Radiation astronomy/Galaxies

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This Galaxy Evolution Explorer (GALEX) image of the spiral galaxy Messier 81 is in ultraviolet light. Credit: NASA/JPL-Caltech/J. Huchra (Harvard-Smithsonian CfA).

The radiation astronomy of galaxies generally is about the galaxy as a radiated or radiation emitting astronomical object. The stellar aspects of individual galaxies are in galaxies of stars.

Galaxy clusters[edit]

This is a map of the Boötes Supercluster. Credit: Atlas of the Universe.{{free media}}

"There are a couple of [prominent] superclusters in Bootes over 800 million light years away but this region of the sky is more famous for the large Boötes Void that lies next to them."[1]

The Boötes Supercluster is a super cluster of galaxies located in the direction of the Boötes constellation bordering the Super Corona of the Northern Crown with which it is probably connected by a filament of galaxies, and with the Void of Boötes, an area of the universe with a minimum concentration of galaxies (less than one hundred have been identified) with a diameter of about 300 million light years. In Boötes there are two concentrations of galaxy clusters called SCL 349 and SCL 351, placed respectively at 830 million and 1 billion light years from Earth.[2][3]

Some clusters of galaxies from Boötes Supercluster
Name of cluster (Abell) R.A. Dec. Redshift (z) Distance (million light years) Richness class
Abell 1775 13h 41m 9s +26° 22′ ″ 0,0705 950 2
Abell 1781 13h 4m 5s +29° 51′ ″ 0,0606 820 0
Abell 1795 13h 49m 0s +26° 35′ ″ 0,0619 840 2
Abell 1800 13h 49m 7s +28° 04′ ″ 0,0743 1000 0
Abell 1825 13h 58m 0s +20° 39′ ″ 0,0583 790 0
Abell 1827 13h 58m 2s +21° 42′ ″ 0,0642 870 1
Abell 1828 13h 58m 4s +18° 23′ ″ 0,0611 840 1
Abell 1831 13h 59m 2s +27° 59′ ″ 0,0603 815 1
Abell 1861 14h 07m 5s +27° 49′ ″ uncertain uncertain 1
Abell 1873 14h 11m 7s +28° 09′ ″ 0,0764 1025 0
Abell 1898 14h 20m 6s +25° 09′ ″ 0,0762 1025 1

High-velocity galaxies[edit]

The irregular galaxy NGC 1427A is passing through the Fornax cluster at nearly 600 kilometers per second (400 miles per second). Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).

"The irregular galaxy NGC 1427A is a spectacular example of the resulting stellar rumble. Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometers per second or nearly 400 miles per second."[4]

"Galaxy clusters, like the Fornax cluster, contain hundreds or even thousands of individual galaxies. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A, which give the galaxy an overall arrowhead shape that appears to point in the direction of the galaxy's high-velocity motion."[4]

Active galactic nuclei[edit]

The image shows NGC 1700, the Elliptical Galaxy and Rotating Disk by the Chandra X-ray Observatory. Credit: Thomas S. Statler, Brian R. McNamara.{{free media}}

NGC 1700 is an elliptical galaxy of the Hubble type E4 pec in the constellation Eridanus south of the ecliptic. The galaxy has an angular extent of 3.3 '× 2.1', an apparent brightness of +11.2 mag. It is about 180 million light years away from the solar system and has a diameter of about 215,000 light years.[5] It has an active galactic nucleus.


This shows the Milky Way edge on with the Large Magellanic Cloud. Credit: ESO/S. Brunier.{{free media}}

Two clouds occur in the image above: the Large Magellanic Cloud, second lowest object on the right below the galaxy, and the lowest diffuse object on the right, the Small Magellanic Cloud. Passing your cursor over image on Commons finds these two clouds.

