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==Further investigations== |
==Further investigations== |
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Having been important in convincing people of the existence of dark matter, recent work on galaxy rotation curves provides some of its greatest challenges. The [[Tully-Fisher relation]] shows for spiral galaxies that rotation is related to galaxy luminosity, which in turn is dependent on the amount of matter in the stars of the galaxy. Thus the amplitude of the galaxy rotation curve is related to the galaxy's visible mass. This is a surprising result for dark matter, since it was assumed that ''a priori'' there was no direct link between the amount of dark matter and the amount of visible matter |
Having been important in convincing people of the existence of dark matter, recent work on galaxy rotation curves provides some of its greatest challenges. The [[Tully-Fisher relation]] shows for spiral galaxies that rotation is related to galaxy luminosity, which in turn is dependent on the amount of matter in the stars of the galaxy. Thus the amplitude of the galaxy rotation curve is related to the galaxy's visible mass. This is a surprising result for dark matter, since it was assumed that ''a priori'' there was no direct link between the amount of dark matter and the amount of visible matter,<ref>http://www.scientificamerican.com/article.cfm?id=dark-matter-doubts&page=2</ref> though state-of-the-art cosmological and [[galaxy formation]] simulations of dark matter with normal [[baryon#Baryonic matter | baryonic]] matter included show that baryonic matter traces dark matter structures.<ref>{{cite journal |last1=Weinberg |first1=David H. |last2=et |first2=al. |year=2008 |title=Baryon Dynamics, Dark Matter Substructure, and Galaxies |journal=The Astrophysical Journal |volume=678 |issue=1 |pages=6-21 |doi=10.1086/524646 |url=http://adsabs.harvard.edu/abs/2008ApJ...678....6W |accessdate=13 September 2012}}</ref><ref>{{cite journal |last1=Duffy |first1=Alan R. |last2=al. |first2=et |year=2010 |title=Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles |journal=Monthly Notices of the Royal Astronomical Society |volume=405 |issue=4 |pages=2161-2178 |doi=10.1111/j.1365-2966.2010.16613.x |url=http://adsabs.harvard.edu/abs/2010MNRAS.405.2161D |accessdate=13 September 2012}}</ref> Additionally, detailed investigations of the rotation curves of [[low surface brightness galaxies]] (LSB galaxies) in the 1990s<ref>{{cite journal | author=W. J. G. de Blok, S. McGaugh| title=The dark and visible matter content of low surface brightness disc galaxies | journal=[[Monthly Notices of the Royal Astronomical Society]] | year=1997 | volume=290 | pages=533–552|arxiv = astro-ph/9704274 |bibcode = 1997MNRAS.290..533D }} available online at the [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1997MNRAS.290..533D Smithsonian/NASA Astrophysics Data System]</ref> and of their position on the [[Tully-Fisher relation]]<ref>{{cite journal | author=M. A. Zwaan, J. M. van der Hulst, W. J. G. de Blok, S. McGaugh| title = The Tully-Fisher relation for low surface brightness galaxies: implications for galaxy evolution | journal=[[Monthly Notices of the Royal Astronomical Society]] | year=1995 | volume=273 | pages=L35–L38|arxiv = astro-ph/9501102 |bibcode = 1995MNRAS.273L..35Z }} available online at the [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995MNRAS.273L..35Z Smithsonian/NASA Astrophysics Data System]</ref> showed that LSB galaxies had to have [[dark matter halo]]es that are more extended and less dense than those of HSB galaxies and thus surface brightness is related to the halo properties. Such dark matter-dominated [[dwarf galaxies]] may hold the key to solving the [[dwarf galaxy problem]] of [[structure formation]]. |
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Further challenges to dark matter theory, or at least its most popular form - [[cold dark matter]] (CDM), came from analysis of the centres of low surface brightness galaxies. Numerical simulations based on CDM gave predictions of the shape of the rotation curves in the centre of dark-matter dominated systems, such as these galaxies. Observations of the actual rotation curves did not show the predicted shape.<ref>{{cite journal | author=W. J. G. de Blok, A. Bosma| title=High-resolution rotation curves of low surface brightness galaxies | journal=Astronomy & Astrophysics | year=2002 | volume=385 | issue=3 | pages=816–846| doi=10.1051/0004-6361:20020080 | bibcode=2002A&A...385..816D|arxiv = astro-ph/0201276 }} available online at the [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...385..816D Smithsonian/NASA Astrophysics Data System]</ref> This [[cuspy halo problem]] of cold dark matter is considered a |
