Light echo

Last updated
Reflected light following path B arrives shortly after the direct flash following path A but before light following path C. B and C have the same apparent distance from the star as seen from Earth. Light Echo Corrected.png
Reflected light following path B arrives shortly after the direct flash following path A but before light following path C. B and C have the same apparent distance from the star as seen from Earth.

A light echo is a physical phenomenon caused by light reflected off surfaces distant from the source, and arriving at the observer with a delay relative to this distance. The phenomenon is analogous to an echo of sound, but due to the much faster speed of light, it mostly only manifests itself over astronomical distances.

Contents

For example, a light echo is produced when a sudden flash from a nova is reflected off a cosmic dust cloud, and arrives at the viewer after a longer duration than it otherwise would have taken with a direct path. Because of their geometries, light echoes can produce the illusion of superluminal motion. [1]

Explanation

The distance traveled from one focus to another, via some point on the ellipse, is the same regardless of the point selected. Ellipse Animation Small.gif
The distance traveled from one focus to another, via some point on the ellipse, is the same regardless of the point selected.
Direct light from a stellar outburst (white spot) reaches the observer (path 0) followed by light reflected off particles on progressively wider paraboloids (1-5): the observed disc apparently initially expands faster than light but the illusion is due to light reflecting off different unrelated particles (ANIMATION) Light echo principle.svg
Direct light from a stellar outburst (white spot) reaches the observer (path 0) followed by light reflected off particles on progressively wider paraboloids (1–5): the observed disc apparently initially expands faster than light but the illusion is due to light reflecting off different unrelated particles (ANIMATION)

Light echoes are produced when the initial flash from a rapidly brightening object such as a nova is reflected off intervening interstellar dust which may or may not be in the immediate vicinity of the source of the light. Light from the initial flash arrives at the viewer first, while light reflected from dust or other objects between the source and the viewer begins to arrive shortly afterward. Because this light has only travelled forward as well as away from the star, it produces the illusion of an echo expanding faster than the speed of light. [3]

In the first illustration above, light following path A is emitted from the original source and arrives at the observer first. Light which follows path B is reflected off a part of the gas cloud at a point between the source and the observer, and light following path C is reflected off a part of the gas cloud perpendicular to the direct path. Although light following paths B and C appear to come from the same point in the sky to the observer, B is actually significantly closer. As a result, the echo of the event in an evenly distributed (spherical) cloud for example will appear to the observer to expand at a rate approaching or faster than the speed of light, because the observer may assume the light from B is actually the light from C.

All reflected light rays that originate from the flash and arrive at Earth together will have traveled the same distance. When the rays of light are reflected, the possible paths between the source and Earth that arrive at the same time correspond to reflections on an ellipsoid, with the origin of the flash and Earth as its two foci (see animation to the right). This ellipsoid naturally expands over time.

Examples

V838 Monocerotis

Images showing the expansion of the light echo of V838 Monocerotis. Credit: NASA/ESA. V838 Monocerotis expansion.jpg
Images showing the expansion of the light echo of V838 Monocerotis. Credit: NASA/ESA.

The variable star V838 Monocerotis experienced a significant outburst in 2002 as observed by the Hubble Space Telescope. The outburst proved surprising to observers when the object appeared to expand at a rate far exceeding the speed of light as it grew from an apparent visual size of 4 to 7 light years in a matter of months. [3] [4]

Supernovae

Using light echoes, it is sometimes possible to see the faint reflections of historical supernovae. Astronomers calculate the ellipsoid which has Earth and a supernova remnant at its focal points to locate clouds of dust and gas at its boundary. Identification can be done using laborious comparisons of photos taken months or years apart, and spotting changes in the light rippling across the interstellar medium. By analyzing the spectra of reflected light, astronomers can discern chemical signatures of supernovae whose light reached Earth long before the invention of the telescope and compare the explosion with its remnants, which may be centuries or millennia old. The first recorded instance of such an echo was in 1936, but it was not studied in detail. [4]

An example is supernova SN 1987A, the closest supernova in modern times. Its light echoes have aided in mapping the morphology of the immediate vicinity [5] as well as in characterizing dust clouds lying further away but close to the line of sight from Earth. [6]