Cosmic rays[edit]

The flux of cosmic-ray particles is a function of their energy. Credit: Sven Lafebre, after Swordy.[6]
Lots of cosmic rays in this one had to be removed manually even after running automatic noise removal over it. Credit: Judy Schmidt.{{free media}}

At right is an image indicating the range of cosmic-ray energies. The flux for the lowest energies (yellow zone) is mainly attributed to solar cosmic rays, intermediate energies (blue) to galactic cosmic rays, and highest energies (purple) to extragalactic cosmic rays.[6]

There is "a correlation between the arrival directions of cosmic rays with energy above 6 x 1019 electron volts and the positions of active galactic nuclei (AGN) lying within ~75 megaparsecs."[7]

The "propagation of relativistic electrons is sufficiently well constrained that the observed distribution may have direct bearing on the distribution of cosmic rays. Observations near 5 GHz trace cosmic ray electrons that propagate 1 to 3 kpc from their sources before losing their energy. Supernova remnants are plausible candidates for cosmic ray production given their expected energy outputs and their surface number densities in galactic disks."[8]

"The sample of galaxies [includes] NGC 5005 [shown on the left]."[8]


The secondary antiprotons (p) then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium. The antiproton cosmic ray energy spectrum is now measured reliably and is consistent with this standard picture of antiproton production by cosmic ray collisions.[9]


As of December 5, 2011, "Voyager 1 is about ... 18 billion kilometers ... from the [S]un [but] the direction of the magnetic field lines has not changed, indicating Voyager is still within the heliosphere ... the outward speed of the solar wind had diminished to zero in April 2010 ... inward pressure from interstellar space is compacting [the magnetic field] ... Voyager has detected a 100-fold increase in the intensity of high-energy electrons from elsewhere in the galaxy diffusing into our solar system from outside ... [while] the [solar] wind even blows back at us."[10]


This is a Chandra X-ray observatory image of NGC 5813. Credit: NASA.
Chandra X-ray Observatory image is NGC 5846 with superimposed contours of Hα+[N ii] emission. Credit: .

NGC 5846 is the foremost galaxy of the large galaxy group known as the NGC 5846 group which includes NGC 5813, NGC 5831, NGC 5845, and NGC 5854.[11] The group has two subgroups, one centered around the elliptical NGC 5813 and the other around NGC 5846, lying at a projected separation of 600 kpc.[12] The group is part of the Virgo III Groups, a very obvious chain of galaxy groups on the left side of the Virgo cluster, stretching across 40 million light years of space.[13]

In the image on the left of NGC 5846, white crosses mark the detected CO cloud positions.[14]

The galaxy has complex X-ray morphology[15] that is considered to be the result of AGN outflows. Two inner bubbles in the hot gas, at a distance of 600 pc from the center and filled with radio emission, are clear indications of recent AGN feedback. A weak radio source, elongated in the NE–SW direction, connects the inner cavities. X-ray-bright rims surround the inner X-ray bubbles.[16] Many X-ray knots are visible, suggesting cooling sites. The scenario indicated by the Chandra observation is that of an AGN outflow, compressing and cooling the gas[17] in the central ~2 kpc (20" at the distance of NGC 5846).[14]


This beautiful galaxy is M81, or NGC 3031. Credit: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data: NASA/JPL/Caltech/S. Willner (Harvard-Smithsonian Center for Astrophysics.

M81 is the beautiful galaxy tilted at an oblique angle on to our line of sight, giving a "birds-eye view" of the spiral structure, in the image on the right.

Messier 81 aka NGC 3031 or Bode's Galaxy is a spiral galaxy about 12 million light-years away, with a diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major.[18]

Close to Earth, M81 has a large size, and an active galactic nucleus (which harbors a 70 million M[19] supermassive black hole. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers.[20]

Messier 81 is located approximately 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group.[20][21]

Messier 81 and Messier 82 can both be viewed easily using binoculars and small telescopes.[20][21] The two objects are generally not observable to the unaided eye, although highly experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a very dark sky.[20] Telescopes with apertures of 8 inches (20 cm) or larger are needed to distinguish structures in the galaxy.[21]


The telescope photograph of the Great Andromeda Nebula is taken around 1899. Credit: Isaac Roberts.{{free media}}
NGC 5846 (right) and NGC 5850 (left) are by the Schulman Telescope at Mount Lemmon SkyCenter. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona.{{free media}}

Still much further away from the Earth than the Sun or Neptune are the many stars and nebulae that make up the Milky Way. Beyond the confines of our galaxy is the Andromeda Galaxy shown at the top of the page.

