The unusually high luminosity may be related to the onset of relativistic jet pointing to the direction of observation and moving at a velocity of 99.5% of the speed of light.

M87 Jet In another 2011 report, the jet from M87 has been observed in great detail with the Very Long Baseline Array (VLBA), which has resolution 400 times finer than the HST. Since black hole is not a clean eater, about half of the food (the in falling star) is left behind in the accretion disk close to the black hole. Interaction of the rotating "crumbs" and magnetic field produces the jet. The observation finds a jet of high-energy particles starts as a broad flow at

Figure 05-02k M87 Jet
[view large image]

a distance about 14 to 23 times the Schwarzschild radius of the black hole. The jet becomes more cylindrical and faster with distance from the black hole. Figure 05-02k (a) is a schematic diagram showing different sections of the jet that have different particle densities observed at
different frequencies, from about 200 GHz (more dense) to 5 GHz (less dense); diagram b shows the actual jet obtained with the VLBA at a frequency of 15 GHz (yellow is brightest, blue is faint). The orientation of the disk is vertical in these pictures. This observation is in broad agreement with the magneto-hydrodynamics description in earlier study. The discrepancy is in the distance of 100,000 Schwarzschild radius from the black hole predicted by theory and observed in quasars. One of the explanations ascribes the difference to viewing direction.

Black Hole Correlations Black Hole Formation M87 used to hold the title of harbouring the most massive black hole with a mass of 6.3x109 Msun. The record was broken in 2011 with the detection of 2 super massive black holes in NGC 3842 (within the Abell 1367 cluster) and NGC 4889 (within the Coma cluster) both with a mass of about 1010 Msun. These giant galaxies all sit at the bottom of the cluster's gravitational potential well collecting a lot

Figure 05-02l Black Hole Correlations

Figure 05-02m Black Hole Formation [view large image]

of the material falling inward. Estimate of black hole mass depend on either the measurement of stellar (or gas, or maser emission from molecular clouds etc.) velocity
dispersion or V-band galactic bulge luminosity Lv. The data points usually fall on (or near) a straight line of a logarithmic plot (see the formula for the solid black line in Figure 05-02l, b). However, the super massive black holes doesn't seem to follow such relationship (Figure 05-02l, a and b). The deviation is explained by proposing that the super massive black hole was formed by merger of black holes while the lighter variety was grown by accretion of cold gas (Figure 05-02m). BTW, the BCG (in Figure 05-02l) is the abbreviation of Brightest Cluster Galaxies, and MW denotes Milky Way.

The discovery of a 2 billion solar mass supermassive black hole at a mere 770x106 years after the Big Bang (Figure 05-02n) has
Supermassive Black Hole Formation presented astronomers with a problem to explain how could it be created so soon and so big? It is known that the acquisition of material by the black hole is always balanced by ejection of some of the in-falling matter. It has been calculated that a black hole sucking in matter continuously at its maximal rate would double its mass Mbh every 50x106 years, i.e., Mbh=M0x277/5~ 4x104M0, where M0 is the initial mass. This is too slow for a seed

Figure 05-02n Supermassive Black Hole [view large image]

black hole of stellar mass to grow into billion-sun in 770x106 years (Figure 05-02o,a).


Super Black Hole Formation Two different theories has been proposed to resolve the problem. They all try to make the basic component from solar-mass black hole to middleweight one with mass of 105 to 2x106 Msun, i.e., to bump up M0. One theory suggests that black hole at the center of a star cluster could grow quickly enough to middleweight seed hole (Figure 05-02o,b). The other one involves collapse of primordial gas cloud (Figure 05-02o,c). The former scenario predicts more leftover middleweight seed holes to survive into the present age. Since only a few hundred such holes among 2x105 galaxies have been found so far, it seems that the odd is against the bottom-up version at least for now (see also related observation in Baby Galaxy).

