Editing Galaxy rotation curve (section)

==History==
{{more science citations needed |section|date=December 2016}}
[[Vesto Slipher]] made the first measurements related to galaxy rotation curves in 1914 when observing the Andromeda galaxy.<ref name="Mambrini-2021">{{Citation |last=Mambrini |first=Yann |title=Introduction |date=2021 |work=Particles in the Dark Universe: A Student’s Guide to Particle Physics and Cosmology |pages=1–22 |editor-last=Mambrini |editor-first=Yann |url=https://link.springer.com/chapter/10.1007/978-3-030-78139-2_1 |access-date=2025-04-26 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-78139-2_1 |isbn=978-3-030-78139-2}}</ref> Slipher observed that the stars on the left side of the galaxy's bulge were approaching at speeds of around 320 km/s, faster than those on the right, which were moving at about 280 km/s. This suggested that the galaxy's disc was rotating in such a way that it appeared to be spinning toward us.<ref>{{Cite web |title=Vesto Slipher |url=https://www.roe.ac.uk/~jap/slipher/ |access-date=2025-04-26 |website=www.roe.ac.uk}}</ref><ref name="Mambrini-2021" />

In 1918 [[Francis G. Pease|Francis Pease]] 
determined the rotation speed within the central region of Andromeda.<ref name="Mambrini-2021" /> His findings were represented by the formula <math>\displaystyle V_c = -0.48 r - 316</math>, where <math>V_c</math> is the measured circular speed (in km/s) at a distance <math>r</math> from the center of Andromeda's bulge. The results indicated that the central part of the galaxy rotates at a constant angular speed.<ref name="Mambrini-2021" />

In 1932, [[Jan Oort|Jan Hendrik Oort]] became the first to report that measurements of the stars in the [[solar neighborhood]] indicated that they moved faster than expected when a mass distribution based upon visible matter was assumed, but these measurements were later determined to be essentially erroneous.<ref name="Oxford1999">{{cite book
|title=Oxford Dictionary of Scientists
|date=1999
|publisher=Oxford University Press
|location=Oxford|isbn=978-0-19-280086-2
|url=https://books.google.com/books?id=AtngooiwXikC}}</ref> In 1939, [[Horace Babcock]] reported in his PhD thesis measurements of the rotation curve for Andromeda which suggested that the mass-to-luminosity ratio increases radially.<ref>{{cite journal |last1=Babcock |first1=H. W. |year=1939 |title=The rotation of the Andromeda Nebula |journal=[[Lick Observatory Bulletin]] |volume=19 |pages=41–51 |bibcode=1939LicOB..19...41B |bibcode-access=free |doi=10.5479/ADS/bib/1939LicOB.19.41B |doi-access=}}</ref> He attributed that to either the absorption of light within the galaxy or to modified dynamics in the outer portions of the spiral and not to any form of missing matter. Babcock's measurements turned out to disagree substantially with those found later, and the first measurement of an extended rotation curve in good agreement with modern data was published in 1957 by Henk van de Hulst and collaborators, who studied M31 with the [[Dwingeloo Radio Observatory]]'s newly commissioned 25-meter [[radio telescope]].<ref>{{cite journal |last=Van de Hulst |first=H.C|display-authors=etal|year=1957 |title=Rotation and density distribution of the Andromeda nebula derived from observations of the 21-cm line |journal=[[Bulletin of the Astronomical Institutes of the Netherlands]] |volume=14 |page=1 |bibcode=1957BAN....14....1V |bibcode-access=free}}</ref> A companion paper by Maarten Schmidt showed that this rotation curve could be fit by a flattened mass distribution more extensive than the light.<ref>{{cite journal |last=Schmidt |first=M|year=1957 |title=Rotation and density distribution of the Andromeda nebula derived from observations of the 21-cm line |journal=[[Bulletin of the Astronomical Institutes of the Netherlands]] |volume=14 |page=17|bibcode=1957BAN....14...17S |bibcode-access=free}}</ref> In 1959, Louise Volders used the same telescope to demonstrate that the spiral galaxy [[Triangulum Galaxy|M33]] also does not spin as expected according to [[Kepler's laws of planetary motion|Keplerian dynamics]].<ref>{{cite journal |last=Volders |first=L. |year=1959 |title=Neutral hydrogen in M 33 and M 101 |journal=[[Bulletin of the Astronomical Institutes of the Netherlands]] |volume=14 |issue=492 |page=323 |bibcode=1959BAN....14..323V |bibcode-access=free}}</ref>

Reporting on [[NGC 3115]], [[Jan Oort]] wrote that "the distribution of mass in the system appears to bear almost no relation to that of light... one finds the ratio of mass to light in the outer parts of NGC 3115 to be about 250".<ref>Oort, J.H. (1940), [http://adsabs.harvard.edu/abs/1940ApJ....91..273O Some Problems Concerning the Structure and Dynamics of the Galactic System and the Elliptical Nebulae NGC 3115 and 4494]</ref>  On page 302–303 of his journal article, he wrote that "The strongly condensed luminous system appears imbedded in a large and more or less homogeneous mass of great density" and although he went on to speculate that this mass may be either extremely faint dwarf stars or interstellar gas and dust, he had clearly detected the dark matter halo of this galaxy.

