Galaxies

Contents

Types of Galaxies¹

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.

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

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]

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]

Active Galaxies²

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, extremely red objects (ERO), and quasars 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.

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 galaxy 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]

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 05-01f is an artist's illustration of the X-ray AGN.

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

Seyfert Galaxies

		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 impression that matters
Figure 05-02a Seyfert Galaxy Face-on [view large image]	Figure 05-02b Seyfert Galaxy Edge-on [view large image]	are falling into a hole.

Radio Galaxies

	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 in 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
	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. By superimposing the radio images taken by the Very Long Baseline Array (VLBA) to the gamma-ray sky map produced by the
Figure 05-03 Radio Galaxy	Large Area Telescope (LAT), astronomers are able to confirm that the gamma-ray emission from the core of AGN is associated with the radio jet.

Extremely Red Objects (ERO)

		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 image containing over 100,000 galaxies (Figure 05-04b). Many of the faint red objects in the background (against a relatively
Figure 05-04a ERO [view large image]	Figure 05-04b Infrared Galaxies [view large image]	nearby spiral galaxy) are massive galaxies at distances of over 10 billion light-years.

Quasars

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

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

their spectra, the overall spectrum is consistent with synchrotron 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 (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.

Black Holes⁴

		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 05-06a 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 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]

		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]

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]

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.

Formation and Evolution of Galaxy^5,6

		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 galaxies (clusters of galaxies) are still in the process of assembling.
Figure 05-08a Initial Formation [view large image]	Figure 05-08b Proto- galaxies [large image]	The opposing view is the "top down" process in which large clump breaks up into smaller units. It is referred to as the

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.

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]

After the initial phase of galaxy formation, there was an era of cosmic fireworks: galaxies collided and merged (see Figure 05-08e), 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 transited from a small number of bright

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

objects to a large number of dimmer ones. Computer simulations suggest that such shift may be a direct consequence of cosmic expansion.

		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 05-08f shows the evolution subsequent to the initial phase. Figure 05-08g shows another evolutuon simulation with cold dark matter.
Figure 05-08f Evolution History [view large image]	Figure 05-08g Evolution Simulation [view large image]

	This finding is consistent with the progression of mass among the types of galaxy as shown in Figure 05-08h, 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 05-08h 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

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 05-08i 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 Evolution of Galaxies

Figure 05-08j shows the evolution of galaxies in the form of various astronomical objects as we look back in time:

Nearby individual galaxies (triangles). Clusters of galaxies such as the Coma cluster. SDSS (Sloan Digital Sky Survey) galaxies. Superclusters (blank strips: planes of the Milkyway blocking light from outside). GRBs (gamma-ray bursts) with known distances (asterisks). Boundary between cosmic deceleration and acceleration at z = 0.7554. SDSS quasars. SN 1997ff, the most distant type Ia supernova at z = 1.7. GRB 050904, most distant GRB at z = 6.295. Most distant SDSS quasar at z = 6.42. Most distant SDF (Subaru Deep Field) galaxy at z = 6.578 (to z = 6.96 in 2006). Epoch of first star formation with z ~ 20. Cosmic microwave background at z ~ 1100.

Figure 05-08j Evolution of Galaxies [view large image]

The lookback time T (in billions of years) is computed from the red shift z by the formula: T = 13.7 x [(1 + z)2 - 1] / [(1 + z)2 + 1].

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 supernovae), have low metal content, and are in the form of amorphous blob. The 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 is 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 slowly until, after nearly 13 billion years, it had enough density to form stars. The images on top of Figure 05-09a 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 05-09a 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. More HST observations in 2008 indicate that star formation in

I Zwicky 18 began at least one billion - and perhaps up to 10 billion - years ago, making the galaxy no more remarkable than most of its neighbors.

Another baby galaxy in the early universe has been detected in 2005. It is located at a point about 800 million years after the Big Bang. It has a mass eight times that in the Milky Way. The discovery was surprising, since astronomers have long theorized that galaxies form when stars gradually cluster together, with small galaxies preceding bigger galaxies. Now the new evidence suggests that the process of galaxy formation started really very early on. This galaxy, known as HUDF-JD2 was found by researchers using NASA's Hubble and Spitzer space telescopes. It is the smaller red object in Figure 05-

Figure 05-09b HUDF-JD2 [view large image]

09b. Other survey found massive galaxies originated 700 million years after the Big Bang, whereas the supposedly old dwarf galaxies showed the most recent bouts of star formation just 4 billion years ago. Theoretical consideration also suggests that the proto-

galaxies in the early universe may have fizzled out by supernova (as shown in Figure 05-08b) and thus unable to support the "merger". These new evidences have turned the "Inside-out Theory" cherished by astronomers in the 1990's upside down. It is at least incomplete, if not entirely wrong.