Another example is the SN 1572 supernova observed on Earth in 1572, where in 2008, faint light-echoes were seen on dust in the northern part of the Milky Way. [7] [8]

Light echoes have also been used to study the supernova that produced the supernova remnant Cassiopeia A. [7] The light from Cassiopeia A would have been visible on Earth around 1660, but went unnoticed, probably because dust obscured the direct view. Reflections from different directions allow astronomers to determine if a supernova was asymmetrical and shone more brightly in some directions than in others. The progenitor of Cassiopeia A has been suspected as being asymmetric, [9] and looking at the light echoes of Cassiopeia A allowed for the first detection of supernova asymmetry in 2010. [10]

Yet other examples are supernovae SN 1993J [11] and SN 2014J. [12]

Light echo from the 1838-1858 Great Eruption of Eta Carinae were used to study this supernova imposter. A study from 2012, which used light echo spectra from the Great Eruption, found that the eruption was colder compared to other supernova imposters. [13]

Cepheids

Light echoes from RS Puppis propagate through its reflection nebula.

Light echoes were used to determine the distance to the Cepheid variable RS Puppis to an accuracy of 1%. [14] Pierre Kervella at the European Southern Observatory described this measurement as so far "the most accurate distance to a Cepheid". [15]

Nova Persei 1901

In 1939, French astronomer Paul Couderc published a study entitled "Les Auréoles Lumineuses des Novae" (Luminous Haloes of the Novae). [16] Within this study, Couderc published the derivation of echo locations and time delays in the paraboloid, rather than ellipsoid, approximation of infinite distance. [16] However, in his 1961 study, Y.K. Gulak queried Couderc's theories: "It is shown that there is an essential error in the proof according to which Couderc assumed the possibility of expansion of the bright ring (nebula) around Nova Persei 1901 with a velocity exceeding that of light." [17] He continues: "The comparison of the formulas obtained by the author, with the conclusions and formulas of Couderc, shows that the coincidence of the parallax calculated according to Coudrec's scheme, with parallaxes derived by other methods, could have been accidental." [17]

ShaSS 622-073 system

An echo of ShaSS 073 galaxy's light detected by ESO's VLT Survey Telescope. An echo of light ShaSS 073, ShaSS 622, ShaSS 622-073 (cropped).jpg
An echo of ShaSS 073 galaxy's light detected by ESO's VLT Survey Telescope.

The ShaSS 622-073 system is composed of the larger galaxy ShaSS 073 (seen in yellow in the image on the right) and the smaller galaxy ShaSS 622 (seen in blue) that are at the very beginning of a merger. The bright core of ShaSS 073 has excited with its radiation a region of gas within the disc of ShaSS 622; even though the core has faded over the last 30,000 years, the region still glows brightly as it re-emits the light. [18]

Quasar light and ionisation echoes

A Hubble Space Telescope image of NGC 5972, a quasar ionisation echo. NGC 5972.jpg
A Hubble Space Telescope image of NGC 5972, a quasar ionisation echo.

Since 2009 objects known either as quasar light echoes or quasar ionisation echoes have been investigated. [19] [20] [21] [22] [23] [24] A well studied example of a quasar light echo is the object known as Hanny's Voorwerp (HsV). [25]

HsV is made entirely of gas so hot –about 10,000 degrees Celsius  –that astronomers felt it had to be illuminated by something powerful. [26] After several studies of light and ionisation echoes, it is thought they are likely caused by the 'echo' of a previously-active AGN that has shut down. Kevin Schawinski, a co-founder of the website Galaxy Zoo, stated: "We think that in the recent past the galaxy IC 2497 hosted an enormously bright quasar. Because of the vast scale of the galaxy and the Voorwerp, light from that past still lights up the nearby Voorwerp even though the quasar shut down sometime in the past 100,000 years, and the galaxy's black hole itself has gone quiet." [26] Chris Lintott, also a co-founder of Galaxy Zoo, stated: "From the point of view of the Voorwerp, the galaxy looks as bright as it would have before the black hole turned off –it's this light echo that has been frozen in time for us to observe." [26] The analysis of HsV in turn has led to the study of objects called Voorwerpjes and Green bean galaxies.