Further challenges to dark matter theory, or at least its most popular form - [[cold dark matter]] (CDM), came from analysis of the centres of low surface brightness galaxies. Numerical simulations based on CDM gave predictions of the shape of the rotation curves in the centre of dark-matter dominated systems, such as these galaxies. Observations of the actual rotation curves did not show the predicted shape.<ref>{{cite journal | author=W. J. G. de Blok, A. Bosma| title=High-resolution rotation curves of low surface brightness galaxies | journal=Astronomy & Astrophysics | year=2002 | volume=385 | issue=3 | pages=816–846| doi=10.1051/0004-6361:20020080 | bibcode=2002A&A...385..816D|arxiv = astro-ph/0201276 }} available online at the [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...385..816D Smithsonian/NASA Astrophysics Data System]</ref> This so-called [[cuspy halo problem]] of cold dark matter is considered a tractable issue by theoretical cosmologists.<ref>de Blok, W. G. ''The Core Cusp Problem''. "Dwarf Galaxy Cosmology" special issue of Advances in Astrophysics. 2009. [http://arxiv.org/abs/0910.3538].</ref> |
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That dark matter theory continues to be supported as an explanation for galaxy rotation curves is because the evidence for dark matter is not solely derived from these curves. It has been uniquely successful in simulating the formation of the large scale structure seen in the distribution of galaxies and in explaining the dynamics of groups and clusters of galaxies.<ref>Peter, Annika H. G. ''Dark Matter: A Brief Review''. Proccedings of Science. 2012.</ref> Dark matter also correctly predicts the results of [[gravitational lens]]ing observations, see especially the [[Bullet Cluster]]. |
That dark matter theory continues to be supported as an explanation for galaxy rotation curves is because the evidence for dark matter is not solely derived from these curves. It has been uniquely successful in simulating the formation of the large scale structure seen in the distribution of galaxies and in explaining the dynamics of groups and clusters of galaxies.<ref>Peter, Annika H. G. ''Dark Matter: A Brief Review''. Proccedings of Science. 2012.</ref> Dark matter also correctly predicts the results of [[gravitational lens]]ing observations, see especially the [[Bullet Cluster]]. |
Revision as of 17:28, 17 September 2012
The rotation curve of a galaxy (also called a velocity curve) is the plot of the orbital speed (in km/s) of the stars or gas in the galaxy on the y-axis against the distance from the center of the galaxy on the x-axis.
A general observation of galaxy rotation can be stated as: galaxies with a central bulge in their disk have a rotation curve which is flat from near the centre to the edge (line B in illustration), i.e. stars are observed to revolve around the centre of these galaxies at a constant speed over a large range of distances from the centre of the galaxy. However, it was expected that these galaxies would have a rotation curve that slopes down from the centre to the edge (dotted line A in illustration), in the same way as other systems with most of their mass in the centre, such as the Solar System of planets or the moons of Jupiter, following the prediction of Kepler's Laws. Something else is needed to account for the dynamics of galaxies besides a simple application of the laws of gravity to the observed matter. It is also observed that galaxies with a uniform distribution of luminous matter have a rotation curve sloping up from center to edge. Most low surface brightness galaxies (LSB galaxies) rotate with a rotation curve that slopes up from the center, indicating little core bulge.
The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the ones predicted assuming a centrally-dominated mass that follows the luminous material observed. If masses of galaxies are derived solely from the luminosities and the mass-to-light ratios in the disk and core portions of spiral galaxies are assumed to be close to that of stars, the masses derived from the kinematics of the observed rotation do not match. This discrepancy can be accounted for if there exists a large amount of dark matter that permeates the galaxy and extends into the galaxy's halo.