[[File:Rotation curve (Milky Way).svg|thumb|400px|[[Galaxy rotation curve]] for the Milky Way – vertical axis is speed of rotation about the galactic center; horizontal axis is distance from the galactic center in kpcs; the sun is marked with a yellow ball; the observed curve of speed of rotation is blue; the predicted curve based upon stellar mass and gas in the Milky Way is red; scatter in observations roughly indicated by gray bars, the difference is due to dark matter.<ref name=Koupelis_Kuhn2007>{{cite book |title=In Quest of the Universe |first1=Theo |last1=Koupelis |first2=Karl F. |last2=Kuhn |page=[https://archive.org/details/inquestofunivers00koup/page/492 492], Fig. 16–13 |url=https://archive.org/details/inquestofunivers00koup |url-access=registration |isbn=978-0-7637-4387-1 |date=2007 |publisher=Jones & Bartlett Publishers}}</ref><ref name="Jones1">{{Cite book |last1=Jones |first1=Mark H. |url=https://books.google.com/books?id=36K1PfetZegC&q=Milky+Way+%22rotation+curve%22&pg=PA20 |title=An Introduction to Galaxies and Cosmology |last2=Lambourne |first2=Robert J. |last3=Adams |first3=David John |date=2004 |publisher=Cambridge University Press |isbn=978-0-521-54623-2 |page=21; Fig. 1.13 |access-date=October 27, 2020 |archive-url=https://web.archive.org/web/20230326143523/https://books.google.com/books?id=36K1PfetZegC&q=Milky+Way+%22rotation+curve%22&pg=PA20 |archive-date=March 26, 2023 |url-status=live}}</ref> Note that close to the center the speed is actually proportional to the distance, going to zero at the center, as shown in a reference.<ref name="Schneider">{{Cite book |last=Peter Schneider |url=https://books.google.fr/books?id=uP1Hz-6sHaMC&&pg=PA5 |title=Extragalactic Astronomy and Cosmology |date=2006 |publisher=Springer |isbn=978-3-540-33174-2 |at=page&nbsp;4, Fig.&nbsp;1.4 |access-date=October 27, 2020 |archive-url=https://web.archive.org/web/20230326143010/https://www.google.com/books/edition/Extragalactic_Astronomy_and_Cosmology/uP1Hz-6sHaMC?hl=en&gbpv=1&bsq=rotation+Milky+way&pg=PA100&printsec=frontcover |archive-date=March 26, 2023 |url-status=live}}</ref>]]

The [[Carnegie telescope]] (Carnegie Double Astrograph) was intended to study this problem of Galactic rotation.<ref>{{Cite journal|title=1947PASP...59..182S Page 182|journal=Publications of the Astronomical Society of the Pacific|bibcode=1947PASP...59..182S |last1=Shane |first1=C. D. |year=1947 |volume=59 |issue=349 |page=182 |doi=10.1086/125941 |doi-access=free }}</ref>

Oort also did work on [[Milky Way#Galactic rotation|motion inside the Milky Way]], and tried to determine what are known as the [[Oort constants]], but did not find very accurate values. With space telescopes like [[Hipparcos]] and [[Gaia (spacecraft)|Gaia]] it has been possible to study the rotation of the Milky Way much more accurately.

In the late 1960s and early 1970s, [[Vera Rubin]], an astronomer at the Department of Terrestrial Magnetism at the [[Carnegie Institution of Washington]], worked with 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.<ref name="Rubin1970">{{Cite journal |last1=Rubin |first1=V. |last2=Ford |first2=W. K. Jr. |year=1970 |title= Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions |journal=[[The Astrophysical Journal]] |volume=159 |page=379 |bibcode= 1970ApJ...159..379R |bibcode-access=free |doi= 10.1086/150317 |s2cid=122756867 |doi-access=}}</ref> Together with fellow staff-member [[Kent Ford (astronomer)|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,<ref>{{Cite journal |last1=Rubin |first1=V.C. |last2= Thonnard |first2=N. |last3= Ford  |first3=W.K. Jr. |year=1978 |title= Extended rotation curves of high-luminosity spiral galaxies. IV – Systematic dynamical properties, SA through SC  |journal=[[The Astrophysical Journal Letters]] |volume=225 |pages=L107–L111 |bibcode= 1978ApJ...225L.107R |bibcode-access=free  |doi= 10.1086/182804  |doi-access=}}</ref> and that this implied that galaxy masses grow approximately linearly with radius well beyond the location of most of the stars (the [[galactic bulge]]). Rubin presented her results in an influential paper in 1980.<ref name="Rubin1980">{{Cite journal |last1=Rubin  |first1=V. |last2=Thonnard |first2=N. |last3=Ford  |first3=W. K. Jr.  |year=1980 |title=Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc) |journal=[[The Astrophysical Journal]] |volume=238 |page=471 |bibcode=1980ApJ...238..471R |bibcode-access=free |doi=10.1086/158003 |doi-access=}}</ref> These results suggested either that [[Gravity|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. Although initially met with skepticism, Rubin's results have been confirmed over the subsequent decades.<ref>{{cite journal |last1=Persic |first1=M. |last2=Salucci |first2=P. |last3=Stel |first3=F. |year=1996 |title=The universal rotation curve of spiral galaxies – I. The dark matter connection |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=281 |issue=1 |pages=27–47 |arxiv=astro-ph/9506004 |bibcode=1996MNRAS.281...27P |bibcode-access=free |doi=10.1093/mnras/278.1.27 |doi-access=free}}</ref>

If [[Newtonian mechanics]] is assumed to be correct, it would follow that most of the mass of the galaxy had to be in the galactic bulge near the center and that the stars and gas in the disk portion should orbit the center at decreasing velocities with radial distance from the galactic center (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). It is also observed that galaxies with a uniform distribution of luminous matter have a rotation curve that rises from the center to the edge, and most [[Low-surface-brightness galaxy|low-surface-brightness galaxies]] (LSB galaxies) have the same anomalous rotation curve.

The rotation curves might be explained by hypothesizing the existence of a substantial amount of matter permeating the galaxy outside of the central bulge that is not emitting light in the [[Mass-to-light ratio|mass-to-light]] ratio of the central bulge. 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 cluster]]s.