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Theory of Spiral Arm Formation

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 wave of excess density (density wave10) 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 05-10a 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

Figure 05-10a Density Wave [view large image]

being the result of a temporary compression of 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 05-10a).




The original density wave soon ran into the difficulty of energy dissipation by setting up shock waves in the interstellar gas. Complex mechanism was proposed to resolve the problem without much success. This unsatisfying state of affairs changed only in the 1980s and 1990s when gas is included into the simulations. Since gas constitutes only a few percent of the mass of spiral galaxies, it was not taken into account in previous modeling. It turns out that the gas has a disproportionate dynamical role. As soon as gas is included into the simulation, it produces a rich variety of galactic morphologies. The torque exerted by the stellar bar acts as a giant stirrer, continuously driving a spiral structure in the gas. The spiral did not fade away as it had in earlier simulations. It also explains the presence of dust lanes on the

Figure 05-10b Reinvigoration [view large image]

leading edge of spiral arms, the high rate of star formation in galactic center, and the refueling of the central black hole. Figure 05-10b shows the cycles of reinvigoration by the accretion of intergalactic gas into the system starting from an initial disk of stars, gas, and dust.


Recent study suggests that the wave action works not by literally pushing the stars together but by subtly choreographing their orbits. As a star trundles around the galaxy on an ellipse, the ellipse itself is moving. This secondary motion is the raw material for the density wave (see Figure 05-10c). As shown in diagram a, no wave motion occurs when orbits are oriented randomly. Bar wave arises when orbits align as shown in diagram b. Gravity causes the ellipses to move in unison, maintaining their alignment. The wave front is the area of greatest star density, which occurs along the major axis

		of the ellipses. Spiral wave arises when the ellipses move in unison but are not perfectly aligned; each ellipse is slightly skewed compared with its neighbors. The density of stars is highest where the ellipses crowd together as shown in diagram c. Diagram d shows the barred spiral pattern when orbits near the center of the galaxy are aligned but those farther out are skewed. It is thought that the alignment is caused by spontaneous
Figure 05-10c Wave Patterns [view large image]	Figure 05-10d Wave Anatomy [view large image]	gravitational instability. Because gravity in galaxies is not a fixed external force but a product of the stars themselves, waves

can be self-reinforcing. The process starts when stellar orbits become aligned by chance. Amplified by proximity, the gravity of the stars modifies the rotation speed of the ellipses. Faster ones slow down and slower ones speed up, so they bring themselves into sync. When a star enters the wave, gravity locks it in temporarily; after a while, it becomes unlocked and exits. Stars coming in the other side of the wave ensure that the structure persists. The corotation circle marks the radius where the wave accompanies the stars at the same velocity. Stars' orbits moving at the Lindblad resonance have a certain synchrony with the wave. It plays a distinct role in shaping the orbits and delimiting the density wave (Figure 05-10d).

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The Milky Way⁷

			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 exposure time, it appears like a silvery river across the sky as shown in
Figure 05-11 View from Death Valley [view large image]	Figure 05-12 All Sky View [large image]	Figure 05-13a NGC7331 [large image]	Figure 05-11. It is a view looking from inside the galactic disk. The all sky (panoramic) view in different regions of the electromagnetic spectrum is shown in

Figure 05-12 (in false colours). If we could fly away from the Milkyway and look back, the view would be similar to the spiral galaxy NGC 7331 as depicted in Figure 05-13a. Similar in size to our own Milky Way, spiral galaxy NGC 7331 lies about 50 million light-years away toward the constellation Pegasus. It contains a mixture of young stars in the bluer regions and an older population in the yellowish center. The total mass of NGC 7331 and the Milky Way is estimated to be several 1011 solar mass. The first painting featuring a prominent MilkyWay is probably the "Flight into Egypt" of the holy family by Adam Elsheimer in 1609 (see Figure 05-13b).

Figure 05-13b Milkyway, Artist's View [view large image]

In the Middle Ages, it is called the "Jacob's Ladder" leading to heaven in his dream. The MilkyWay was also known as "Silvery River" in other folklore.

		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 05-14a. The radius of the visible disk is about 20 kpc with the Sun located 15 kpc from the center. Thickness of the disk is only about
Figure 05-14a The Milky Way [view large image]	Figure 05-14b The Components [view large image]	1 kpc. Figure 05-14b shows the location of the various components within the Milky Way up to 100 kpc from the center.

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.