Though dark matter is by far the most accepted explanation for the resolution to the galaxy rotation problem, other proposals have been offered with varying degrees of success. Of the possible alternatives, the most notable is Modified Newtonian Dynamics (MOND), which involves modifying the laws of gravity.[2]
History and description of the problem
In 1932, Jan Hendrik Oort was the first to report measurements that the stars in the Solar neighborhood moved faster than expected when a mass distribution based upon the visible matter was assumed. A few years later, Horace Babcock reported in his PhD thesis measurements of the rotation curve for Andromeda which suggested that the mass-to-luminosity ratio increases radially.[3] He, however, attributed it to either absorption of light within the galaxy or modified dynamics in the outer portions of the spiral and not to any form of missing matter. In 1959, Louise Volders demonstrated that spiral galaxy M33 does not spin as expected according to Keplerian dynamics.[4] Following this, in the late 1960s and early 1970s, Vera Rubin, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington presented findings based on a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to a greater degree of accuracy than had ever before been achieved.[5] Together with fellow staff-member Kent Ford, Rubin announced at a 1975 meeting of the American Astronomical Society the discovery that most stars in spiral galaxies orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the location with most of the stars (the galactic bulge), a result independently found in 1978.[6] Rubin presented her results in an influential paper in 1980.[7] These results suggest that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Met with skepticism, Rubin insisted that the observations were correct.
Based on Newtonian mechanics and assuming, as was originally thought, that most of the mass of the galaxy had to be in the galactic bulge near the center, matter (such as stars and gas) in the disk portion of a spiral should orbit the center of the galaxy similar to the way in which planets in the solar system orbit the sun, i.e. where the average orbital speed of an object at a specified distance away from the majority of the mass distribution would decrease inversely with the square root of the radius of the orbit (the dashed line in Fig. 1).
Observations of the rotation curve of spirals, however, do not bear this out. Rather, the curves do not decrease in the expected inverse square root relationship but are "flat", i.e. outside of the central bulge the speed is nearly a constant (the solid line in Fig. 1). If a substantial amount of matter far from the center of the galaxy that is not emitting light in the mass-to-light ratio of the central bulge is assumed, the rotation curves can be explained. The material responsible for the extra mass was dubbed, "dark matter", the existence of which was first posited in the 1930s by Jan Oort in his measurements of the Oort constants and Fritz Zwicky in his studies of the masses of galaxy clusters, though these proposals were left unexplored until after Rubin's work was accepted as correct. The existence of non-baryonic cold dark matter (CDM) is today a major feature of the Lambda-CDM model that describes the cosmology of the universe.
Further investigations
Having been important in convincing people of the existence of dark matter, recent work on galaxy rotation curves provides some of its greatest challenges. The Tully-Fisher relation shows for spiral galaxies that rotation is related to galaxy luminosity, which in turn is dependent on the amount of matter in the stars of the galaxy. Thus the amplitude of the galaxy rotation curve is related to the galaxy's visible mass. This is a surprising result for dark matter, since it was assumed that a priori there was no direct link between the amount of dark matter and the amount of visible matter,[8] though state-of-the-art cosmological and galaxy formation simulations of dark matter with normal baryonic matter included show that baryonic matter traces dark matter structures.[9][10] Additionally, detailed investigations of the rotation curves of low surface brightness galaxies (LSB galaxies) in the 1990s[11] and of their position on the Tully-Fisher relation[12] showed that LSB galaxies had to have dark matter haloes that are more extended and less dense than those of HSB galaxies and thus surface brightness is related to the halo properties. Such dark matter-dominated dwarf galaxies may hold the key to solving the dwarf galaxy problem of structure formation.
Further challenges to dark matter theory, or at least its most popular form - cold dark matter (CDM), came from analysis of the centres of low surface brightness galaxies. Numerical simulations based on CDM gave predictions of the shape of the rotation curves in the centre of dark-matter dominated systems, such as these galaxies. Observations of the actual rotation curves did not show the predicted shape.[13] This so-called cuspy halo problem of cold dark matter is considered a tractable issue by theoretical cosmologists.[14]
That dark matter theory continues to be supported as an explanation for galaxy rotation curves is because the evidence for dark matter is not solely derived from these curves. It has been uniquely successful in simulating the formation of the large scale structure seen in the distribution of galaxies and in explaining the dynamics of groups and clusters of galaxies.[15] Dark matter also correctly predicts the results of gravitational lensing observations, see especially the Bullet Cluster.
Alternatives to dark matter
There are a limited number of attempts to solve the problem of galaxy rotation curves without invoking dark matter.
One of the most discussed alternatives is MOND (Modified Newtonian Dynamics), originally proposed by Mordehai Milgrom as a phenomenological explanation back in 1983 but which has been seen to have predictive power in accounting for galaxy rotation curves. This posits that the physics of gravity changes at large scale but, until recently, was not a relativistic theory. However, this changed with the development by Jacob Bekenstein of the tensor–vector–scalar gravity (TeVeS) theory,[16][2] enabling gravitational lensing to be covered by the theory.
