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[[Image:GalacticRotation2.svg|thumb|right|300px|Tipica curva di rotazione di una [[galassia spirale]]: A indica la curva ''predetta'' e B quella osservata. La discrepanza fra le due è attribuita alla presenza di [[materia oscura]].]]

|nome = NGC 3

La '''curva di rotazione''' di una [[galassia]] può essere rappresentata da un [[grafico]] in cui sull'asse delle [[ordinata|ordinate]] è posta la [[velocità orbitale]] delle [[stella|stelle]] o del [[gas]] di una galassia, mentre sull'asse delle ascisse la distanza dal centro della galassia. Le stelle ruotano attorno al centro delle galassie con una velocità costante in un ampio intervallo di distanze dal centro della galassia. Quindi esse ruotano ad una velocità molto più elevata di quella che ci si aspetterebbe se esse fossero sottoposte soltanto al [[potenziale#Potenziale gravitazionale|potenziale newtoniano]].

|immagine =

|image_text =

|scopritore = [[Albert Marth]]

|anno = 1864

|epoca = J2000.0

|costellazione = Pesci

|ra = {{RA|00|07|16,8}}<ref name="ned">{{cita web | titolo=NASA/IPAC Extragalactic Database: risultato per NGC 3 | url=http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=ngc+3&img_stamp=YES | lingua=en | accesso=20-12-2009 }}</ref>

|dec = {{DEC|+08|18|06}}<ref name="ned"/>

|pre_dist_al =

|dist_al = 159,8

|misura = milioni

|note_dist_al =

|pre_dist_pc =

|dist_pc = 49

|misura_pc = Mpc

|note_dist_pc = <ref name="ned"/>

|appmag_v = 14,2<ref name="ned"/>

|dimensione_v = 1,1 × 0,6<ref name="ned"/>

|luminosita_sup =

|redshift = 0,013009<ref name="ned"/>

|angolo_posizione =

|v_radiale = 3900&nbsp;±&nbsp;49

|tipo = Galassia lenticolare

|classe = S0<ref name="ned"/>

|massa =

|pre_dimensioni_al =

|dimensioni_al =

|note_dimensioni_al =

|pre_dimensioni_pc =

|dimensioni_pc =

|note_dimensioni_pc =

|absmag_v = -19,3

|note =

|nomi = [[Uppsala General Catalogue|UGC]]&nbsp;58, [[Morphological Catalogue of Galaxies|MCG]]&nbsp;+01-01-037, [[2MASS|2MASX]]&nbsp;J00071680+0818058, [[Principal Galaxies Catalogue|PGC]]&nbsp;565

}}

'''NGC 3''' è una [[galassia lenticolare]] di quattordicesima [[magnitudine apparente|magnitudine]] visibile nella [[costellazione]] dei [[Pesci (costellazione)|Pesci]] e distante circa 49 [[parsec|Mpc]] (159,8 milioni di [[anno luce|anni luce]])<ref name="ned"/> dalla [[Terra]].

La discrepanza osservata fra la velocità di rotazione della materia del disco di una galassia e quella predetta dalla [[principi della dinamica|meccanica newtoniana]] considerando solo la materia visibile, è il cosiddetto ''problema della rotazione delle galassie''. Attualmente si ipotizza che questa discrepanza palesi la presenza di [[materia oscura]] che permea le galassie e si estende attorno ad esse in una sorta di [[Alone galattico#Materia oscura nell'alone|alone]]. Teorie alternative, come le [[teorie MOND]], cercano di spiegare la discrepanza con una modifica della [[legge di gravità]] ad intensità molto basse<ref name="Milgrom07">{{cita pubblicazione |titolo=The MOND Paradigm | autore=Mordehai Milgrom | url=http://arxiv.org/abs/0801.3133v2 | anno=2007 | rivista=ArXiv preprint}}. L'articolo è il riassunto di un discorso tenuto alla conferenza ''XIX Rencontres de Blois "Matter and energy in the Universe: from nucleosynthesis to cosmology"''.</ref> .

Fu scoperta da [[Albert Marth]] nel [[1864]] utilizzando un [[telescopio]] [[telescopio riflettore|riflettore]] di 48 [[pollice (unità di misura)|pollici]] (121,9 [[centimetro|cm]]).