See also

Related Research Articles

<span class="mw-page-title-main">Supernova</span> Explosion of a star at its end of life

A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

<span class="mw-page-title-main">SN 1987A</span> 1987 supernova event in the constellation Dorado

SN 1987A was a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately 51.4 kiloparsecs from Earth and was the closest observed supernova since Kepler's Supernova. 1987A's light reached Earth on February 23, 1987, and as the earliest supernova discovered that year, was labeled "1987A". Its brightness peaked in May, with an apparent magnitude of about 3.

<span class="mw-page-title-main">Kepler's Supernova</span> Supernova visible from Earth in the 17th century

SN 1604, also known as Kepler's Supernova, Kepler's Nova or Kepler's Star, was a Type Ia supernova that occurred in the Milky Way, in the constellation Ophiuchus. Appearing in 1604, it is the most recent supernova in the Milky Way galaxy to have been unquestionably observed by the naked eye, occurring no farther than 6 kiloparsecs from Earth. Before the adoption of the current naming system for supernovae, it was named for Johannes Kepler, the German astronomer who described it in De Stella Nova.

<span class="mw-page-title-main">Lynx (constellation)</span> Constellation in the northern celestial hemisphere

Lynx is a constellation named after the animal, usually observed in the Northern Celestial Hemisphere. The constellation was introduced in the late 17th century by Johannes Hevelius. It is a faint constellation, with its brightest stars forming a zigzag line. The orange giant Alpha Lyncis is the brightest star in the constellation, and the semiregular variable star Y Lyncis is a target for amateur astronomers. Six star systems have been found to contain planets. Those of 6 Lyncis and HD 75898 were discovered by the Doppler method; those of XO-2, XO-4, XO-5 and WASP-13 were observed as they passed in front of the host star.

<span class="mw-page-title-main">Cassiopeia (constellation)</span> Constellation in the northern celestial hemisphere

Cassiopeia is a constellation and asterism in the northern sky named after the vain queen Cassiopeia, mother of Andromeda, in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, and it remains one of the 88 modern constellations today. It is easily recognizable due to its distinctive 'W' shape, formed by five bright stars.

<span class="mw-page-title-main">Centaurus A</span> Radio galaxy in the constellation Centaurus

Centaurus A is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.

<span class="mw-page-title-main">NGC 300</span> Galaxy in the constellation Sculptor

NGC 300 (also known as Caldwell 70) is a spiral galaxy in the constellation Sculptor. It is one of the closest galaxies to the Local Group, and probably lies between the latter and the Sculptor Group. It is the brightest of the five main spirals in the direction of the Sculptor Group. It is inclined at an angle of 42° when viewed from Earth and shares many characteristics of the Triangulum Galaxy. It is 94,000 light-years in diameter, somewhat smaller than the Milky Way, and has an estimated mass of (2.9 ± 0.2) × 1010M☉.

<span class="mw-page-title-main">Messier 81</span> Spiral galaxy in the constellation Ursa Major

Messier 81 (also known as NGC 3031 or Bode's Galaxy) is a grand design spiral galaxy about 12 million light-years away in the constellation Ursa Major. It has a D25 isophotal diameter of 29.44 kiloparsecs (96,000 light-years). Because of its relative proximity to the Milky Way galaxy, large size, and active galactic nucleus (which harbors a 70 million M☉ supermassive black hole), Messier 81 has been studied extensively by professional astronomers. The galaxy's large size and relatively high brightness also makes it a popular target for amateur astronomers. In late February 2022, astronomers reported that M81 may be the source of FRB 20200120E, a repeating fast radio burst.

<span class="mw-page-title-main">Messier 74</span> Face-on spiral galaxy in the constellation Pisces

Messier 74 is a large spiral galaxy in the equatorial constellation Pisces. It is about 32 million light-years away from Earth. The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a grand design spiral galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. Its relatively large angular size and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study spiral arm structure and spiral density waves. It is estimated that M74 hosts about 100 billion stars.