Another, similar, alternative is the relativistic modified gravity (MOG) theory, also called scalar–tensor–vector gravity (STVG), of John Moffat.[17] Brownstein and Moffat [18] applied MOG and MOND to the question of galaxy rotation curves, and demonstrated excellent fits to a large sample of over 100 low surface brightness (LSB), high surface brightness (HSB) and dwarf galaxies.[19] Each galaxy rotation curve fit was made without dark matter, using only the available photometric data (stellar matter and visible gas) and a two-parameter mass distribution model which made no assumption regarding the mass to light ratio. The MOG results were compared to MOND and were nearly indistinguishable right out to the edge of the rotation curve data, where MOND predicts a forever flat rotation curve, but MOG predicts an eventual return to the familiar inverse-square gravitational force law.
Although these alternatives are not considered by the astronomical community to be as convincing as the dark matter model,[20][21] gravitational lensing studies have been proposed as the means to separate the predictions of the different theories. Indeed, gravitational lensing by the Bullet Cluster was reported to provide the best current evidence for the nature of dark matter[22][23] and to provide "evidence against some of the more popular versions of Modified Newtonian Dynamics (MOND)" as applied to large galactic clusters.[24] In retort, Milgrom, the original proposer of MOND, posted an rejoinder online[25] that claims MOND correctly accounts for the dynamics of galaxies outside of galaxy clusters, and removes the need for most dark matter in clusters, leaving twice as much matter as is visible, which Milgrom expects to be simply unseen ordinary matter rather than exotic cold dark matter.
Some Quantum Gravity theories also give alternative explanations, see alternative theories to dark matter.
See also
- Vera Rubin
- Unsolved problems in physics
- Nonsymmetric gravitational theory
- Dark matter
- Long-slit spectroscopy
Footnotes
- ^ "The generally accepted explanation of the mass discrepancy is the proposal that spiral galaxies consist of a visible component surrounded by a more massive and extensive dark component .." is stated in the introduction of the article: K.G. Begeman, A.H. Broeils, R.H.Sanders (1991). "Extended rotation curves of spiral galaxies: dark haloes and modified dynamics". Monthly Notices of the Royal Astronomical Society. 249: 523–537. Bibcode:1991MNRAS.249..523B.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) available online at the Smithsonian/NASA Astrophysics Data System. Also Figure 1 of the article has numerous galactic rotation curves qualitatively similar to this one. - ^ a b For an extensive discussion of the data and its fit to MOND see Mordehai Milgrom (2007). "The MOND Paradigm". arXiv:0801.3133 [astro-ph]. This paper is a talk presented at the XIX Rencontres de Blois "Matter and energy in the Universe: from nucleosynthesis to cosmology".
- ^ Babcock, H, 1939, “The rotation of the Andromeda Nebula”, Lick Observatory bulletin ; no. 498
- ^ L. Volders. "Neutral hydrogen in M 33 and M 101". Bulletin of the Astronomical Institutes of the Netherlands. 14 (492): 323–334.
- ^ V. Rubin, W. K. Ford, Jr (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions". Astrophysical Journal. 159: 379. Bibcode:1970ApJ...159..379R. doi:10.1086/150317.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) - ^ A. Bosma, "The distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types", PhD Thesis, Rijksuniversiteit Groningen, 1978, available online at the Nasa Extragalactic Database
- ^ V. Rubin, N. Thonnard, W. K. Ford, Jr, (1980). "Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc)". Astrophysical Journal. 238: 471. Bibcode:1980ApJ...238..471R. doi:10.1086/158003.
{{cite journal}}
: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link) - ^ http://www.scientificamerican.com/article.cfm?id=dark-matter-doubts&page=2
- ^ Weinberg, David H.; et, al. (2008). "Baryon Dynamics, Dark Matter Substructure, and Galaxies". The Astrophysical Journal. 678 (1): 6–21. doi:10.1086/524646. Retrieved 13 September 2012.
- ^ Duffy, Alan R.; al., et (2010). "Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles". Monthly Notices of the Royal Astronomical Society. 405 (4): 2161–2178. doi:10.1111/j.1365-2966.2010.16613.x. Retrieved 13 September 2012.