Figure 05-02o Supermassive Black Hole Formation

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Seyfert Galaxies

A Seyfert is a spiral or barred-spiral galaxy with a bright compact nucleus. In short exposure images, the outer parts of the galaxy are not seen and the nucleus appears almost star-like, so that, in this respect, a Seyert nucleus resembles a quasar. Although not usually strong radio emitters, Seyfert nuclei radiate strongly over a wide range of wavelength producing strong gamma-ray emission up to 1 million ev. Its intensity peaks at an emission line near 450 nm They are less luminous than quasars, but are brighter than
Seyfert galaxy Face-on Seyfert galaxy Edge-on most normal spirals (about 100 times more luminous than the Milky Way). Across the spectrum, the tremendous brightness of Seyferts can change over periods of just days to months and Seyfert galaxies like NGC 7742 in Figure 05-03a are suspected of harboring massive black holes at their cores. Figure 05-03b shows the edge-on view of another Seyfert galaxy M106, which conveys an impression that matters are falling into a hole.

Figure 05-03a Seyfert Galaxy Face-on [view large image]

Figure 05-03b Seyfert Galaxy Edge-on [view large image]

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Radio Galaxies

Radio Emission Radio galaxies are so named because they are powerful sources of radio emission that radiate much more strongly at radio wavelengths than do conventional galaxies as shown in the upper diagram of Figure 05-04. Whereas normal galaxies emit blackbody radiation, the radio emission is generated by a mechanism called synchrotron radiation. Cygnus A was the first radio galaxy identified in 1951. It is shown in the lower diagram of Figure 05-04. In a typical radio galaxy, most of the emission comes from two huge lobes located far beyond and on either side of the visible galaxy. The radio-emitting lobes are believed to be clouds of energetic charged particles that have been expelled from the nucleus of the central galaxy, the jets are streams of additional energetic particles, which have been accelerated in the nucleus and are surging outward toward the lobes, producing "hot spots" (represented by red colour) where they plow into the leading edges of the lobes.
Radio Emission This material typically spans a region of space five to ten times larger than the visible galaxy, and sometimes far larger than that. The overall luminosities can be up to several thousand times that of the Milky Way. Strong radio emissions are usually associated with elliptical galaxies - such as M87 (Virgo A) - or disturbed galaxies such as Centaurus A3. This kind of objects is sometimes referred to as AGN for Active Galaxy Nucleus. By superimposing the radio images taken by the Very Long Baseline Array (VLBA) to the gamma-ray sky map produced by the Large Area Telescope (LAT),

Figure 05-04 Radio Galaxy

astronomers are able to confirm that the gamma-ray emission from the core of AGN is associated with the radio jet.

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Quasars

Quasar During the early 1960s, some radio sources were shown to coincide in position with objects that looked like stars. These became known as quasars (quasi-stellar radio source). It was later discovered that only about one in ten of these objects is a strong radio emitter, the radio-quiet type is named quasi-stellar object (QSO). The term quasar is still widely used to describe both kinds of objects. Figure 05-05a shows the quasar 3C273 (3C denotes the third Cambridge Catalogue of radio sources) discovered in 1962. The radio, optical, and X-ray images are displayed in the top from left to right. The lower picture is a drawing of a quasar. These objects have high redshift, some of which translate into distance well in excess of 10 billion light-years. In order to appear as bright as they do, quasars must be extremely luminous at more than ten thousands times the luminosity of the normal galaxies. Quasars radiate strongly over a wide range of wavelenghts, and although emission lines are present in their spectra, the overall spectrum is consistent with synchrotron

Figure 05-05a Quasar 3C273
[view large image]

emission. Their powerful energy sources are compact and variable, with some quasars


Blazar varying substantially in brightness over periods as short as a few days. Some has a jet (e.g, 3C273), or pair of jets emerging from their centers similar to the radio galaxies. There are many more high redshift quasars than low redshift ones. No known quasar has a redshift less than 0.06, and quasar numbers seem to be highest at redshifts of around 2-3. It follows that quasar activity must have been more prevalent among galaxies billions of years ago, when the universe was younger than it is now.

There is a class of objects called BL Lacertae objects or blazars (Figure 05-05b). They are star-like radio sources, similar in appearance to quasars, but with no obvious emission lines in their featureless spectra. They may be quasars seen almost end-on with the jet pointing to the line of sight. Astronomers divide blazars roughly into two groups: lower-energy, relatively nearby BL Lacertae objects and higher-energy, distant soruces. More than 1000 blazars have been catgaloged.

Figure 05-05b Blazar [view large image]

It is possible that redshift of the blazars may be masked by the approaching jet, which shifts the light to shorter wavelength.