<span class="mw-page-title-main">Cassiopeia A</span> Supernova remnant in the constellation Cassiopeia

Cassiopeia A (Cas A) is a supernova remnant (SNR) in the constellation Cassiopeia and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. The supernova occurred approximately 11,000 light-years (3.4 kpc) away within the Milky Way; given the width of the Orion Arm, it lies in the next-nearest arm outwards, the Perseus Arm, about 30 degrees from the Galactic anticenter. The expanding cloud of material left over from the supernova now appears approximately 10 light-years (3 pc) across from Earth's perspective. It has been seen in wavelengths of visible light with amateur telescopes down to 234 mm (9.25 in) with filters.

<span class="mw-page-title-main">NGC 4261</span> Galaxy in the constellation Virgo

NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. It was discovered April 13, 1784 by the German-born astronomer William Herschel. The galaxy is a member of its own somewhat meager galaxy group known as the NGC 4261 group, which is part of the Virgo Cluster.

<span class="mw-page-title-main">Hanny's Voorwerp</span> Astronomical object appearing as a bright blob, discovered by Hanny van Arkel

Hanny's Voorwerp, is a rare type of astronomical object called a quasar ionization echo. It was discovered in 2007 by Dutch schoolteacher Hanny van Arkel while she was participating as a volunteer in the Galaxy Zoo project, part of the Zooniverse group of citizen science websites. Photographically, it appears as a bright blob close to spiral galaxy IC 2497 in the constellation Leo Minor.

<span class="mw-page-title-main">NGC 4666</span> Spiral galaxy in the constellation Virgo

NGC 4666 is a spiral galaxy in the equatorial constellation of Virgo, located at a distance of approximately 55 megalight-years from the Milky Way. It was discovered by the German-born astronomer William Herschel on February 22, 1784. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. John L. E. Dreyer described it as "bright, very large, much extended 45°±, pretty suddenly brighter middle". It is a member of an interacting system with NGC 4668 and a dwarf galaxy, and belongs to a small group that also includes NGC 4632.

<span class="mw-page-title-main">IC 2497</span> Spiral galaxy in the constellation Leo Minor

IC 2497 is a spiral galaxy close to the intergalactic cloud Hanny's Voorwerp. The galaxy is a former quasar, whose light lit up Hanny's Voorwerp, which is now a light echo of that extinct quasar. It is about 45,000–70,000 light-years (14,000–21,000 pc) away from Hanny's Voorwerp. The quasar shut down sometime in the last 70,000 years. This revises current theories of quasar operation, as the quasar is quiescent, shutting down much faster than was thought possible, and is much cooler than predicted. The galaxy is currently 100 to 10,000 times dimmer than it was when its quasar burned into Hanny's Voorwerp.

<span class="mw-page-title-main">NGC 5584</span> Galaxy in the constellation Virgo

NGC 5584 is a barred spiral galaxy in the constellation Virgo. It was discovered July 27, 1881 by American astronomer E. E. Barnard. Distance determination using Cepheid variable measurements gives an estimate of 75 million light years, whereas the tip of the red-giant branch approach yields a distance of 73.4 million light years. It is receding with a heliocentric radial velocity of 1,637 km/s. It is a member of the Virgo III Groups, a series of galaxies and galaxy clusters strung out to the east of the Virgo Supercluster of galaxies.

<span class="mw-page-title-main">SN 2014J</span> Supernova in Messier 82

SN 2014J was a type-Ia supernova in Messier 82 discovered in mid-January 2014. It was the closest type-Ia supernova discovered for 42 years, and no subsequent supernova has been closer as of 2023. The supernova was discovered by chance during an undergraduate teaching session at the University of London Observatory. It peaked on 31 January 2014, reaching an apparent magnitude of 10.5. SN 2014J was the subject of an intense observing campaign by professional astronomers and was bright enough to be seen by amateur astronomers.

<span class="mw-page-title-main">SN Refsdal</span> Supernova that has been lensed

SN Refsdal is the first detected multiply-lensed supernova, visible within the field of the galaxy cluster MACS J1149+2223. It was named after Norwegian astrophysicist Sjur Refsdal, who, in 1964, first proposed using time-delayed images from a lensed supernova to study the expansion of the universe. The observations were made using the Hubble Space Telescope.