Of the Local Group, “[i]ts two dominant galaxies, the Milky Way and Andromeda (M31), are separated by a distance of ~700 kpc and are moving toward each other with a radial velocity of about -117 km s-1 (Binney & Tremaine 1987, p. 605).”[22] making Andromeda one of the few blueshifted galaxies. The Andromeda Galaxy and the Milky Way are thus expected to collide in about 4.5 billion years, although the details are uncertain since Andromeda's tangential velocity with respect to the Milky Way is only known to within about a factor of two.[23] A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy.[24] Such events are frequent among the galaxies in galaxy groups. The fate of the Earth and the Solar System in the event of a collision are currently unknown. If the galaxies do not merge, there is a small chance that the Solar System could be ejected from the Milky Way or join Andromeda.[25]

NGC 5846 is an elliptical galaxy (type E0-1[26]) located in the constellation Virgo at a distance of 93 ± 32 Mly (28.5 ± 9.8 Mpc)[26] from Earth, which, given its apparent dimensions, means that NGC 5846 is about 110,000 light years across. It lies near 110 Virginis and is part of the Herschel 400 Catalogue.[27]

NGC 5846 is a giant elliptical galaxy with a round shape and a low luminosity active galactic nucleus, whose categorisation is ambiguous, having features that are observed both in LINER and HII regions.[28] NGC 5846 apparently harbors a supermassive black hole with estimated mass 1.1±0.1×109
based on the central velocity dispersion.[16][29]

NGC 5846 harbors a large number of globular clusters; over 1,200 have been detected in images by Hubble Space Telescope.[30] The specific frequency of occurrence is similar to other elliptical galaxies in groups as is the metallicity with bimodial distribution, roughly of [Fe/H]=-1.2 and -0.2.[31] Their typical effective radii are in the range of 3 - 5 pc, with the largest clusters located in the central regions; seven of the globular clusters have X-ray counterparts, which are among the most luminous X-ray sources in NGC 5846, and they are mostly in the central region, optically luminous, compact and belong to the red subpopulation.[32]


Messier 106 is one of the brightest and nearest spiral galaxies to our own. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), and R. Gendler (for the Hubble Heritage Team).{{free media}}

Messier 106 (NGC 4258) is an intermediate spiral galaxy in the constellation Canes Venatici at a distance of about 22 to 25 million light-years, contains an active nucleus classified as a Type 2 Seyfert, and the presence of a central supermassive black hole demonstrated from radio astronomy observations of the rotation of an accretion disk of molecular gas orbiting within the inner light-year around the black hole.[33]

M106 has a water vapor megamaser (the equivalent of a laser operating in microwave instead of visible light and on a galactic scale) that is seen by the 22-GHz line of ortho-H2O that evidences dense and warm molecular gas that give M106 its characteristic purple color.[34] Water masers are useful to observe nuclear accretion disks in active galaxies, enabling the first case of a direct measurement of the distance to a galaxy, thereby providing an independent anchor for the cosmic distance ladder.[35][36] M106 has a slightly warped, thin, almost edge-on Keplerian disc which is on a subparsec scale that surrounds a central area with mass 4 × 107 M.[37]

It is one of the largest and brightest nearby galaxies, similar in size and luminosity to the Andromeda Galaxy.[38] The supermassive black hole at the core has a mass of 39,000,000±0.1 solar mass.[39]

M106 has also played an important role in calibrating the cosmic distance ladder: Cepheid variables from other galaxies could not be used to measure distances since they cover ranges of metallicities different from the Milky Way's, but M106 contains Cepheid variables similar to both the metallicities of the Milky Way and other galaxies' Cepheids, by measuring the distance of the Cepheids with metallicities similar to our galaxy, recalibration of the other Cepheids with different metallicities, a key fundamental step in improving quantification of distances to other galaxies in the universe, was possible.[40]


The Pinwheel Galaxy, a.k.a. Messier 101 or NGC 5457, is a face-on spiral galaxy about 27 million light-years away in the constellation Ursa Major. Credit: Jim Keller.{{free media}}
This is one of the largest and most detailed photo of a spiral galaxy NGC 5457 that has been released from Hubble. Credit: European Space Agency & NASA.{{free media}}

For this image on the right, the DSS red channel was mapped as red, and the DSS blue channel was mapped as cyan. North is to the left, and the field of view is approximately 43x43 arcminutes.