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Extremely Red Objects (ERO)

The Hercules Deep Field provides a detailed view of hundreds of distant galaxies. One particular object called Extremely Red Object (ERO, now renamed to "Hot DOG" - for Hot Dust-Obscured Galaxies) is marked with the yellow box as shown in Figure 05-06a. This type of galaxies is generally faint in the visible light, but can be very bright in the infrared. The six images below show how different the same object can appear from visible blue light (left-most image), to well into the infrared (far-right). This object appears to have achieved its extreme red color because the bulk of its star formation has been reddened with a thick layer of dust. This galaxy is believed to lie about 9 billion light years away, at a time when the universe was only a third of its present age. It is estimated that this galaxy has around 100 billion stars and may in fact be a very distant mirror -- an analog of our own Milky Way Galaxy in its formative years. Combining data over a period of 3 years obtained at UKIRT, astronomers in 2008 have produced an
ERO Infrared Galaxies Hot DOG image containing over 100,000 galaxies (Figure 05-06b). Many of the faint red objects in the background (against a relatively nearby spiral galaxy) are massive galaxies

Figure 05-06a ERO
[view large image]

Figure 05-06b Infrared Galaxies [view large image]

Figure 05-06c Hot DOG
[view large image]

at distances of over 10 billion light-years.

In 2012, news reported that the Wide-field Infrared Survey Explorer (WISE) designed for detecting infrared oddities over all sky, has found millions of supermassive black holes at distance of about 10 billion light years away. In addition, there are about 1000 dust-obscured galaxies with very high temperature dubbed "Hot DOG" (inside circles, Figure 05-06c). These are the same type of astronomical objects called ERO. Further observations are needed to determine the evolutionary sequence between the supermassive black hole and Hot DOG (see "Formation and Evolution of Galaxy").

Furthest Galaxies Furthest Cluster of Galaxies In August 2009 the rejuvenated Hubble Telescope took an infrared deep field image (Figure 05-07a). It shows many small galaxies with redshift of up to 8.5 corresponding to 13.1 billion light years from us or about 600 million years after the Big Bang. Their size and mass are about 1/20 and 1/100 of those of the Milky Way respectively. Although detected in the near infrared region of the spectrum, they are intrinsically blue (before the redshift) - meaning that they may be

Figure 05-07a Furthest Galaxies [view large image]

Figure 05-07b Furthest Cluster of Galaxies [view large image]

deficient in heavier elements, i.e., made with primordial matter, and as a result, quite free of the dust that reddens light through scattering. The discovery of these galaxies so near the
beginning of the "reionization epoch" with the seemingly insufficient radiation output raises the possibility that there were more efficient processes to ionize the neutral hydrogen in an even earlier epoch unobserved and unknown to us yet.

Figure 05-07b shows the X-ray image of the furthest galaxy cluster (appeared as blue diffuse gas) at redshift 1.9 corresponding to 3.7 billion years after the Big Bang. The delayed appearance of the cluster together with the smaller size of the early galaxies lend support to the "bottom up" theory, which suggests smaller units merge and form larger ones.

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Formation and Evolution of Galaxy5,6

Galaxy Formation Supermassive Black Hole The most accepted view on the formation and evolution of large scale structure is that it was formed as a consequence of the growth of primordial fluctuations by gravitational instability. Galaxies can form in a "bottom up" process in which smaller units merge and form larger units. It is referred to as the "Inside-out Theory" or "Merger" as shown in the upper half of Figure 05-08a. In the present epoch, large concentrations of galaxies (clusters of galaxies) are still in the process of assembling. The opposing view is the "top down" process in which large clump breaks up into smaller units. It is referred to as the "Outside-in Theory" as shown in the lower half of Figure

Figure 05-08a BH, Initial Formation [view large image]

Figure 05-08b BH, Supermassive

05-08a. The figure also shows the kind of objects the NGST (Next Generation Space Telescope) will detect according to the two opposing theories. The "bottom up" theory have been given
a boost in 2008 by the first ever detection of the infant protogalaxies with an unprecedented 92-hour session on the European Southern Observatory's Very Large Telescope. These protogalaxies were irregularly shaped and with low star-formtion rates, but the stars that did form were massive and consequently exploded as supernovae. The image shows a group of protogalaxies in the process of merging. (See a movie of "Galaxy Formation")

By 2013 the bottom up theory becomes less sustainable with the discovery that supermassive black hole with mass over billion Msun is common at an age about 750 - 900 million years after the Big Bang. It just did not have enough time to amass for such behemoth. Various schemes have been proposed to resolve the discrepancy, but each one has its own problem (see Figure 05-08b).