<span class="mw-page-title-main">NGC 4424</span> Spiral galaxy in the constellation Virgo

NGC 4424 is a spiral galaxy located in the equatorial constellation of Virgo. It was discovered February 27, 1865 by German astronomer Heinrich Louis d'Arrest. This galaxy is located at a distance of 13.5 million light years and is receding with a heliocentric radial velocity of 442 km/s. It has a morphological class of SB(s)a, which normally indicates a spiral galaxy with a barred structure (SB), no inner ring feature (s), and tightly-wound spiral arms (a). The galactic plane is inclined at an angle of 62° to the line of sight from the Earth. It is a likely member of the Virgo Cluster of galaxies.

<span class="mw-page-title-main">NGC 4070</span> Galaxy in the constellation Coma Berenices

NGC 4070 is an elliptical galaxy located 340 million light-years away in the constellation Coma Berenices. NGC 4070 was discovered by astronomer William Herschel on April 27, 1785. It was rediscovered by John Herschel on April 29, 1832 and was listed as NGC 4059. The galaxy is a member of the NGC 4065 Group.

References

  1. ↑ Bond, H. E.; et al. (2003). "An energetic stellar outburst accompanied by circumstellar light echoes". Nature . 422 (6930): 405–408. arXiv: astro-ph/0303513 . Bibcode:2003Natur.422..405B. doi:10.1038/nature01508. PMID   12660776. S2CID   90973.
  2. ↑ "Anatomy of a light echo".
  3. 1 2 Britt, R. R.; Bond, H. (26 March 2003). "Hubble Chronicles Mysterious Outburst with 'Eye-Popping' Pictures". Space.com . Archived from the original on 2006-11-25. Retrieved 2007-04-17.
  4. 1 2 "Hubble watches light echo from mysterious erupting star". European Space Agency. 26 March 2007. Retrieved 2017-05-15.
  5. ↑ Sugerman, B. E. K.; Crotts, A. P. S.; Kunkel, W. E.; Heathcote, S. R.; Lawrence, S. S. (2005). "A New View of the Circumstellar Environment of SN 1987A". The Astrophysical Journal . 627 (2): 888–903. arXiv: astro-ph/0502268 . Bibcode: 2005ApJ...627..888S . doi: 10.1086/430396 .
  6. ↑ Malin, D. "The light echo of supernova 1987A, AAT 66". Australian Astronomical Observatory.
  7. 1 2 Semeniuk, I. (23 January 2008). "Supernova 'echoes' are a window to the galaxy's past". New Scientist . Retrieved 2017-05-15.
  8. ↑ Krause, O.; Tanaka, M.; Usuda, T.; Hattori, T.; Goto, M.; Birkmann, S.; Nomoto, K. (2008). "Tycho Brahe's 1572 supernova as a standard type Ia explosion revealed from its light echo spectrum". Nature . 456 (7222): 617–619. arXiv: 0810.5106 . Bibcode:2008Natur.456..617K. doi:10.1038/nature07608. PMID   19052622. S2CID   4409995.
  9. ↑ Wheeler, J. C.; Maund, J. R.; Couch, S. M. (2007). "The Shape of Cas A". The Astrophysical Journal . 677 (2): 1091–1099. arXiv: 0711.3925 . Bibcode: 2008ApJ...677.1091W . doi: 10.1086/528366 .
  10. ↑ Rest, A.; et al. (2011). "Direct Confirmation of the Asymmetry of the Cas A SN Explosion with Light Echoes". The Astrophysical Journal . 732 (1): 3. arXiv: 1003.5660 . Bibcode: 2011ApJ...732....3R . doi: 10.1088/0004-637X/732/1/3 .
  11. ↑ Sugerman, B.; Crotts, A. (2002). "Multiple Light Echoes from Supernova 1993J". The Astrophysical Journal Letters . 581 (2): L97. arXiv: astro-ph/0207497 . Bibcode: 2002ApJ...581L..97S . doi: 10.1086/346016 .
  12. ↑ Crotts, A. (2015). "Light Echoes From Supernova 2014J in M82". The Astrophysical Journal Letters . 804 (2): L37. arXiv: 1409.8671 . Bibcode: 2015ApJ...804L..37C . doi: 10.1088/2041-8205/804/2/L37 .