The Pinwheel Galaxy is a face-on spiral galaxy distanced 21 million light-years (six megaparsecs)[41] away from Earth in the constellation Ursa Major.

M101 is a large galaxy, with a diameter of ~170,000 ly in diameter[42] It has around a trillion stars, twice the number in the Milky Way.[43] It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.[44]

M101 has a high population of H II regions, many are very large and bright, usually accompanying the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form, are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hot superbubbles.[45] In a 1990 study, 1264 H II regions were cataloged in the galaxy.[46] Three are prominent enough to receive New General Catalogue numbers - NGC 5461, NGC 5462, and NGC 5471.[47]

M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress interstellar hydrogen gas, which then triggers strong star formation activity in M101's spiral arms that can be detected in ultraviolet images.[48]

M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV.[49] As stated above, the gravitational interaction between M101 and its satellites may have triggered the formation of the grand design pattern in M101. M101 has also probably distorted the companion galaxy NGC 5474.[49] M101 and its companion galaxies comprise most or possibly all of the M101 Group.[50][51][52][53]


The galaxy lies 13 million light-years away in the southern constellation Circinus. Credit: NASA, Andrew S. Wilson (University of Maryland); Patrick L. Shopbell (Caltech); Chris Simpson (Subaru Telescope); Thaisa Storchi-Bergmann and F. K. B. Barbosa (UFRGS, Brazil); and Martin J. Ward (University of Leicester, U.K.).{{free media}}

"Resembling a swirling witch's cauldron of glowing vapors, the black hole-powered core of a nearby active galaxy appears in this colorful NASA Hubble Space Telescope image. The galaxy lies 13 million light-years away in the southern constellation Circinus."[54]

"This galaxy is designated a type 2 Seyfert, a class of mostly spiral galaxies that have compact centers and are believed to contain massive black holes. Seyfert galaxies are themselves part of a larger class of objects called Active Galactic Nuclei or AGN. AGN have the ability to remove gas from the centers of their galaxies by blowing it out into space at phenomenal speeds. Astronomers studying the Circinus galaxy are seeing evidence of a powerful AGN at the center of this galaxy as well."[54]

"Much of the gas in the disk of the Circinus spiral is concentrated in two specific rings - a larger one of diameter 1,300 light-years, which has already been observed by ground-based telescopes, and a previously unseen ring of diameter 260 light-years."[54]

"In the Hubble image, the smaller inner ring is located on the inside of the green disk. The larger outer ring extends off the image and is in the plane of the galaxy's disk. Both rings are home to large amounts of gas and dust as well as areas of major "starburst" activity, where new stars are rapidly forming on timescales of 40 - 150 million years, much shorter than the age of the entire galaxy."[54]

"At the center of the starburst rings is the Seyfert nucleus, the believed signature of a supermassive black hole that is accreting surrounding gas and dust. The black hole and its accretion disk are expelling gas out of the galaxy's disk and into its halo (the region above and below the disk). The detailed structure of this gas is seen as magenta-colored streamers extending towards the top of the image."[54]

"In the center of the galaxy and within the inner starburst ring is a V-shaped structure of gas. The structure appears whitish-pink in this composite image, made up of four filters. Two filters capture the narrow lines from atomic transitions in oxygen and hydrogen; two wider filters detect green and near-infrared light. In the narrow-band filters, the V-shaped structure is very pronounced. This region, which is the projection of a three-dimensional cone extending from the nucleus to the galaxy's halo, contains gas that has been heated by radiation emitted by the accreting black hole. A "counter-cone," believed to be present, is obscured from view by dust in the galaxy's disk. Ultraviolet radiation emerging from the central source excites nearby gas causing it to glow. The excited gas is beamed into the oppositely directed cones like two giant searchlights."[54]

"Located near the plane of our own Milky Way Galaxy, the Circinus galaxy is partially hidden by intervening dust along our line of sight. As a result, the galaxy went unnoticed until about 25 years ago. This Hubble image was taken on April 10, 1999 with the Wide Field Planetary Camera 2."[54]


This image shows a cluster of yellow galaxies near the middle of the photograph. Credit: W.N. Colley and E. Turner (Princeton University), J.A. Tyson (Bell Labs, Lucent Technologies) and STScl/NASA.

