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The Milky Way (+ 2022 Update, 2023 Update )

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 the Milky Way in panorama and top views.

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

Figure 05-12 All Sky View

Milky Way Panorama View Milky Way Top View

Figure 13a Parnorama View of the Milky Way [view large image]

Top View

See lot of astronomical images 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 1.5x1012 solar mass. The first painting featuring a prominent MilkyWay is probably the "Flight into Egypt" of

Figure 05-13b NGC7331 [view large image]

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

the holy family by Adam Elsheimer in 1609 (Figure 05-13c). In the Middle Ages, it is called the "Jacob's Ladder" leading to heaven in his dream. It is 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,
Milky Way Neighborhood Milky Way Streams 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 05-15a 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 reached all the way in. Recent surveys of the sky in near-infrared

Figure 05-15b Barred Milky Way

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
Barred Milky Way, Infrared Barred Milky Way, Latest Version 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,

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

Figure 05-15d Latest Version [large image]

partial arm called the Orion Arm, or Spur (see an artist's rendition in Figure 05-15d). 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 central black hole. The structure is strikingly

Figure 05-16b Bubbles and Beams from Milky Way Center [view large image]

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

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 05-16d1). The study concludes that the radio lobes are formed by star formation (rather than black hole driven) with outflow from the Milky Way's central 200 pc. The process transports a huge amount of

Figure 05-16d1 MW Bubbles in Radio Frequency

Figure 05-16d2 Milky Way Bubbles

magnetic energy, about 1055 ergs (~ 10 Msun), into the galactic halo. Figure 05-16d2 is a 2013 interpretation of the bubbles in terms of winds of hot gas from young star clusters. See "X-Ray Surveys of the Galactic Black Hole, 2019".
[Update 2022] [End of Update 2022]

[Update 2023]

Figure 05-16e5 shows another radio view of Milky Way center in colors instead of the "black and white" diagram in
Figure 05-16e3,a. It presents the colors as "spectral index"

= d[logF()] / d[log()] in general with F() = , = logF() / log() for power law of the spectrum,
where F() is the flux density = rate of flow of the radiation per unit area per unit frequency .

As shown in Figure 05-16e6, interpretation of the colors becomes easier by assuming a power law for the spectral index at least for portion of the spectrum (see upper left insert). In such case, a spectral index of less than zero indicates that the intensity of radiation decreases with increasing frequency, while a spectral index greater than zero signifies that the intensity of radiation increases with increasing frequency. A spectral index of zero means the intensity of radiation is independent of frequency. In other words, at a given frequency, the higher value of is related to higher flux density from the source.
Milky Way Center, Re-visit Spectral Index For example, the region in blue associates with higher flux density in Figure 05-16e6,b. While in Figure 05-16e5, objects in red to light-green colors have negative from synchrotron radiation (showing the magnetic field lines as well); These objects would fade away in high frequency such as the infrared (see Figure 05-16e4).

Figure 05-16e5 Milky Way Center, Re-visit

Figure 05-16e6 Spectral Index
[view large image]

Objects with deep-green to blue color have positive are thermal emission from molecular clouds (designated by Sgr A, B, C, D) and dust.

Radio Arc at Milky Way Center, Jet Bending of 3C465 Origin of the radio arc in Figure 05-16e5 (see also its enlargement in Figure 05-16e7) is still not well understood. It is a subject of active research and debate among astronomers. Large-scale supernova explosion, and multiple outbursts from Sgr A* over several thousand years ago have been suggested.

However, comparison with the case of distorted jets in 3C465 (see Figure 05-16e8, and "A high-resolution view of the jets in 3C 465") indicates that the arc could be the result of interaction with the surrounding medium such as dense molecular cloud.

Figure 05-16e7 Radio Arc, Milky Way Center

Figure 05-16e8 [view large image]
Bent Jet of 3C465

(see "X-ray Gas Associated with the Galactic Center Radio Arc"). The bent jet and its synchrotron radiation have betrayed the secret of its origin (see a video on "Bent Jets from Black Holes").

