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Galaxies

Contents

Types of Galaxies
Active Galaxies
Seyfert Galaxies
Radio Galaxies
Quasars
Extremely Red Objects (ERO)
Blackholes
Formation and Evolution of Galaxy
The Milky Way
Blackhole at the Milky Way Center
Dark Matter in the Milky Way
References
Index

Types of Galaxies1

Galaxies are systems of stars, gas and dust. They exist in a wide variety of shapes and sizes. The simplest classification scheme, which was devised by Edwin Hubble, recognizes four basic types - elliptical, spiral, barred spiral, and irregular and arranges these 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". 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 05-01a Types of Galaxies, Photo Image [view large image]

 The classification for the spirals is further subdivided into five luminosity classes: from I
galaxy types (most luminous) to V (least luminous). Figure 05-01a shows some real images for the different types of galaxies; while Figure 05-01b 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

Figure 05-01b Types of Galaxies, Schematic [view large image]

 systematic rotation. Their individual member stars and globular clusters move around their centers in
galaxy merger random directions. 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 05-01c).

Figure 05-01c Galaxy Formation [view large image]

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Active Galaxies2

The types of galaxies in Figure 05-01a and b 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, extremely red objects (ERO), and quasars in a rough order of ascending redshift (distance).

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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 wavelengths. They are less
Seyfert galaxy Face-on Seyfert galaxy Edge-on luminous than quasars, but are brighter than 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 05-02a are suspected of harboring massive black holes at their cores. Figure 05-02b shows the edge-on view of another Seyfert galaxy M106, which conveys an

Figure 05-02a Seyfert Galaxy Face-on [view large image]

Figure 05-02b Seyfert Galaxy Edge-on [view large image]

 impression that matters are falling into a hole.

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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 05-03. 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 at the lower diagram of Figure 05-03. 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
Radio Emission lobes. 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.

Figure 05-03 Radio Galaxy

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Extremely Red Objects (ERO)

ERO The Hercules Deep Field provides a detailed view of hundreds of distant galaxies. One particular object called Extremely Red Object (ERO) is marked with the yellow box as shown in Figure 05-04. 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.

Figure 05-04 ERO [view large image]

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Quasars

3C273 Quasar X-ray 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
Quasar object (QSO). The term quasar is still widely used to describe both kinds of objects. Figure 05-05 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 05-05 The 3C273 Quasar

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

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Blackholes4

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 = 2xGxM/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 05-06a shows a schematic diagram of the Schwarzschild geometry.

Figure 05-06a Black Hole
[view large image]

Figure 05-06b 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 05-06b. 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 05-07a shows a model of the black hole.

Figure 05-07a AGN Model [view large image]

Figure 05-07b 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 05-07c 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 much larger warm component (T=320K, red) is well resolved. The arrows indicate the
Black Hole NGC4261 Black Hole NGC1068 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 05-07b NGC4261
[view large image]

Figure 05-07c 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 05-07d is an artist's conception of a supermassive black hole with matter swirling into it.

Figure 05-07d Supermassive Black Hole [view large image]

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Formation and Evolution of Galaxy5,6

Galaxy Formation Galaxy Evolution 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" in the upper half of Figure 05-08a. In the present epoch, large concentrations of

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

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

 galaxies ("clusters") are still 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" in the lower half of Figure 05-08a. The figure also shows the kind of objects the NGST (Next Generation Space Telescope) will detect according to the two opposing theories.
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

Figure 05-08c Power Spectrum for Density Fluctuation

 motion of the particles. Thus only large scale fluctuations survive to the 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 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 (protogalaxies). 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 05-08a).

After the initial phase of galaxy formation, there was an era of cosmic fireworks: galaxies collided and merged (see Figure 05-08d), 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 transitioned from a small number of bright objects to a large number of dimmer ones. Computer simulations 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 galxies grows hotter and more diffuse, it does not gravitationally collapse as readily into the galaxy's potential well. A few billion years
Merger Simulation from now, the smaller galaxies that are active today will have consumed much of their fuel, and the total cosmic output of radiation 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 05-08b shows the evolution subsequent to the initial phase.
Table 05-01 summarizes the evolutionary sequence.