- ^ W. J. G. de Blok, S. McGaugh (1997). "The dark and visible matter content of low surface brightness disc galaxies". Monthly Notices of the Royal Astronomical Society. 290: 533–552. arXiv:astro-ph/9704274. Bibcode:1997MNRAS.290..533D. available online at the Smithsonian/NASA Astrophysics Data System
- ^ M. A. Zwaan, J. M. van der Hulst, W. J. G. de Blok, S. McGaugh (1995). "The Tully-Fisher relation for low surface brightness galaxies: implications for galaxy evolution". Monthly Notices of the Royal Astronomical Society. 273: L35–L38. arXiv:astro-ph/9501102. Bibcode:1995MNRAS.273L..35Z.
{{cite journal}}
: CS1 maint: multiple names: authors list (link) available online at the Smithsonian/NASA Astrophysics Data System - ^ W. J. G. de Blok, A. Bosma (2002). "High-resolution rotation curves of low surface brightness galaxies". Astronomy & Astrophysics. 385 (3): 816–846. arXiv:astro-ph/0201276. Bibcode:2002A&A...385..816D. doi:10.1051/0004-6361:20020080. available online at the Smithsonian/NASA Astrophysics Data System
- ^ de Blok, W. G. The Core Cusp Problem. "Dwarf Galaxy Cosmology" special issue of Advances in Astrophysics. 2009. [1].
- ^ Peter, Annika H. G. Dark Matter: A Brief Review. Proccedings of Science. 2012.
- ^ J. D. Bekenstein (2004). "Relativistic gravitation theory for the modified Newtonian dynamics paradigm". Physical Review D. 70 (8): 083509. arXiv:astro-ph/0403694. Bibcode:2004PhRvD..70h3509B. doi:10.1103/PhysRevD.70.083509.
- ^ J. W. Moffat (2006). "Scalar tensor vector gravity theory". Journal of Cosmology and Astroparticle Physics. 3 (03): 4. arXiv:gr-qc/0506021. Bibcode:2006JCAP...03..004M. doi:10.1088/1475-7516/2006/03/004.
- ^ http://www.arxiv.org/abs/astro-ph/0506370 astro-ph/0506370
- ^ J. R. Brownstein and J. W. Moffat (2006). "Galaxy Rotation Curves Without Non-Baryonic Dark Matter". Astrophysical Journal. 636 (2): 721. arXiv:astro-ph/0506370. Bibcode:2006ApJ...636..721B. doi:10.1086/498208.
- ^ http://www.bbc.co.uk/sn/tvradio/programmes/horizon/missing.shtml
- ^ http://chandra.harvard.edu/press/02_releases/press_102202.html
- ^ M. Markevitch, A. H. Gonzalez, D. Clowe, A. Vikhlinin, L. David, W. Forman, C. Jones, S. Murray, and W. Tucker. "Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56". arXiv:astro-ph/0309303. Bibcode:2004ApJ...606..819M. doi:10.1086/383178.
{{cite journal}}
: Cite journal requires|journal=
(help)CS1 maint: multiple names: authors list (link) - ^ M. Markevitch, S. Randall, D. Clowe, A. Gonzalez and M. Bradac (16 - 23 July 2006). "Dark Matter and the Bullet Cluster" (PDF). 36th COSPAR Scientific Assembly. Beijing, China.
{{cite conference}}
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suggested) (help)CS1 maint: multiple names: authors list (link) abstract only - ^ http://hea-www.harvard.edu/LUNCH_TALKS/abstracts.html lunch-time talk at Harvard University by Scott Randall on 31 May 2006, abstract only
- ^ http://www.astro.umd.edu/~ssm/mond/moti_bullet.html
External links
Bibliography
- V. Rubin, W. K. Ford, Jr (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions". Astrophysical Journal. 159: 379. Bibcode:1970ApJ...159..379R. doi:10.1086/150317.
{{cite journal}}
: CS1 maint: multiple names: authors list (link)- This was the first detailed study of orbital rotation in galaxies.
- V. Rubin, N. Thonnard, W. K. Ford, Jr, (1980). "Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc)". Astrophysical Journal. 238: 471. Bibcode:1980ApJ...238..471R. doi:10.1086/158003.
{{cite journal}}
: CS1 maint: extra punctuation (link) CS1 maint: multiple names: authors list (link)- Observations of a set of spiral galaxies gave convincing evidence that orbital velocities of stars in galaxies were unexpectedly high at large distances from the nucleus. This paper was influential in convincing astronomers that most of the matter in the universe is dark, and much of it is clumped about galaxies.
- Galactic Astronomy, Dmitri Mihalas and Paul McRae.W. H. Freeman 1968.