"This Hubble Space Telescope image [at right] shows several blue, loop-shaped objects that actually are multiple images of the same galaxy. They have been duplicated by the gravitational lens of the cluster of yellow, elliptical and spiral galaxies - called 0024+1654 - near the photograph's center. The gravitational lens is produced by the cluster's tremendous gravitational field that bends light to magnify, brighten and distort the image of a more distant object. How distorted the image becomes and how many copies are made depends on the alignment between the foreground cluster and the more distant galaxy, which is behind the cluster."[55]

"In this photograph, light from the distant galaxy bends as it passes through the cluster, dividing the galaxy into five separate images. One image is near the center of the photograph; the others are at 6, 7, 8, and 2 o'clock. The light also has distorted the galaxy's image from a normal spiral shape into a more arc-shaped object. Astronomers are certain the blue-shaped objects are copies of the same galaxy because the shapes are similar. The cluster is 5 billion light-years away in the constellation Pisces, and the blue-shaped galaxy is about 2 times farther away."[55]

"Though the gravitational light-bending process is not new, Hubble's high resolution image reveals structures within the blue-shaped galaxy that astronomers have never seen before. Some of the structures are as small as 300 light-years across. The bits of white imbedded in the blue galaxy represent young stars; the dark core inside the ring is dust, the material used to make stars. This information, together with the blue color and unusual "lumpy" appearance, suggests a young, star-making galaxy."[55]

"The picture was taken October 14, 1994 with the Wide Field Planetary Camera-2. Separate exposures in blue and red wavelengths were taken to construct this color picture."[55]


The sky image of NGC 5831 is obtained by the Sloan Digital Sky Survey, DR14 with SciServer. Credit: Sloan Digital Sky Survey.{{free media}}
Response of SDSS filters are for the imaging camera. Credit: Mamoru Doi, Masayuki Tanaka, Masataka Fukugita, James E. Gunn, Naoki Yasuda, Zeljko Ivezic, Jon Brinkmann, Ernst de Haars, Scott J. Kleinman, Jurek Krzesinski, and R. French Leger.{{free media}}
The image shows the arrangement of the CCDs and filters on the SDSS-III camera. Credit: Mamoru Doi, Masayuki Tanaka, Masataka Fukugita, James E. Gunn, Naoki Yasuda, Zeljko Ivezic, Jon Brinkmann, Ernst de Haars, Scott J. Kleinman, Jurek Krzesinski, and R. French Leger.{{free media}}

The SDSS imaging camera collects photometric imaging data using an array of 30 SITe/Tektronix 2048 by 2048 pixel CCDs arranged in six columns of five CCDs each, aligned with the pixel columns of the CCDs themselves.[56]

SDSS-III used the same filter system as the original SDSS with central wavelengths of the six filters:

  1. u 3551 Å (ultraviolet)
  2. g 4686 Å (green)
  3. r 6166 Å (red)
  4. i 7480 Å (infrared)
  5. z 8932 mÅ (Z band).[56]


This is an image of H 0323+022 using the red (R) filter. Credit: Renato Falomo, ESO NTT.

A BL Lacertae object or BL Lac object is a type of active galaxy with an active galactic nucleus (AGN) and is named after its prototype, BL Lacertae. In contrast to other types of active galactic nuclei, BL Lacs are characterized by rapid and large-amplitude flux variability and significant optical polarization.

All known BL Lacs are associated with core dominated radio sources, many of them exhibiting superluminal motion.

QSO B0323+022 is a BL Lacertae object. The image at right is taken with the ESO New Technology Telescope (NTT) using the R filter.


This is a three-color far-infrared image of M51, the Whirlpool Galaxy. Credit: ESA and the PACS Consortium.{{free media}}
NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope joined forces to create this striking composite image of Messier 104. Credit: NASA/JPL-Caltech and The Hubble Heritage Team (STScI/AURA).{{free media}}

Huge, cold clouds of gas and dust in our own galaxy, as well as in nearby galaxies, glow in far-infrared light. This is due to thermal radiation of interstellar dust contained in molecular clouds.