Figure 05-08d Simulation of Galactic Merger [view large image]
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.
< 0.1 > 25 First stars, supernovea. Formation of black holes, production of heavy elements.
< 0.5 > 8.0 Protogalaxies. Protogalaxies drew in matter.
< 1.0 > 4.70 Baby galaxies. Galaxies took shape.
< 3.0 > 1.75 Quasar, supermassive black holes. Galaxies collided and merged, bursts of star formation.
< 6.0 > 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

Baby Galaxy Near the year end of 2004, it is reported that the NASA orbiting telescope Galaxy Evolution Explorer has discovered about three dozens bright and compact galaxies (Baby Galaxies) within one billion light years from the Milky Way. These objects emit strong ultraviolet light (from newborn stars and exploding supernovea), have low metal content, and are in the form of amorphous blob. These galaxy's gas contains just 2% of the Sun's abundance of heavy elements, or metals - the most pristine galactic gas seen since the big bang (star-forming regions in the Milky Way contain 100 to 200 more of these elements than the baby galaxy). This kind of astronomical objects are thought to exist more than 10 billion years ago (a few billion years after the Big Bang). Such nearby baby galaxies probably started out as a small gas cloud in a relatively empty region of space. It grew very very slowly until, after nearly 13 billion years, it had enough density to form stars. The top of Figure 05-09 compares the mature and newborn galaxies in visible and ultraviolet lights. The lower image is the baby galaxy I Zwicky 18, at a distance of only 45 million light years. The galaxy's proximity allowed Hubble's eagle-eyed

Figure 05-09 Baby Galaxy [view large image]

 Advanced Camera for Surveys to resolve a few thousand of its estimated 20,000 stars. The stars' colour and brightness suggest that none are more than 500 million years old.


The Milky Way7

Milkyway1 Milkyway2 NGC3370 There are about 4x1010 galaxies in the universe. Among this multitude of galaxies, 34% are spirals, 20% are ellipticals, and 54% are irregular. We happen to live in an ordinary spiral galaxy called the Milky Way. On a clear night and with the aid of long

Figure 05-10 MW, Earth's View
[view large image]

Figure 05-11 MW, All Sky View [view large image]

Figure 05-12 NGC3370
[view large image]

 exposure time, it appears like a silvery bridge across the sky as shown in Figure 05-10. It is a view looking from inside the galactic disk. An all sky (panoramic) view is
shown in Figure 05-11. If we could fly away from the Milky Way and look back, the view would be similar to the spiral galaxy NGC 3370 as depicted in Figure 05-12. Similar in size to our own Milky Way, spiral galaxy NGC 3370 lies about 100 million light-years away toward the constellation Leo. It contains a mixture of young stars in the bluer regions and an older population in the yellowish center. The total mass of NGC 3370 and the Milky Way is estimated to be several 1011 solar mass.
Milkyway Milkyway Components The main components of the Milky Way consist of a nucleus at the center, a nuclear bulge, a disk in the form of spiral arms winding around this nucleus, and a halo, which covers both the nucleus, the disk, and contains a spherical distribution of globular clusters as shown in Figure 05-13. The radius of the visible disk is about 20 kpc with the Sun located 15 kpc from the center. Figure 05-14 shows the location of the various components within the Milky Way up to 100 kpc from the center.