		An August 2008 report from the Sloan Digital Sky Survey indicates that there are many stellar streams crisscrossing the Milky Way halo. They are the stars torn from disrupted satellite galaxies that have merged with the Milky Way. Figure 05-16a is a theoretical model of a galaxy like the Milky Way showing many trails of stars . The region shown is about 1 million light-years on a side; the Sun is just 25,000 light-years from the center of the galaxy and would appear close to the center of this picture.
Figure 05-15 Milky Way Neighborhood [large image]	Figure 05-16a Milky Way Streams [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. The Milky Way is now known as a barred spiral. The evidence, at first indirect, began to accumulate in 1975: stars and gas tracked in the middle of the Milky Way did not follow the orbits they would if the spiral pattern

Figure 05-16b Barred Milky Way [view large image]

reached all the way in. Recent surveys of the sky in near-infrared light have revealed the bar directly and dispelled the remaining doubts (Figure 05-16b).

The latest (2008) view of the Milky Way is presented by the Spitzer Space Telescope in infrared. Figure 05-16c shows just a small part of the mosaic toward the Milky Way center in which the green filaments are light (false-color) from complex molecules - polycyclic aromatic hydrocarbons (PAHs) - that on Earth are the common, sooty products of incomplete combustion. The PAHs are found in star forming regions, along with reddish emission from graphite dust particles. Blue specks throughout the picture are individual Milky Way stars. The new data also reveal a structure

		different from the traditional view. It finds that the Milky Way is a barred spiral with only two major arms - the Scutum-Centaurus and Perseus arms. They contain the greatest densities of both young, bright stars, and older, so-called red-giant stars. The two minor arms, Sagittarius and Norma, are filled with gas and pockets of young stars. The solar system lies near a small, partial arm called the Orion Arm,
Figure 05-16c Milky Way, Infrared [view large image]	Figure 05-16d Latest Version [large image]	or Orion Spur (see an artist's rendition in Figure 05-16d).

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Black Hole 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

Figure 05-16e Sagittarius [view large image]

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 unavoidable

part of a galaxy's formation and evolution. The black hole at the Milky Way center is located at SgrA* toward a point in the constellation Sagittarius (Figure 05-16e). 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 05-17a 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 05-17a 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

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

with other stars are thought to cause the system near the black hole to spiral inward toward the Galactic Center.

In 2005, the Chandra X-ray Observatory discovered more than 50 massive stars (with mass = 30 - 50 MSun) in a big dust ring around the central black hole of the Milky Way at less than a light year's distance (Figure 05-17b). According to the standard model for star formation, gas clouds from which stars form should have been ripped apart by tidal forces from the supermassive black hole. Evidently, the gravity of a dense disk of gas around Sagittarius A* offsets the tidal forces and allows stars to form. Since there are no low mass stars in this region, it strongly suggests that the massive stars must have formed where we see them now - around the black hole. They would not be members of star cluster migrating from a more distant location.The discovery may also explain the large amounts of heavy elements such as oxygen and iron observed in the disks of supermassive black holes.

Figure 05-17b Massive Stars near the BH [view large image]

Using one of the Paranal Observatory's very large telescopes in northern Chile and a sophisticated infrared camera, astronomers patiently followed the orbit of a particular star, designated S2, as it came within about 17 light-hours of the

		center of the Milky Way (about 3 times the radius of Pluto's orbit). Their results convincingly show that S2 is moving under the influence of the enormous gravity of an unseen object that must be extremely compact -- a super-massive black hole with over 3 million times the mass of the Sun. The deep near-infrared image in Figure 05-17c shows the crowded inner 2 light-years of the Milky Way with the exact position of the galactic center indicated by arrows.
Figure 05-17c BH at MW Center [view large image]	Figure 05-17d Stars near BH [view large image]	Figure 05-17d is a time-lapse movie in infrared light detailing how stars in the central light-year of our Galaxy have moved over a period of eight years. The yellow mark

at the image center represents the location of Sgr A*. The rapid stellar motion is consistent with a black hole over one million times the mass of our Sun confining to a region less than 1/5 of a light-year across at the center of the Milkyway. The computation to obtain the mass of the black hole with elementary formulas is illustrated in footnote 1.



		It is reported in November 2005 that compact radio source at wavelength of 3.5 mm from gas in a region just 8 light minutes (~1.44x1013 cm) across, centered on the Milky Way black hole, was measured by the Very long Baseline Array, which is a system of ten radio telescopes scattered across the United States. Figure 05-17e shows a super-resolution image with a circular beam of 0.2 mas (major axis size) from which an east-west elongated structure can be seen. The Schwarzschild radius Rs is
Figure 05-17e Radio SgrA* [view large image]	Figure 05-17f BH Shadow [view large image]	1.2 x 1012 cm according to the latest estimated value for the mass of the central black hole (~ 4 x 106 Msun). Thus, the size of the unresolved beam (the circle) is 2.5 Rs with

the black hole just hidden inside. A 2007 measurement at a wavelength of 1.3 mm (which offers better resolution and hence finer detail) indicates that SgrA* arises in a region offset from the central black hole.