As observational resolution has improved to angular unit of second ("), it is now possible to measure the orientation of the filament in the Milky Way Center in term of Position Angle (PA), which is defined as the angle between the filament and the vertical axis (latitude) perpendicular to the galactic plane (see Figure 05-16e9).
PA Definition Filament PA Figure 05-16ea shows the PA with respect to Galactic north (PA = 0°), traced by different colors. It displays the full range of filament between 0° and 180°. The colors are used in order to distinguish between the filaments with positive PA (red) and negative PA (blue). The distribution suggests that the filaments oriented perpendicular to the Galactic plane tend to be long. On the other hand, filaments with Pas running parallel to the Galactic plane are short in yellow color.

Figure 05-16e9 PA Definition

Figure 05-16ea Filament PA [view large image]

Short filament PA distribution is orthogonal to the PA distribution of long filaments which are aligned close to the Galactic north-south orientation. Figure 05-16-be,a shows the PA distribution of short (L < 66") filaments. They appear mainly in two peaks near 70° and -10°. The filaments in the peak at PAs ~70 with a width of ~40 are distributed mainly within ±20° of the Galactic plane, whereas the filaments that peak around -10° are oriented closer to the Galactic north-south direction, having a distribution similar to that of the long filaments.
Filaments, Short The majority of short filaments have positive spectral indices consistent with thermal emission though it cannot exclude nonthermal filaments similar to the filaments of the Radio Arc near ~0.2o. Figure 05-16-be,b also indicates that some short filaments are nonthermal with a steep negative spectral indices (see Update 2023).

Figure 05-16eb Filaments, Short [view large image]

It is noted that vertical filaments dominated by length L > 66" have non-thermal spectra.

While the long filaments align with the magnetic field which is more or less perpendicular to the galactic plane and emitting non-thermal radiation; the short filaments have an entirely opposite characteristics associated with thermal radiation and parallel to the galactic plane. Those short filaments surrounding the Sgr E molecular cloud on the galactic plane about 2o East of Sgr A* provided a vivid example (see Figure 05-16ec). See original article : "The Population of the Galactic Center Filaments"

[End of Update 2023]

Here's a series of images progressively zoom into the central black hole SgrA* from 1 kpc to 3x10-6 pc:

[view large image]

Frame #2 has a corresponding X-ray image showing the Fermi bubbles. See "X-ray astronomy comes of age" for more info about the X-ray observation.

Frame #4 of "Sgr A West" has 2 enlarged images [see ] from "Sgr A* - The Supermassive Black Hole in the Milky Way", and "The episodic and multiscale Galactic Centre" (for the 3-D schematic).

Frame #7 shows blueshifted H30 line emission, and continuum emission in white contours. The Bondi circle marks the boundary where surrounding medium (gas, dust) is likely to fall in. See "A cool accretion disk around the Galactic Centre black hole".

Frame #8 zooms into SgrA* at 86 GHz. See "Lifting the veil on the black hole at the heart of our Galaxy".
BTW, the absence of jet seems to present a problem with the frame #8 image. Actually, there would be no jet if the Alfven velocity vA = B/(4)1/2 is less than the escape velocity (see "Magnetohydrodynamics (MHD) and the Formation of Jets"). In addition, the intensity of the Synchrotron radiation also depends on the frequency (see "Synchrotron Radiation" and a video of the M87 jet).

Frame #9 is the Milky Way's central black hole taken by the same collaboration using the Event Horizon Telescope (radio) in producing the M87 black hole. The one shown to the public is an average over thousands of face-on images during the observations. In addition to show a ring of radiation around a darker disk, the resulting image contained three brighter ‘knots’, which are probably artefacts of the interferometry technique used by the EHT. Since the signals vary in time scales of 5 to 15 minutes, the size of the object must be smaller than 3x10-6 pc, otherwise the variations would not occur. It is consistent with the Schwarzschild radius of 4x10-7 pc for the black hole (see more data and graphs about the size for various objects about the black hole). Here's a version with the artefacts removed : Milky Way Black Hole within 3x10-6 pc, see [a video], and "Black hole at the centre of our Galaxy", 12 May 2022.

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