Figure 05-13 The Milky Way
[view large image]

Figure 05-14 The Components
[view large image]
Milky Way Neighborhood The Milky Way does not exist in isolation and is not a finished work as perceived by astronomers many years ago. It is observed that most galaxies formed from the merging of smaller precursors, and in the case of the Milky Way, we can observe the final stages of this process. As shown in Figure 05-15, the Milky Way is tearing apart small satellite galaxies (such as the Large and Small Magellanic Clouds) and incorporating their stars. Meanwhile hot intergalactic gas clouds are continually arriving from intergalactic space. The evidence for the continuing accretion of gas by the Milky Way involves high-velocity clouds (HVCs) - mysterious clumps of hydrogen, up to 10 million solar mass and 10,000 ly across, moving rapidly (from 90 to 400 km/sec) through the outer regions of the galaxy. These materials form the reservoir from which the Milky Way can draw on to make new stars.

Figure 05-15 Milky Way Neighbor-
hood [view large image]
The disk of the Milky Way8 exhibits a spiral structure, which shows up in the distribution of objects populating the disk component. These objects include, the HI regions of neutral hydrogen atoms, the population I objects such as young stars, diffuse star-forming nebulae9, H II regions of ionized hydrogen atoms and open star clusters. These population I objects are very young, in contrast to the very old population II objects in the Milky Way's Halo (globular clusters and old stars, including older planetary nebulae). The arms of the Milky Way, at least near the solar neighborhood in our Galaxy, are typically named for the constellations where more prominent parts of them are situated. The solar system is trundleing around at nearly 200 km/sec in the Local or Orion Arm - a spur in between the more substantial Sagittarius and Perseus arms.
Density Wave The formation of the spiral patterns is still a mystery because the simple model of differential rotation (rotational speed varies with distance) would produce tightly wound spirals (within 500 million years) in contrary to observation. A generally accepted mechanism for producing the spiral structure involves the density wave10 which is thought to be a gravitational disturbance that gently travels around the galaxy compressing gas in its wake. This compressed gas triggers star formation and helps to explain why we see the concentration of bright young stars and clusters in the spiral arms. Figure 05-16 is a schematic model illustrating the action of a density wave, which causes stars and interstellar gas and dust to bunch up temporarily, with the spiral arm being the result of a temporary compression of

Figure 05-16 Density Wave
[view large image]

 material. The mechanism for the generation of density wave is unclear, but it is thought to be similar to the traffic jam on highway (see Figure 05-16, actually a slowly moving truck is more appropriate in the example).

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Blackhole at the Milky Way Center11

It is found recently that the central black hole invariably comprises about 0.5% of the mass of the stars in the spheroid of the galaxies. This is referred to as the Magorrian relation, the same relationship is also applicable to the black holes in some globular clusters. A galaxy's spheroid is the round central bulge in a spiral galaxy, or the whole galaxy with an elliptical. It seems that once the black hole reaches a particular maximum mass, it would shut off its own growth by forcing orbits of surrounding stars to become more circular. This keeps them out of the black hole's powerful grip. If this process happened commonly, most galaxies must have undergone a bright quasarlike phase in their youth, when the central black hole was growing by ingesting material and producing quasarlike symptoms. The development of a central black hole may be an
SgrA unavoidable part of a galaxy's formation and evolution. The black hole in the Milky Way center is located at SgrA* toward a point in the constellation Sagittarius. It shows no sign of motion, has a mass of 3.7 million suns, its size is smaller than the solar system and is in a much quieter phase comparing to the AGN's or quasars. The upper portion of Figure 05-17 shows a 400 by 900 light-years mosaic of Chandra X-ray images toward the central region of the Milky Way. It reveals thousands of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the Milky Way center is located inside the bright white patch at the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high). The lower part of Figure 05-17 is a blow-up of the central region. It shows four (A-D) of the large number of variable x-ray sources - likely black holes or neutron stars in binary star systems - swarming around the Milky Way's central supermassive black hole. While four sources may not make a swarm, these all lie within only three light-years of Sgr A* (the bright source just above C). Repeated gravitational interactions with other stars are thought to cause the black hole systems to spiral inward toward the Galactic Center region.