"One of the most interesting discoveries made by IRAS was of galaxies with far-infrared luminosities of 1011 - 1012 L, and LIR/LB ~ 10 - 100 (Soifer et al. 1984). [...] Only a few examples of this type of object, namely Arp 220, NGC 3690, Mrk 231, and NGC 6240, have ever been studied from the ground in detail."[57]

"Three-color far-infrared image of M51, the Whirlpool Galaxy." includes "Red, green and blue correspond to the 160-micron, 100-micron and 70-micron wavelength bands of the Herschels Photoconductor Array Camera and Spectrometer, PACS instruments."[58]

"Glowing light from clouds of dust and gas around and between the stars is visible clearly. These clouds are a reservoir of raw material for ongoing star formation in this galaxy. Blue indicates regions of warm dust that is heated by young stars, while the colder dust shows up in red."[58]

In the image on the left, "NASA/ESA Hubble Space Telescope and NASA's Spitzer Space Telescope joined forces to create this striking composite image of one of the most popular sights in the universe. Messier 104 is commonly known as the Sombrero galaxy because in visible light, it resembles the broad-brimmed Mexican hat. However, in Spitzer's striking infrared view, the galaxy looks more like a "bull's eye"."[59]

"Spitzer's full view shows the disk is warped, which is often the result of a gravitational encounter with another galaxy, and clumpy areas spotted in the far edges of the ring indicate young star-forming regions."[59]

"The Sombrero galaxy is located some 28 million light-years away. Viewed from Earth, it is just six degrees south of its equatorial plane. Spitzer detected infrared emission not only from the ring, but from the center of the galaxy too, where there is a huge black hole, believed to be a billion times more massive than our Sun."[59]

"The Spitzer picture is composed of four images taken at 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) microns. The contribution from starlight (measured at 3.6 microns) has been subtracted from the 5.8 and 8-micron images to enhance the visibility of the dust features."[59]

"The Spitzer picture [of] four images [was] taken [in] June 2004 and January 2005."[60]


The ALMA observations — shown here in red, pink and yellow — were tuned to detect carbon monoxide molecules. Credit: ALMA (ESO/NAOJ/NRAO). Visible light image: the NASA/ESA Hubble Space Telescope.

Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre." and "Using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of star formation from earliest collapse to stellar birth.

These wavelengths are sometimes called Terahertz radiation, since they have frequencies of the order of 1 THz.

"The Antennae Galaxies (also known as NGC 4038 and 4039) are a pair of distorted colliding spiral galaxies about 70 million light-years away, in the constellation of Corvus (The Crow). This view combines ALMA observations, made in two different wavelength ranges during the observatory’s early testing phase, with visible-light observations from the NASA/ESA Hubble Space Telescope."[61]

"The Hubble image is the sharpest view of this object ever taken and serves as the ultimate benchmark in terms of resolution. ALMA observes at much longer wavelengths which makes it much harder to obtain comparably sharp images. However, when the full ALMA array is completed its vision will be up to ten times sharper than Hubble."[61]


The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). Credit: NASA Hubble Space Telescope.{{free media}}

"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[62]

"Phenomena across the universe emit radiation spanning the entire electromagnetic spectrum — from high-energy gamma rays, which stream out from the most energetic events in the cosmos, to lower-energy microwaves and radio waves."[63]

"Microwaves, the very same radiation that can heat up your dinner, are produced by a multitude of astrophysical sources, including strong emitters known as masers (microwave lasers), even stronger emitters with the somewhat villainous name of megamasers, and the centers of some galaxies. Especially intense and luminous galactic centers are known as active galactic nuclei. They are in turn thought to be driven by the presence of supermassive black holes, which drag surrounding material inwards and spit out bright jets and radiation as they do so."[63]

"The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). MCG+01-38-005 is a special kind of megamaser; the galaxy’s active galactic nucleus pumps out huge amounts of energy, which stimulates clouds of surrounding water. Water’s constituent atoms of hydrogen and oxygen are able to absorb some of this energy and re-emit it at specific wavelengths, one of which falls within the microwave regime. MCG+01-38-005 is thus known as a water megamaser!"[63]

"Astronomers can use such objects to probe the fundamental properties of the universe. The microwave emissions from MCG+01-38-005 were used to calculate a refined value for the Hubble constant, a measure of how fast the universe is expanding. This constant is named after the astronomer whose observations were responsible for the discovery of the expanding universe and after whom the Hubble Space Telescope was named, Edwin Hubble."[63]