Density Fluctuation The difference between the "bottom up" (inside-out) and "top down" (outside-in) point of view is related to whether the universe is composed with cold dark matter (CDM, slow moving) or hot dark matter (HDM, fast moving). In the former scenario there is fluctuation in the power spectrum over a wide range of physical scales as shown in Figure 05-08c. It increases with smaller scales, therefore structure formed first with small objects, which then merge to form ever larger structures. This is called ``bottom up'' structure formation. The observations strongly favour this scenario over its competitor: ``top down'' structure formation. The proto-typical ``top down'' scenario is structure formation in a universe dominated by hot dark matter. Hot dark matter cannot support fluctuations on small length scales - they are washed out with the rapid motion of the particles. Thus only large scale fluctuations survive to the

Figure 05-08c Power Spectrum for Density Fluctuation

present epoch. Structure forms first large scale objects which fragment into smaller objects.


The early universe was a barren wasteland of hydrogen, helium, and a touch of lithium, containing none of the elements necessary for life as we know it. From those primordial gases were born giant stars a few hundred times as massive as the Sun, burning their fuel at such a prodigious rate that they lived for only about 3 million years before exploding. Those explosions spewed elements like
Protogalaxy Evolution carbon, oxygen and iron into the void at tremendous speeds. By the remarkably young age of 275 million years, the universe was substantially seeded with metals thrown off by exploding stars. That seeding process was aided by the structure of the infant universe, where small protogalaxies less than one-millionth the mass of the Milky Way clustered together into vast filamentary structures. Giant stars form at the intersections of these great filaments of primordial hydrogen, forming the nuclei of the first galaxies - the protogalaxies (Figure 05-08d). The small sizes and distances between those protogalaxies allowed an individual supernova to rapidly seed a significant volume of star forming space. New simulations show that the first, "greatest generation" of stars spread incredible amounts of such heavy elements like carbon, oxygen and iron across thousands of light-years of space, thereby seeding the cosmos with the stuff of life.

Figure 05-08d Protogalaxy Evolution
[view large image]

Mass Assembly Galaxies used to be considered as "island universes" that formed in the distant past and have since evolved in isolation from their surroundings. Such concept was replaced by the hierarchical growth scenario, which enrols galaxy interactions, collisions and mergers to shape the mature galaxies in the current epoch. Lately in the 2010s, observations reveal that the decline in galaxy formation is not matched by the decrease of atomic hydrogen - only half of the latter has been consumed by the galaxies. The latest explanation proposes that the hydrogen fuel is supplied by ionized hydrogen in the intergalactic environment. Such novel idea is supported by the recent (2013) detection of neutral hydrogen in space between the Andromeda and Triangulum galaxies. The detection of 21 cm radio emission from neutral hydrogen is much easier although the neutral variety is only about 1% of the ionized one. The new discovery adds one more element into the process of galaxy assembly.

Figure 05-08e Galactic Mass Assembly [view large image]

Figure 05-08e is the numerical simulation of galaxy formation and evolution, in which green denotes higher ionized hydrogen density and black is lower (a-c are zoom in views).

Merger Simulation After the initial phase of galaxy formation, there was an era of cosmic fireworks: galaxies collided and merged (see Figure 05-08f), powerful black holes in quasars sucked in huge whirlpools of gas, and stars were born in unrivaled profusion. The activity of star formation peaked about four to six billion years. Since then galactic mergers became much less common, the gargantuan black holes were replaced by numerous moderate ones, star formation continued but mostly in the low mass variety. In other words, the contents of the universe have transitted from a small number of bright objects to a large number of dimmer ones. Computer simulations

Figure 05-08f Simulation of Galactic Merger [view large image]

suggest that such shift may be a direct consequence of cosmic expansion.