==History and description of the problem==

==Osservazione==

In 1959,

(INSERIRE UNA BREVE DESCRIZIONE OSSERVATIVA DELL'OGGETTO: SE È POSSIBILE OSSERVARLO AD OCCHIO NUDO O CON UN TELESCOPIO ETC. SE NON DISPONI DI QUESTI DATI, NON INSERIRE LA SEZIONE.)

Louise Volders demonstrated

that spiral galaxy [[Triangulum_Galaxy|M33]] does not spin

as expected according to [[Keplerian|Keplerian dynamics]],<ref>{{cite journal | author=L. Volders | title=Neutral hydrogen in M 33 and M 101 | journal=Bulletin of the Astronomical Institutes of the Netherlands | volume=14 | number=492 | pages=323–334}}</ref>

a result which was extended to many other spiral galaxies

during the seventies.<ref>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 [http://nedwww.ipac.caltech.edu/level5/March05/Bosma/frames.html Nasa Extragalactic Database]</ref> Based on this model, [[matter]] (such as [[star]]s 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, that is, according to [[Newtonian mechanics]]. Based on this, it would be expected that 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). At the time of the discovery of the discrepancy, it was thought that most of the mass of the galaxy had to be in the [[galactic bulge]], near the center.

==Caratteristiche==

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" &ndash; outside of the central bulge the speed is nearly a constant function of radius (the solid line Fig. 1). The explanation that requires the [[Occam's Razor|least adjustment]] to the [[physics|physical laws of the universe]] is that there is 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. This extra mass is proposed by astronomers to be due to [[dark matter]] within the [[Dark matter halo|galactic halo]], the existence of which was first posited by [[Fritz Zwicky]] some 40 years earlier in his studies of the masses of [[galaxy cluster]]s. Presently, there are a large number of pieces of observational evidence that point to the presence of [[cold dark matter]], and its existence is a major feature of the present [[Lambda-CDM model]] that describes the [[physical cosmology|cosmology]] of the [[universe]].

(INSERIRE UNA DESCRIZIONE DELL'OGGETTO, SPIEGANDO DI CHE SI TRATTA, QUALI SONO LE SUE PARTICOLARITÀ, L'ETÀ, LA STRUTTURA, ETC. SE NON DISPONI DI QUESTI DATI, NON INSERIRE LA SEZIONE, SEBBENE SIA FORTEMENTE CONSIGLIATO REPERIRE QUALCHE DATO.)

[[File:NGC_0003_2MASS.jpg|thumb|left|160px|NGC 3 ripresa nel vicino [[radiazione infrarossa|infrarosso]] da [[2MASS]].]]

==Further investigations==

==Note==

Having been important in convincing people of the existence of dark matter, recent work on galaxy rotation curves provides some of its greatest challenges. 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}} 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}} 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 these did not behave as expected. These galaxies had to be dominated by dark matter in a surprising fashion. However, such dark matter-dominated [[dwarf galaxies]] may hold the key to solving the [[dwarf galaxy problem]] of [[structure formation]].

<references/>

==Bibliografia==

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 | pages=816–846| doi=10.1051/0004-6361:20020080}} 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.

===Libri===

* {{en}} {{cita libro | cognome= Lada| nome= C. J. | coautori= N. D. Kylafits| titolo= The Origin of Stars and Planetary Systems| editore= Kluwer Academic Publishers| città= | anno= 1999| id= ISBN 0-7923-5909-7}}

* {{cita libro | cognome= De Blasi| nome= A. | titolo= Le stelle: nascita, evoluzione e morte| editore= CLUEB| città= [[Bologna]]| anno= 2002| id= ISBN 88-491-1832-5}}

===Carte celesti===

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 (as originally proposed by Zwicky). Dark matter also correctly predicts the results of [[gravitational lens]]ing observations.

* {{cita libro | cognome= Tirion, Rappaport, Lovi | titolo=Uranometria 2000.0 - Volume I & II | editore=Willmann-Bell, inc.| città=Richmond, Virginia, USA | anno=1987 | id= ISBN 0-943396-15-8}}

* {{cita libro | cognome= Tirion, Sinnott| titolo=Sky Atlas 2000.0 - Second Edition | editore= Cambridge University Press | città= Cambridge, USA| anno= 1998| id= ISBN 0-933346-90-5}}

* {{cita libro | cognome= Tirion| titolo=The Cambridge Star Atlas 2000.0 | ed=3 | editore= Cambridge University Press | città= Cambridge, USA| anno= 2001| id= ISBN 0-521-80084-6}}

