Utente:Paperoastro/Sandbox3: differenze tra le versioni

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Riga 1:

{{Oggetto non stellare

[[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 6 / NGC 20

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 = NGC 0006 2MASS.jpg

|image_text = NGC 6

|scopritore = [[William Parsons]]

|anno = [[1857]]

|epoca = J2000.0

|costellazione = Andromeda

|ra = {{RA|00|09|32,7}}<ref name="ned">{{en}} {{cita web

| titolo=NGC 20

|sito= [http://nedwww.ipac.caltech.edu/ NASA/IPAC Extragalactic Database]

| url=http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=NGC+20

| accesso=17 febbraio 2010

}}</ref>

|dec = {{DEC|+33|18|31}}<ref name="ned"/>

|dist_al = 210

|misura = milioni

|pre_dist_pc =

|dist_pc = 64,5

|misura_pc = Mpc

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

|appmag_v = 14,04<ref name="ned"/>

|dimensione_v = 1,5'&nbsp;×&nbsp;1,5'<ref name="ned"/>

|luminosita_sup =

|redshift = 0,016581<ref name="falco1999">{{en}} {{cita pubblicazione

| cognome = Falco | nome = Emilio E. | coautori = Kurtz, Michael J.; Geller, Margaret J.; Huchra, John P.; Peters, James; Berlind, Perry; Mink, Douglas J.; Tokarz, Susan P.; Elwell, Barbara | anno = 1999

| titolo = The Updated Zwicky Catalog (UZC)

| rivista = Publications of the Astronomical Society of the Pacific | volume = 111 | numero = 758 | pagine =438-452

| doi = 10.1086/316343 | url = http://adsabs.harvard.edu/abs/1999PASP..111..438F

| accesso = 18 febbraio 2010

}}</ref>

|angolo_posizione = 140°<ref name="ugc">{{en}} {{cita web

| titolo= NGC 20 | url=http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-out.add=.&-source=VII/26D/catalog&recno=84

|sito= [[Uppsala General Catalogue|Uppsala General Catalogue of Galaxies]] (UGC) (Nilson 1973)

| editore= VizieR Service | accesso=18 febbraio 2010

}}</ref>

|v_radiale = 4971&nbsp;±&nbsp;49

|tipo = Galassia ellittica

|classe = S0/E

|massa =

|pre_dimensioni_al =

|dimensioni_al =

|note_dimensioni_al =

|pre_dimensioni_pc =

|dimensioni_pc =

|note_dimensioni_pc =

|absmag_v = -20,01

|note =

|nomi = [[New General Catalogue|NGC]]&nbsp;6, [[Uppsala General Catalogue|UGC]]&nbsp;84, [[Morphological Catalogue of Galaxies|MCG]]&nbsp;+05-01-036, [[2MASS|2MASX]]&nbsp;J00093270+3318310, [[Principal Galaxies Catalogue|PGC]]&nbsp;679

}}

'''NGC 20''' (a cui successivamente è stato assegnato anche il nome di '''NGC 6''') è una [[galassia]] [[galassia ellittica|ellittica]] di quattordicesima [[magnitudine apparente|magnitudine]] visibile nella [[costellazione]] di [[Andromeda (costellazione)|Andromeda]] e distante dalla [[Terra]] circa 64,5&nbsp;[[parsec|Mpc]] (210 milioni di [[anno luce|anni luce]]).

==Storia delle osservazioni==

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> .

NGC 20 fu scoperta il [[18 settembre]] [[1857]] da [[William Parsons]] con un [[telescopio]] [[telescopio riflettore|riflettore]] da 72 [[pollice (unità di misura)|pollici]] (182,9 [[centimetro|cm]]). Nel [[1888]] [[John Dreyer]] la incluse nel [[catalogo NGC]] con il nome di NGC&nbsp;20 e la descrisse come una debole galassia. Dreyer annotò inoltre la presenza di una [[stella]] di decima magnitudine apparentemente attaccata all'oggetto<ref name="ngc">{{en}} {{cita libro

| autore=J.L.E. Dreyer | wkautore=John Dreyer

| titolo=New general catalogue of nebulæ and clusters of stars (1888) - Index catalogue (1895) - Second index catalogue (1908)

| url=http://www.ngcicproject.org/legacy/ngconline/default.htm | formato=jpg |accesso=24 gennaio 2010

| annooriginale=1888 | anno=1962 | editore=Memoirs of the Royal Astronomical Society | città=Londra | pagine=14

}}</ref>.

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==History and description of the problem==

Nella notte del [[20 settembre]] [[1885]] [[Lewis Swift]] osservò sei oggetti, che vennero poi inseriti da Dreyer nel suo catalogo con il nome di NGC&nbsp;6, NGC&nbsp19, NGC&nbsp21, NGC&nbsp7831 e NGC&nbsp7836. NGC&nbsp;6 fu descritto come un (ri-trovare l'articolo!!!!!!

