Galaxies

The Milky Way (+ 2022 Update, 2023 Update )

		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

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

		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.

		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]

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).

		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.

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 :

		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.

		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".

The MeerKAT telescopes have collected more data about the radio bubbles as it came online in 2018. Here's some of the details (see pictorial illustrations in Figure 05-16e3) :



	The MeerKAT telescope array was launched in 2018. It is part of the Square Kilometre Array (SKA). Diagram-e in Figure 05-16e1 is a composite image bringing together the two SKA sites under a shared sky with South Africa’s KAT-7 and MeerKAT telescopes to the left and Australia’s ASKAP telescope on the right.
Figure 05-16e1 SKA [view large image]



• Figure 05-16e3 shows the inner region of the Milky Way from 500 pc to about 50 pc.
  
  
  
  
  1. Diagram-a is the panoramic view spanning about 500 pc x 300 pc. It is composed from a mosaic of 20 different observations taken over three years (see "Mysterious strands at the heart of the Milky Way", 2022). There are many radio features including supernova remnants, compact star-forming regions, and the large population of mysterious radio filaments. The broad feature running vertically through the image is the radio bubbles, spanning ~ 300 pc across the center of the Galaxy. It is bound by the more prominent "radio arc" and its extension on one side and an obscure boundary running in an imaginary line near "S9" to "C3" (see more in diagram-b).
     
     Here are some examples for visualizing the unit of distance in parsec (pc) :

     The distance from the Sun to the nearest star - Alpha Centauri, is 1.34 pc. 1 pc = 3x1018 cm. 1 ly = 1018 cm = (1/3) pc. At Milky Way center, 1o 144 pc (measured from the Sun at a distance of ~ 8 kpc). Sun-Earth distance = 1 AU = 5x10-6 pc = 1.5x1013 cm. Inner radius of accretion disk ~ 0.24 AU = 1.2x10-6 pc (~ distance from Sun to Mercury). Schwarzschild radius for the Milky Way black hole rs = 0.08 AU = 4x10-7 pc.

     Figure 05-16e2 Milky Way Black Hole, Explained

     See Figure 05-16e2 for a comparison of the orbital size between the inner solar system and the inner region of the Milky Way Black Hole; and the "M87 Black Hole" which is about 1.55x103 times larger both in mass and rs.

     
     
  2. Diagram-b shows the radio bubbles in a frame of 290 pc x 450 pc (about 1/2 the width of diagram-a, see "Bipolar Radio Bubbles in the Galactic Center"). The dashed grey line shows the outline of the radio bubbles (as traced over an isophote). The location of the strong radio source Sgr A* is marked, along with the positions of the giant molecular cloud complexes Sgr B2 and Sgr C. The most prominent magnetized filaments are marked in red, including the longest filaments making up the "radio arc", and the corresponding counterpart near Sgr C, that along with the radio arc are coincident with the latitudinal boundaries of the radio bubbles. Note that many of the red lines or marks represent complexes consisting of multiple parallel filaments. The dashed blue line marks the X-ray bubble as mentioned earlier. Similar to the image of some planetary nebular, only the edge of the expanding bubble is visible because of a thicker layer of emitters is aligned to the line of sight .
  
  
  3. Diagram-c is in a frame of ~ 60 pc x 80 pc zooming ever closer to SgrA*. This image is taken from "Shock-Generated Turbulence In the Innermost 50 pc of the Galaxy Center" published in 2014. It is a 21 cm radio continuum image with 11 arcsec resolution. Except for the Radio Arc and part of the Sgr A region which are non-thermal, all the radio emission is thermal bremsstrahlung (braking radiation). The 38 positions observed by with the Spitzer telescope in the mid-infrared are indicated by square boxes. At a distance of about 25 pc in the Galactic plane are the Quintet and the Arches star clusters. Most of the stars are young (age ~ 2 Myr) and massive (10 - 50) M_Sun. The Arches and Sickle gas filaments are also visible.
  
  

  Diagram-d reveals thousand of magnetic filaments upon further analysis of Diagram-a (one week after its publication, see "1,000 mysterious strands revealed at the Milky Way’s heart"). The image shows a huge population of the filaments, 10 times more than previously thought. Some stretch up to 50 pc in length. It is highly organized. At times they come in clusters and in pairs, at times criss-crossing, at times curving, with some evenly spaced out “like strings on a harp. It is suggested that the filaments are related to past activity of the Milky Way’s central supermassive black hole rather than coordinated bursts of supernovae. The filaments also could be related to enormous, radio-emitting bubbles, which was discovered in 2019. The

  Figure 05-16e3 Milky Way Center [view large image]

  filaments are magnetized (although about 1000 times weaker than the Earth's), they are amplified along the filaments amid a weaker magnetic field permeating the central 100 pc, a primary characteristic all the filaments share (see details in "Galactic Center Filaments").

  Taking a circular area of about 10⁴ pc² within the boundary of the bubble at the galactic equator, 1000 magnetic filaments going through it would be averaged to about 10 pc² per filament. Comparing this number with the 1.3 pc² without another star (as drawn by an imaginary circle from the Earth to Alpha Centauri), the spacing between the filaments is actually rather sparse. The typical distance between stars in a globular cluster is about 0.3 pc which amounts to ~ 0.07 pc² per star - showing a very compact packing.


• Diagram-e is a mid-infrared image of the Milky Way center (see "Downtown Milky Way"). Stars are formed progressively according to the ordering of the materials listed below:
  

  Green areas - organic molecules emissions often referred to as Central Molecular Zone, CMZ (Figure 05-16e4, where 1 Jy = 10-23 erg/s-cm2-Hz). Molecules found in the zone include carbon monoxide, methanol, isocyanic acid, hydrogen cyanide and silicon monoxide. It is an exceptional and extreme environment where the gas temperatures, velocity dispersions, magnetic field strengths, pressure, etc. are much

  Figure 05-16e4 CMZ (Central Molecular Zone)

  higher than in the Galactic disk. Red areas - emissions from dust. Dark areas - dense star forming clouds (such as Sgr B2).

  Note that the elements C, N, O, and Si in the molecules can be produced only in large stars and dispersed through subsequent supernova remnants (SNR). Thus, the environment must have gone through many stellar cycles.

		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.

		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").

		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]

	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.

A model is proposed for the formation of the short filaments (see schematic diagram in Figure 05-.16ec,b) :

	The accretion disk around the black hole at the Milky Way center has its surface perpendicular to the East-West direction. An outflow of something similar to a pair of jet points to both the East and West directions. This high velocity outflow exerts RAM pressure to objects such as magnetic field, molecular cloud and compact HII region creating the short filaments.
Figure 05-16ec Sgr E and Model [view large image]	In addition, this model also explains fewer number of short filaments on the other side (the West side), because the motion of the clouds is East-ward countering the RAM pressure. This interpretation is also consistent with the previous conjecture of "jet bending".

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