  13. ↑ Rest, A.; Prieto, J. L.; Walborn, N. R.; Smith, N.; Bianco, F. B.; Chornock, R.; Welch, D. L.; Howell, D. A.; Huber, M. E.; Foley, R. J.; Fong, W. (Feb 2012). "Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption". Nature. 482 (7385): 375–378. arXiv: 1112.2210 . doi:10.1038/nature10775. ISSN   0028-0836. PMID   22337057. S2CID   205227548.
  14. ↑ Kervella, P.; Mérand, A.; Szabados, L.; Fouqué, P.; Bersier, D.; Pompei, E.; Perrin, G. (2008). "The long-period Galactic Cepheid RS Puppis: I. A geometric distance from its light echoes". Astronomy & Astrophysics. 480 (1): 167–178. arXiv: 0802.1501 . Bibcode:2008A&A...480..167K. doi:10.1051/0004-6361:20078961. ISSN   0004-6361. S2CID   14865683.
  15. ↑ "Light echoes whisper the distance to a star" (Press release). European Southern Observatory. 11 February 2008. Retrieved 2015-10-18.
  16. 1 2 Couderc, P. (1939). "Les Auréoles Lumineuses des Novae". Annales d'Astrophysique . 2: 271–302. Bibcode: 1939AnAp....2..271C .
  17. 1 2 Gulak, Y. K. (1961). "Remarks on the Explanation of the Propagation of a Light Wave around Nova Persei 1901". Soviet Astronomy . 4: 653. Bibcode: 1961SvA.....4..653G .
  18. ↑ "An echo of light". www.eso.org. Retrieved 2 April 2018.
  19. ↑ Lintott, C. J.; et al. (2009). "Galaxy Zoo: 'Hanny's Voorwerp', a quasar light echo?". Monthly Notices of the Royal Astronomical Society . 399 (1): 129–140. arXiv: 0906.5304 . Bibcode: 2009MNRAS.399..129L . doi: 10.1111/j.1365-2966.2009.15299.x .
  20. ↑ Keel, W. C.; et al. (2015). "HST Imaging of Fading AGN Candidates. I. Host-galaxy Properties and Origin of the Extended Gas". The Astronomical Journal . 149 (5): 23. arXiv: 1408.5159 . Bibcode: 2015AJ....149..155K . doi: 10.1088/0004-6256/149/5/155 .
  21. ↑ Schirmer, M.; Diaz, R.; Holhjem, K.; Levenson, N. A.; C. Winge, C. (2013). "A Sample of Seyfert-2 Galaxies with Ultraluminous Galaxy-wide Narrow-line Regions: Quasar Light Echoes?". The Astrophysical Journal . 763 (1): 19. arXiv: 1211.7098 . Bibcode: 2013ApJ...763...60S . doi: 10.1088/0004-637X/763/1/60 .
  22. ↑ Davies, R. L.; Schirmer, M.; Turner, J. E. H. (2015). "The "Green Bean" Galaxy SDSS J224024.1--092748: Unravelling the emission signature of a quasar ionization echo". Monthly Notices of the Royal Astronomical Society . 449 (2): 1731–1752. arXiv: 1502.07754 . Bibcode: 2015MNRAS.449.1731D . doi: 10.1093/mnras/stv343 .
  23. ↑ Schirmer, M.; et al. (2016). "About AGN ionization echoes, thermal echoes, and ionization deficits in low redshift Lyman-alpha blobs". Monthly Notices of the Royal Astronomical Society . 463 (2): 1554–1586. arXiv: 1607.06481 . Bibcode: 2016MNRAS.463.1554S . doi: 10.1093/mnras/stw1819 .
  24. ↑ Schweizer, F.; Seitzer, P.; Kelson, D.; Villanueva, E.; Walth, G. (2013). "The [O III] Nebula of the Merger Remnant NGC 7252: A Likely Faint Ionization Echo". The Astrophysical Journal . 773 (2): 19. arXiv: 1307.2233 . Bibcode: 2013ApJ...773..148S . doi: 10.1088/0004-637X/773/2/148 .
  25. ↑ Rincon, P. (5 August 2008). "Teacher finds new cosmic object". BBC News . Retrieved 2016-09-22.
  26. 1 2 3 "'Cosmic ghost' discovered by volunteer astronomer". Phys.org . 5 August 2008. Retrieved 2016-09-22.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.