Galactic ridge

The galactic ridge is a region of the inner galaxy that is coincident with the galactic plane of the Milky Way.[1] It can be seen from Earth as a band of stars which is interrupted by 'dust lanes'. In these 'dust lanes' the dust in the gaseous galactic disk (or plane) blocks the visible light of the background stars. Due to this, many of the most interesting features of the Milky Way can only be viewed in X-rays. Along with the point X-ray sources which populate the Milky Way, an apparently diffuse X-ray emission concentrated in the galactic plane is also observed. This is known as the galactic ridge X-ray emission (GRXE). These emissions were originally discovered by Diana Worrall and collaborators in 1982, and since then the origins of these emissions have puzzled astrophysicists around the globe.

It was initially believed, due to the difficulty of resolving the GRXE into point sources, that the x-ray emissions were truly diffuse in nature and that their origin might be a Galactic plasma rather than distant stellar sources.[2] It was thought to be caused by low energy cosmic rays interacting with cold gas in the region, which heated up the gas and caused it to emit X-rays.[1] However it was discovered that the temperature of a gas producing such an emission would have to be around tens of millions of degrees. This temperature is far too high for a gas to be bound gravitationally to the galaxy. Therefore, it was proposed that the GRXE might be caused by a large number of extremely remote and outlying stars. In 2009, after decades of attempting to resolve the GRXE, Mikhail Revnivtsev, his partner Sazonov and their colleges managed to resolve approximately 80% of the emissions over the course of 12 days using the Chandra X-ray observatory.[2] During this time period a total of 473 sources of x-ray emission were detected in an area that is significantly smaller than the size of a Full Moon. This is one of the highest densities of x-ray sources ever seen in our Galaxy.[3] Due to this amazing discovery it is now thought that about 80% of the emission comes from discrete sources such as white dwarfs and stars with active coronae.[4]

However, recent work by researchers at the Max Planck Institute for Astrophysics suggests that the GRXE may indeed consist of an additional, diffuse component after all. This diffuse component could arise not from the thermal emission of a very hot plasma but from the reprocessing by the interstellar gas of the X-ray radiation produced by luminous X-ray binary sources located in the Galaxy. X-ray binaries are the most luminous sources of X-rays in galaxies such as the Milky Way. These binary systems emit X-ray radiation when material or substance from a so-called donor star falls into the strong gravitational field of a compact object, such as a neutron star or a black hole. This X-ray radiation illuminates the atoms and molecules in the Galactic interstellar gas, which then scatter the incoming photons in different directions and at different energies. The resulting emission appears truly diffuse to the viewer.[5]

The Galactic Ridge has a width of 5° latitude (b) and ±40° longitude (l) in the Galactic coordinate system.[6]

The first instrument that was able to measure diffuse X-ray emission was the HEAO A2 (High Energy Astrophysical Observatory). However it was created to study the large-scale structure of the galaxy and the universe, and to yield high-quality spatial and spectral data in the X-ray region. Still, the HEAO A2 produced valuable information on discrete X-ray sources such as binary star systems, hot white dwarfs, cataclysmic variables and supernova remnants. The HEAO A2 also allowed for the study of extragalactic objects, for example radio galaxies, Seyfert galaxies, and quasars. Elihu Boldt was the principal investigator of the HEAO A2 instrument, however he worked alongside G. Gamire on the project. The HEAO A2 was launched into space in 1977, where its job was to scan the sky for approximately 17 months. It (the HEAO A2) produced the first low-background, all-sky maps in the 2-60 keV band, and for its time the HEAO A2 produced the best spectra ever obtained over 2-60 keV energy range.[7]


  1. ^ a b Bhat, C. L.; Kifune, T.; Wolfendale, A. W. (November 21, 1985). "A cosmic-ray explanation of the galactic ridge of cosmic X-rays". Nature. 318 (6043): 267–269. Bibcode:1985Natur.318..267B. doi:10.1038/318267a0.
  2. ^ a b Molaro, Margherita; Khatri, Rishi; Sunyaev, Rashid. "New Light on the Origin of the Galactic Ridge X-ray Emission". Max-Planck-Gesellschaft. Retrieved January 14, 2015.
  3. ^ "Galactic Ridge X-Ray Emission Has Millions of Sources". Science 2.0. Retrieved January 19, 2015.
  4. ^ Revnivtsev, M.; Sazonov, S.; Churazov, E.; Forman, W.; Vikhlinin, A.; Sunyaev, R. (April 2009). "Discrete sources as the origin of the Galactic X-ray ridge emission". Nature. 458 (7242): 1142–1144. arXiv:0904.4649. Bibcode:2009Natur.458.1142R. doi:10.1038/nature07946. PMID 19407795.
  5. ^ Molaro, Margherita; Khatri, Rishi; Sunyaev, Rashid. "New Light on the Origin of the Galactic Ridge X-ray Emission". Max-Plank-Gesellschaft. Retrieved January 19, 2015.
  6. ^ Gehrels, N. (August 26–30, 1996). "Gamma Ray Sky Surveys". In Brian J. McLean, Daniel A. Golombek, Jeffrey J. E. Hayes, Harry E. Payne. New horizons from multi-wavelength sky surveys: proceedings of the 179th Symposium of the International Astronomical Union. Baltimore, U.S.A.: Kluwer Academic Publishers. pp. 69–. Bibcode:1998IAUS..179...69G.CS1 maint: Uses editors parameter (link)
  7. ^ Allen, J.; Jahoda, K.; Whitlock, L. "HEAO1 and the A2 Experiment". Nasa's HEASARC. Retrieved January 11, 2015.
Coma Filament

