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Galaxies


The Milky Way

Milkyway1 Milkyway2 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. It is a view looking from inside the galactic disk. The all sky view in different regions of the electromagnetic spectrum is shown in Figure 05-12 (in false colours). Figure 05-13a shows a panorama of the Milky Way. Also check out another panorama of the Milky Way by Axel Mellinger.

Figure 05-11 View from Death Valley [view large image]

Figure 05-12 All Sky View [large image]



Figure 05-13a A Panorama View of the Milky Way [view large image]

See more detail in Serge Brunier's Gallery.

NGC7331 Milkyway3 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 05-13b. 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

Figure 05-13b NGC7331
[view large image]

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

1609 (see Figure 05-13c). 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 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 05-14, 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 Neighborhood Milky Way Streams 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 05-15a 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 05-14 Milky Way Neighborhood [large image]

Figure 05-15a Milky Way Streams [view large image]

Barred Milky Way The disk of the Milky Way 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 nebulae, 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 05-15b 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 05-15b).

The latest (2008) view of the Milky Way is presented by the Spitzer Space Telescope in infrared. Figure 05-15c 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
Barred Milky Way, Infrared Barred Milky Way, Latest Version 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, or Spur (see an artist's rendition in Figure 05-15d).

Figure 05-15c Milky Way,
Infrared [view large image]

Figure 05-15d Latest Version [large image]

While most of the arms are spiraling inward to the center, the 3 kpc arms are expanding at speed more than 50 km/sec.

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, and dark matter. The radius of the visible disk is about 20 kpc with the Sun located 8 kpc from the center. Thickness of the disk is only about 1 kpc. Figure 05-16a shows the recently updated version of the Milky Way as published in the September 2011 issue of the "Astronomy" magazine. Followings are the legends for the various components in the illustration.
Milkyway Components Top View :

1. Galactic Center - A super-massive black hole of 4 million solar mass is sitting at the center. It is about 8 kpc from Earth.
2. Galactic Bar - It is a region about 8.6 kpc long, where stars orbit in narrow elliptical instead of circular paths.
3. Central Molecular Zone - This zone contains dense, turbulent gas that gives rise to new stars at a higher rate than more outlying regions. It is about 7.4 kpc across.
4. Spiral Arm - When orbiting stars and gas enter the arms, they slow down and bunch up triggering star birth.
5. Gas Flow - The gas entering a spiral arm deflects slightly toward the galactic center, where it fuels star birth.
6. Spiral Flow - The movement of gas, dust, and stars forms a spiral pattern, which indicates a "sink" (the black hole) at the galactic center.

Figure 05-16a Milky Way Components

Side View :
7. Galactic Bulge - This is a spherical population of stars orbiting the galactic center.
8. Galactic Disk - It contains most of the galaxy's stars, a majority of which resides within the thin disk with thickness about 0.4 kpc (8a). The thicker disk (8b) is about 2 kpc thick. The galactic disk also contains most of the gas with a warped layer of neutral hydrogen extending to a radius of 21.5 kpc.
9. Disk Stability - As stars form from molecular gas in the disk, they heat up the surrounding gas creating an outward pressure that prevents the disk from collapsing.
10. High-velocity Gas Clouds - At least two dozen large gas clouds and hundreds of smaller ones orbit the Milky Way. Those merged in collisions fuel more star formation.
11. Globular Clusters - At least 158 dense collections of stars orbit in the Milky Way's halo. The dark matter in the extended halo is not shown in the illustration (it's invisible anyway).
Bubbles and Beams from the Center of MW Centaurus A 12. Bubbles and Beams - It was discovered by NASA's Fermi Gamma-ray Space Telescope in 2010 that there are pairs of gamma-ray bubbles and beams emanating up and down from the Milky Ways's center out to a distance of 27000 light-years (Figure 05-16b, by artist's impression). These features indicate that the Milky Way was much more active not long ago. The beams likely resulted from hot matter squeezing through the magnetic field in the galactic center while the bubbles probably formed from the push of material spewing from the

Figure 05-16b Bubbles and Beams from Milky Way Center


Figure 05-16c Centaurus A [view large image]


central black hole. The stucture is strikingly similar to the active radio galaxy Centaurus A as shown in Figure 05-16c (the real thing in various wavelengths, not a rendition by an artist). Even the tilt of the jets at 15o to the rotational axis appears to be the same.

MW Bubbles in Radio Frequency MW Bubbles The real image of the Milky Way bubbles in radio frequency finally appears in an early 2013 issue of Nature. It is taken at 23 GHz showing at least three ridges. The lobes (black solid line) are permeated by strong magnetic fields of up to 15 µG. The field lines (light dark curves) is aligned with the ridges following the curved shape(Figure 05-16d). The study concludes that the radio lobes are formed by star formation (rather than black hole driven) with outflow from the Milky Way's

Figure 05-16d MW Bubbles in Radio Frequency

Figure 05-16e Milky Way Bubbles

central 200 pc. The process transports a huge amount of magnetic energy, about 1055 ergs (~ 10 Msun), into the galactic halo. Figure 05-16e is a 2013 interpretation of the bubbles in terms of winds of hot gas from young star clusters.

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