Figure 05-17 Milky Way Center, SgrA*
[view large image]

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Dark Matter in the Milky Way12

Halo The halo is a spherical cloud of thinly scattered stars and globular clusters. It is the largest component of the Milky Way, extending to radius of about 100 kpc. It contains very little dust or gas. No star formation currently takes place there. This means that the halo contains very few young stars. Most of the halo stars are, in fact, 10 - 14 billion years, which is very close to the age of the galaxy itself. Halo stars are extreme Population II stars. They are very old, have very low metal content, and move in randomly tipped, elliptical orbits.

The motion of objects in the Milky Way is not consistent with the amount of luminous matter, which is not enough to confine these objects inside the Milky Way boundary. The problem can be reconciled if a lot of dark matter still remains in the halo - the original clump of mass - while the cooling of the hydrogen allows ordinary matter to contract, and settled into the disk.

Figure 05-18 Dark Matter Halo

Observations show that the dark matter in a galaxy surrounding the visible matter in a halo is larger and more nearly spherical than the stars and gas. The visible matter density is higher than the dark matter density near the centers of most galaxies, so the dark matter is not very important there. But it extends well beyond the stars and gas, so the outer parts of
Rotation Curve galaxies are essentially all dark matter. Figure 05-18 shows an "artist's impression" of a dark halo surrounding an almost edge-on disk galaxy. Figure 05-14 shows the dark matter distribution extends to 100 kpc. The rotation curve in Figure 05-19 provides a very convincing evidence for the pervasive presence of dark matter in the galactic halo (such as shown in M74). It is a plot of the rotational velocity of an object in the galactic plane versus distance to the center. From observations of starlight alone, the rotational velocity would be expected to fall towards the edge (dashed line). In fact the curve flattens (solid line), suggesting that galaxy is surrounded by a halo of unseen, dark matter.

Figure 05-19 Rotation Curve [view large image]

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    References:

  1. Types of Galaxies -- http://zebu.uoregon.edu/~soper/Galaxies/types.html
  2. Active Galaxies and Quasars -- http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html
  3. Active Galaxies, Centaurus A -- http://hubblesite.org/newscenter/archive/1998/14/
  4. Blackhole, Schwarzschild Geometry -- http://casa.colorado.edu/~ajsh/schwp.html
  5. Formation and Evolution of Galaxy -- http://www.astro.yale.edu/larson/papers/Tenerife91.pdf
  6. Formation of Galaxy, Models -- http://astron.berkeley.edu/~mwhite/modelcmp.html
  7. The Milky Way -- http://www.astro.umd.edu/education/astro/mw/mw.html
  8. Objects in the Disk of the Milky Way -- http://www.astronomynotes.com/ismnotes/s3.htm
  9. Milky Way, Diffuse Nebulae -- http://www.seds.org/messier/diffuse.html
  10. Density Wave -- http://www.astronomynotes.com/ismnotes/s8.htm
  11. Black Hole at the Milky Way Center -- http://www.space.com/scienceastronomy/cosmic_monster_030106.html
  12. Milky Way, Dark Matter -- http://astro.esa.int/SA-general/Projects/GAIA_files/LATEX2HTML/node45.html

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Index

Active galaxies
Active galaxy nucleus (AGN)
Blackhole
BL Lacertae objects (Blazars)
Chandra X-ray image, SgrA*
Cold dark matter
Cygnus A radio source
Dark matter
Density wave
Extremely Red Object (ERO)
Formation of galaxy
Globular clusters
HI regions
HII regions
Hot dark matter
Inside-out theory
Magorrian relation
Milky Way
 Milky Way center, SgrA*
Milky Way halo
OB associations
Outside-in theory
Population I objects
Population II objects
Power spectrum
Quasars
Radio galaxies
Rotation Curve
Schwarzschild geometry
Schwarzschild radius
Seyfert galaxies
Synchrotron radiation
Tuning fork diagram
Types of galaxies
White holes
Worm holes

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