Coma Filament is a galaxy filament. The filament contains the Coma Supercluster of galaxies and forms a part of the CfA2 Great Wall.

Ginga (satellite)

ASTRO-C, renamed Ginga (Japanese for 'galaxy'), was an X-ray astronomy satellite launched from the Kagoshima Space Center on 5 February 1987 using M-3SII launch vehicle. The primary instrument for observations was the Large Area Counter (LAC). Ginga was the third Japanese X-ray astronomy mission, following Hakucho and Tenma. Ginga reentered the Earth's atmosphere on 1 November 1991.

Lynx–Ursa Major Filament

Lynx–Ursa Major Filament (LUM Filament) is a galaxy filament.The filament is connected to and separate from the Lynx–Ursa Major Supercluster.

Milky Way

The Milky Way is the galaxy that contains our Solar System. The name describes the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλαξίας κύκλος (galaxías kýklos, "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.

The Milky Way is a barred spiral galaxy with a diameter between 150,000 and 200,000 light-years (ly). It is estimated to contain 100–400 billion stars and more than 100 billion planets. The Solar System is located at a radius of 26,490 (± 100) light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust. The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, assumed to be a supermassive black hole of 4.100 (± 0.034) million solar masses.

Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed "dark matter". The rotational period is about 240 million years at the radius of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.

Neil Gehrels

Cornelis A. "Neil" Gehrels (October 3, 1952 – February 6, 2017) was an American astrophysicist specializing in the field of gamma-ray astronomy. He was Chief of the Astroparticle Physics Laboratory at NASA's Goddard Space Flight Center from 1995 until his death, and was best known for his work developing the field from early balloon instruments to today's space observatories such as the NASA Swift mission, for which he was the Principal Investigator. He was leading the WFIRST wide-field infrared telescope forward toward a launch in the mid-2020s. He was a member of the National Academy of Sciences and the American Academy of Arts and Sciences.

Gehrels died on February 6, 2017, at the age of 64. On January 10th, 2018, NASA announced that Swift had been renamed the Neil Gehrels Swift Observatory, in his honor..

Perseus–Pegasus Filament

Perseus–Pegasus Filament is a galaxy filament containing the Perseus-Pisces Supercluster and stretching for roughly a billion light years (or over 300/h Mpc). Currently, it is considered to be one of the largest known structures in the universe. This filament is adjacent to the Pisces–Cetus Supercluster Complex.

Radio Galaxy Zoo

Radio Galaxy Zoo (RGZ) is an internet crowdsourced citizen science project that seeks to locate supermassive black holes in distant galaxies. It is hosted by the web portal Zooniverse. The scientific team want to identify black hole/jet pairs and associate them with the host galaxies. Using a large number of classifications provided by citizen scientists they hope to build a more complete picture of black holes at various stages and their origin. It was initiated in 2010 by Ray Norris in collaboration with the Zooniverse team, and was driven by the need to cross-identify the millions of extragalactic radio sources that will be discovered by the forthcoming Evolutionary Map of the Universe survey. RGZ is now led by scientists Julie Banfield and Ivy Wong. RGZ started operations on 17 December 2013.


Tenma, known as ASTRO-B before launch, was Japan's second X-ray astronomy satellite, developed by the Institute of Space and Astronautical Science. It was launched on February 20, 1983 using a M-3S-3 rocket as the vehicle.

Battery failure in July 1984 caused the operation to become limited, and continuing problems lead to the termination of X-ray observation in 1985. It reentered the atmosphere on January 19, 1989.

Ursa Major Filament

Ursa Major Filament is a galaxy filament. The filament is connected to the CfA Homunculus, a portion of the filament forms a portion of the "leg" of the Homunculus.

Active nuclei
Energetic galaxies
Low activity
See also

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