== AlternativesVoci to dark mattercorrelate ==

*[[Oggetti non stellari nella costellazione dei Pesci]]

==Collegamenti esterni==

There are a limited number of attempts to find

[[Dark matter#Alternative explanations|alternative explanations]] to dark matter to explain galaxy rotation curves. One of the most discussed alternatives is [[MOND]] (Modified Newtonian Dynamics), originally proposed as a phenomenological explanation back in 1983 but which has been seen to have predictive power in the rotation curves of LSB galaxies. 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 of the [[tensor-vector-scalar gravity]] (TeVeS) theory<ref>{{cite journal | author=J. D. Bekenstein | title=Relativistic gravitation theory for the modified Newtonian dynamics paradigm | journal=Physical Review D | year=2004 | volume=70 | pages=083509 | doi=10.1103/PhysRevD.70.083509}}</ref>. A more successful alternative is the modified gravity (MOG) theory of Moffat such as [[scalar-tensor-vector gravity]] (STVG)<ref>{{cite journal | author=J. W. Moffat | title=Scalar tensor vector gravity theory | journal=Journal of Cosmology and Astroparticle Physics | year=2006 | volume=3 | pages=4 | doi=10.1088/1475-7516/2006/03/004}}</ref>. Brownstein and Moffat ([http://www.arxiv.org/abs/astro-ph/0506370 astro-ph/0506370]) applied MOG to the question of galaxy rotation curves, and presented the fits to a

large sample of over 100 low surface brightness (LSB), high surface brightness (HSB) and dwarf galaxies<ref>{{cite journal | author=J. R. Brownstein and J. W. Moffat | title= [http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v636n2/63031/brief/63031.abstract.html Galaxy Rotation Curves Without Non-Baryonic Dark Matter]| journal=Astrophysical Journal | year=2006 | volume=636 | pages=721 | doi= 10.1086/498208}}</ref>. Each galaxy

rotation curve was fit without dark matter using only the available photometric data (stellar matter and visible gas)

and alternatively a two-parameter mass distribution model which made no assumption regarding the mass to light

ratio. The results were compared to MOND and were nearly indistinguishably 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 yet considered by the [[astronomy|astronomical]] [[scientific community|community]] to be as convincing as the dark matter model,<ref>See, for example, the review of the development of the subject by [http://www.bbc.co.uk/sn/tvradio/programmes/horizon/missing.shtml BBC science reporters], and this commentary by astronomers involved with the [[Chandra X-Ray Observatory]] [http://chandra.harvard.edu/press/02_releases/press_102202.html]. The astronomical community accepts the existence and presence of dark matter due as well to corroboration by observations unrelated to galaxy rotation curves including [[gravitational lensing]], measurements of the [[cosmic microwave background radiation]], and statistics of the large scale structure of the universe.</ref> gravitational lensing studies may provide the means to separate the predictions of alternative gravity theories from the dark matter explanation.

{{Portale|oggetti del profondo cielo}}

==See also==

* [[Vera Rubin]]

* [[Unsolved problems in physics]]

* [[Nonsymmetric gravitational theory]]

* [[Dark matter]]

<!-- [[Categoria:Oggetti del catalogo NGC|0003]]

==Footnotes==

[[Categoria:Galassie lenticolari|NGC 0003]]

<references />

[[ca:NGC 3]]

== Bibliography ==

[[de:NGC 3]]

[[en:NGC 3]]

* {{cite journal | author=V. Rubin, W. K. Ford, Jr| title= Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions | journal=Astrophysical Journal | year=1970 | volume=159 | pages=379 | doi=10.1086/150317}}

[[eo:NGC 3]]

*: This was the first detailed study of orbital rotation in galaxies.

[[es:NGC 3]]

[[fr:NGC 3]]

* {{cite journal | author=V. Rubin, N. Thonnard, W. K. Ford, Jr, | title= Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to [[Uppsala General Catalogue|UGC]] 2885 (R=122kpc)| journal=Astrophysical Journal | year=1980 | volume=238 | pages=471 | doi=10.1086/158003}}

[[gl:NGC 3]]

*: 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.

[[hr:NGC 3]]

[[hu:NGC 3]]

* ''Galactic Astronomy'', Dmitri Mihalas and [[Paul McRae]].[[W. H. Freeman]] 1968.

[[ko:NGC 3]]

[[lb:NGC 3]]

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