Image is in radio (pink) and X-Ray (cyan) of 3C 75. Credit: X-Ray: NASA / CXC / D.Hudson, T.Reiprich et al. (AIfA); Radio: NRAO / VLA/ NRL.{{free media}}

3C 75 may be X-ray source 2A 0252+060 (1H 0253+058, XRS 02522+060).[64]

"What's happening at the center of active galaxy 3C 75? The two bright sources at the center of this composite x-ray (blue)/ radio (pink) image are co-orbiting supermassive black holes powering the giant radio source 3C 75. Surrounded by multimillion degree x-ray emitting gas, and blasting out jets of relativistic particles the supermassive black holes are separated by 25,000 light-years. At the cores of two merging galaxies in the Abell 400 galaxy cluster they are some 300 million light-years away."[65]


Centaurus A in X-rays shows the relativistic jet. Credit: NASA.

"The structure of relativistic jets in [active galactic nuclei] AGN on scales of light days reveals how energy propagates through jets, a process that is fundamental to galaxy evolution."[66]

Their lengths can reach several thousand[67] or even hundreds of thousands of light years.[68] The hypothesis is that the twisting of magnetic fields in the accretion disk collimates the outflow along the rotation axis of the central object, so that when conditions are suitable, a jet will emerge from each face of the accretion disk. If the jet is oriented along the line of sight to Earth, relativistic beaming will change its apparent brightness. The mechanics behind both the creation of the jets[69][70] and the composition of the jets[71] are still a matter of much debate in the scientific community; it is hypothesized that the jets are composed of an electrically neutral mixture of electrons, positrons, and protons in some proportion.

A relativistic jet emitted from the AGN of M87 is traveling at speeds between four and six times the speed of light.[67]

"The term 'superluminal motion' is something of a misnomer. While it accurately describes the speeds measured, scientists still believe the actual speed falls just below the speed of light."[67]

"It's an illusion created by the finite speed of light and rapid motion".[67]

"Our present understanding is that this 'superluminal motion' occurs when these clouds move towards Earth at speeds very close to that of light, in this case, more than 98 percent of the speed of light. At these speeds the clouds nearly keep pace with the light they emit as they move towards Earth, so when the light finally reaches us, the motion appears much more rapid than the speed of light. Since the moving clouds travel slightly slower than the speed of light, they do not actually violate Einstein's theory of relativity which sets light as the speed limit."[67]


The pseudo-colour image is of the large-scale radio structure of the FRII radio galaxy 3C98. Lobes, jet and hotspot are labelled. Credit: .
Another pseudo-colour image is of the large-scale radio structure of the FRI radio galaxy 3C31. Jets and plumes are labelled. Credit: .

"Over the past 30 years, radioastronomy has revealed a rich variety of molecular species in the interstellar medium of our galaxy and even others."[72]

These regions are non-luminous, save for emission of the 21-cm (1,420 MHz) region spectral line. ... Mapping H I emissions with a radio telescope is a technique used for determining the structure of spiral galaxies.

"In 1974, radio sources were divided into two classes Fanaroff and Riley Class I (FRI), and Class II (FRII).[73]

The distinction was originally made based on the morphology of the large-scale radio emission (the type was determined by the distance between the brightest points in the radio emission): FRI sources were brightest towards the centre, while FRII sources were brightest at the edges.

There is a reasonably sharp divide in luminosity between the two classes: FRIs were low-luminosity, FRIIs were high luminosity.[73]

The morphology turns out to reflect the method of energy transport in the radio source. FRI objects typically have bright jets in the centre, while FRIIs have faint jets but bright hotspots at the ends of the lobes. FRIIs appear to be able to transport energy efficiently to the ends of the lobes, while FRI beams are inefficient in the sense that they radiate a significant amount of their energy away as they travel.

The FRI/FRII division depends on host-galaxy environment in the sense that the FRI/FRII transition appears at higher luminosities in more massive galaxies.[74] FRI jets are known to be decelerating in the regions in which their radio emission is brightest,[75]

The hotspots that are usually seen in FRII sources are interpreted as being the visible manifestations of shocks formed when the fast, and therefore supersonic, jet (the speed of sound cannot exceed c/√3) abruptly terminates at the end of the source, and their spectral energy distributions are consistent with this picture.[76]

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