It is suggested that the appearance of a black hole would be similar to the rendering shown in Figure 05-17f. The event

horizon of a black hole completely absorbs emissions from matter behind when viewed from Earth. The result is a darker circle that could be seen in a sufficiently high-resolution image. The tendency of photons to be flung around the black hole in the direction of its rotation brings about an event-horizon shadow that is off-center, to the right in this picture. A brighter ring around the shadow is formed by light rays, which are strongly deflected by the gravitational pull of the black hole without being absorbed by it. The real image of a black hole has been captured sitting at the center of the face-on spiral galaxy NGC1097 (Figure 05-17g). It is a view looking down

Figure 05-17g NGC1097 [view large image]

from the top. Matter is falling in from bars of stars and gas. The black hole is surrounded by hot gas and bright stars. This mysterious object is completely obscure by the jet stream (in green color) shooting out toward the viewing direction.




By 2007, astronomers can say with almost certainty that a black hole lies at the center of the Milky Way with a mass of some 4 million Suns and a size smaller than the Earth's distance from the Sun.. Figure 05-17h is an artist's impression of the Milky Way center with a black hole surrounding by an accretion disk of gas and dust (yellow and pink rings) and orbiting by dozens of massive, young stars (white and blue). Farther out are clouds and curtains of interstellar dust, which reflect the light from flares occurring when gas falls into the black hole. The region is also home to some very young, very dense star clusters (the blue stars at the upper left).

Figure 05-17h Milky Way Center [view large image]

Figure 05-17i Milkyway Centre Zoom In [view large image]

Figure 05-17i depicts a sequence of images to zoom in to the centre of the Milky Way utilizing the penetration power of different kinds of electromagnetic wave. Figure 05-17j is a blowup of frame # 3 in Figure 05-17i with radio image in red, X-ray image in blue. It shows the radio arc jutting straight out from the Galactic plane and a zoom in view of the Arches, which is the most compact cluster of stars known in the Milky Way. One hypothesis holds that the Radio Arc and the Arches have such geometrical shapes because they contain hot plasma flowing along lines of constant magnetic field. Images from the Chandra X-ray Observatory appear to show this plasma colliding with a nearby cloud of cold gas. The black hole at the Galactic center is hidden inside the bright radio structure.

Figure 05-17j Radio Arc [view large image]

The insert at the lower right is imaged at 20 cm (purple) with the NRAO Very Large Array, tracing H II regions that are illuminated by hot, massive stars, supernova remnants, and synchrotron emission. Emission at 1.1 mm (orange) was observed

with the Caltech Submillimeter Observatory and highlights cold (20-30 K) dust associated with molecular gas. Some of this material will form stars within in the next few million years; the remainder will be blown away (Image courtesy of NRAO/AUI).

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Dark Matter in the Milky Way¹²

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

Figure 05-18 Dark Matter Halo

clump of mass - while the cooling of the hydrogen allows ordinary matter to contract, and settled into the disk.



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 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 05-14b 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]

See more about the gamma-ray sky of the Milky Way in the Appendix.

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Footnotes:

¹ Mass of the black hole M at the Milkyway center can be computed from the velocity of the orbiting star v and the distance r from the black hole:



	1. For a star with mass m moving around the black hole the gravitational attraction must be balanced by the centrifugal repulsion: G m M / r2 = m v2 / r, from which M = r v2 / G where G = 6.67 x 10-8cm3/sec2-gm is the gravitational constant.
Figure 05-20 [vli] Circular Motion

2. If we take r = 0.5 ly = 0.5 x 10¹⁸cm, and the angular displacement = 1^o = /180 = 0.0175 radian, then the distance s covered by the star in t = 8 years = 8 x 3.15 x 10⁷ sec is given by:

s = r = 0.875 x 10¹⁶cm

3. Thus, the tangential velocity of the star:

v = s / t = 3.5 x 10⁷cm/sec

4. Putting everything together we obtain:

M = 0.9 x 10⁴⁰gm = 4.5 x 10⁶ M_sun
where 1 M_sun = 2 x 10³³gm

5. The Schwarzschild radius r_s = 2 G M / c² = 1.4 x 10¹²cm < 1 A.U.
where 1 A.U. = 1.5 x 10¹³cm is the distance from the Sun to the Earth.

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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. Black Hole, 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) Black hole 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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