(Lo stesso oggetto fu osservato)

In 1959,

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.

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]].

NGC 6 = NGC 20. On the night of 20 September 1885, Lewis Swift found

==Further investigations==

six objects. Four of these (NGC 19, NGC 21, NGC 7831, and NGC 7836; see the

notes for these, too) have mean offsets in their positions as published by

Swift, from the correct positions, of +1m 10s in RA and +8m 08s in Dec. A

fifth found later in the night, NGC 801, has offsets of -19s and -0.9m; Swift

clearly ``re-zeroed'' his telescope in the interim.

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]].

Found by Swift (List #2 on 20 Sept 1885) but his position was 1.1 tmin W and 47'

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.

S of NGC 20 = U00084 = M+05-01-036 = CGCG 498-082. His RA offset is identical

to the error in his positions for N19, N21, N7831, N7836 also found the same

evening. Although the dec error is large, his description ("one of 5 st which

point to it is p nr") matches the chain of five stars just following N20, whose

position was accurately given by Schultz. The RNGC lists NGC 6 as nonexistent

but incorrectly equates it with N7831. See NGCBUGS for more info.

--

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.

Schultz's micrometric position for NGC 20 matches U00084 = M+05-01-036 = CGCG

== Alternatives to dark matter ==

498-082. Swift later (20 Sept 1885) entered this galaxy in List #2 (NGC 6) but

his position was 1.1 tmin too far W and 47' too far S. His RA offset is

identical to the error in his positions for N19, N21, N7831, N7836 all found the

same evening. Although the dec error is large, his description ("one of 5 st

which point to it is p nr") matches the chain of 5 stars just following.

--

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.

The sixth object, NGC 6, shares the right ascension offsets of the first four,

==See also==

but its declination is about 45 arcmin too large. It's identity with NGC 20

* [[Vera Rubin]]

is secured by Swift's note ``... one of 5 sts which point to it is pretty

* [[Unsolved problems in physics]]

near.'' The unmistakeable line of five stars stretches about 2 arcmin to the

* [[Nonsymmetric gravitational theory]]

east; Swift's ``pretty near'' star is about 15 arcsec east of the galaxy.

* [[Dark matter]]

-->

==Footnotes==

<references />

== Bibliography Osservazione==

Data la sua debole luminosità, per individuare NGC&nbsp;20 occorre un [[telescopio]] da almeno 250&nbsp;[[millimetro|mm]] di diametro. La galassia appare rotonda, con un piccolo nucleo di aspetto stellare.

Una stella di undicesima magnitudine dista soltanto 30 [[arcosecondo|secondi d'arco]] dal nucleo in direzione est. Sempre in direzione est, a 2,4 [[arcominuto|minuti d'arco]], si trova la stella [[Catalogo_Tycho-2|TYC]]&nbsp;2264-35-1, di magnitudine apparente 10,25<ref name="tycho">{{en}} {{cita web

* {{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}}

| titolo= TYC&nbsp;2264-35-1 | url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=TYC+2264-35-1&NbIdent=1

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

|sito= [[SIMBAD]] Astronomical Database

| editore= [[Centre de Données astronomiques de Strasbourg]] | accesso=24 febbraio 2010

}}</ref>.

La galassia [[NGC 13]] si trova a 12 minuti d'arco verso nord-ovest.

* {{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}}

*: 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.

==Caratteristiche==

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

(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.)

==Note==

<!--

<references/>

[[Category:Astrophysics]]

==Bibliografia==

[[da:Galakserotation]]

===Libri===

[[de:Rotationskurve]]

* {{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| ISBN= 0-7923-5909-7| lingua= en}}

[[en:Galaxy rotation curve]]

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

[[nl:Melkwegstelseldraaiingsprobleem]]

[[ja:銀河の回転曲線問題]]

===Carte celesti===

[[pl:Krzywa rotacji galaktyki]]

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

[[ru:Кривая вращения галактики]]

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

[[fi:Galaksin rotaatiokäyrä]]

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

[[zh:星系自轉問題]]

-->

== Voci correlate ==

*[[Galassia]]

*[[Galassia ellittica]]

*[[New General Catalogue|Catalogo NGC]]

*[[Oggetti non stellari nella costellazione di Andromeda]]

== Altri progetti ==

{{interprogetto|commons=Category:NGC 20|etichetta=NGC 20}}

==Collegamenti esterni==

{{LinksNGC|20}}

{{NavNGC|6}}{{NavNGC|20}}

{{Portale|oggetti del profondo cielo}}

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