As the universe expands, galaxies become more separated and merger become rarer. Furthermore, as the gas surrounding galaxies grows hotter and more diffuse, it does not gravitationally collapse as readily into the galaxy's potential well. A few billion years from now, the smaller galaxies that are active today will have consumed much of their fuel, and the total cosmic output of radiation
Galaxy Evolution Simulation will decline drametically. As the cosmic expansion continues, the dwarf galaxies - which hold only a few million stars each but are the most numerous type of galaxy in the universe - will become the primary hot spots of star formation. Inevitably, though, the universe will darken, and its only contents will be the fossils of galaxies from its past. Figure 05-08g shows the evolution subsequent to the initial phase. Figure 05-08h shows another evolutuon simulation with cold dark matter.

Figure 05-08g Evolution History [view large image]

Figure 05-08h Evolution Simulation [view large image]

It seems that galaxy formation is a very complicated process involving star-forming history, merger history, mass, size, angular momentum, and external environment. But research in 2008 indicates that the mass is the only dominant factor in determining the properties of individual galaxies. A sample of roughly 200 galaxies has been selected from a large, blind sky survey for neutral hydrogen (HI) emission using the hydrogen spectral line at a wavelength of 21 cm. For each galaxy in the catalogue, they measured a number of quantities: 1. the total hydrogen mass; 2. the width of the hydrogen spectral line; 3. the redshift; 4. the inclination with respect to the line of sight; 5. two radii, containing 50% and 90% of the light, respectively; and 6. the optical luminosity in four different colour bands. It is found that the six independent components that they use to describe the galaxies in their sample are all correlated with each other and with a single principal component - the galaxy's mass.
Mass Range of Galaxies This finding is consistent with the progression of mass among the types of galaxy as shown in Figure 05-08i, where the dwarf galaxies are in the lowest mass range of 107 - 108 Msun, the mass of irregular galaxies is in between 108 - 1010 Msun, the range for spiral galaxies is

Figure 05-08i Mass Range of Galaxies [view large image]

1010 - 1012 Msun, while the giant elliptical galaxies is in the range 1012 - 1013 Msun. The mass ranges form a continuous sequence without overlapping. Thus, if the mass of the
galaxy is known, the type would follow according to Figure 05-08i. It is not that straight forward conversely, for if we know the type of the galaxy there is a range of mass within that type.

A single observation in 2009 discovered that elliptical galaxies seem to expand in size from epoch as early as 3 billion years after the Big Bang to present day. However, the mass of such elliptical galaxies remains constant in contradiction to the scaling relationship between mass and effective radius re for nearby galaxies (Figure 05-08j). The picture on the left of the Figure
Elliptical Galaxy shows very compact galaxies evolved to bigger, more diffuse objects and became more abundant. Diagram on the right depicts the scaling relationship for nearby galaxies (the black dots) and the high redshift galaxy 1255-0 (at z = 2.186 in red symbol, dynamical mass refers to mass obtained from velocity dispersion of the stars). Current theory predicts that galaxies evolve according to the scaling law perhaps by merger (the blue arrow). The new observation implies another path as shown by the red arrow.

Figure 05-08j Evolution of Elliptical Galaxy

Further research is required to confirm the atypical observation. It remains to be seen whether we need conventional or novel explanations for the evolution of elliptical galaxies.



Table 05-01 summarizes the evolutionary sequence. The time epoch t is computed from a computer program on cosmological evolution with m = 0.26 and = 0.74.

Epoch (109 years) Red- shift Astronomical Objects Activities
~ 0.38
x 10-3
~ 1090 Cosmic Microwave Background Radiation Transparent to light.
< 0.38
x 10-3
> 1090 None. Dark age.
< 0.05 > 25 First stars, supernovea. Formation of black holes, production of heavy elements.
< 0.6 > 8.0 Protogalaxies. Protogalaxies drew in matter.
< 1.3 > 4.70 Baby galaxies. Galaxies took shape.
< 3.8 > 1.75 Quasar, supermassive black holes. Galaxies collided and merged, bursts of star formation.
< 7.2 > 0.73 ERO (extremely luminous galaxies). Rate of star formation peaked at ~ 5 x 109 year.
< 13.7 > 0 AGN; elliptical, spiral, & irregular galaxies. Small # of bright objects replaced by large # of dimmer ones.
>>13.7 Dwarf galaxies... ... galaxies will disappear with the evaporation of matter.

Table 05-01 Evolution of Galaxies