# Galaxy rotation curve

(Redirected from Galaxy rotation problem)
Rotation curve of the typical spiral galaxy M 33 (yellow and blue points with errorbars) and the predicted one from distribution of the visible matter (white line). The discrepancy between the two curves is accounted for by adding a dark matter halo surrounding the galaxy.[1]

The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the magnitude of the orbital velocities (i.e., the speeds) of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre, typically rendered graphically as a plot.

A general feature of the galaxy rotation curves that have been obtained through measurement to date is that the orbital speed of stars and gas is rising or almost constant as far from the galactic centre as it can be measured: that is, stars are observed to revolve around the centre of the galaxy at increasing or the same speed over a large range of distances from the centre of the galaxy. If disc galaxies have mass distributions similar to the observed distributions of stars and gas then, the orbital speed would always decline at increasing distances in the same way as do other systems with most of their mass in the centre, such as the Solar System or the moons of Jupiter.

The rotation curves of spiral galaxies are also known to be asymmetric. The observational data from each side of a galaxy are generally averaged. RC asymmetry appears to be normal rather than exceptional.[2]

The galaxy rotation problem is the discrepancy between observed galaxy rotation curves and the theoretical prediction, assuming a centrally dominated mass associated with the observed luminous material. When mass profiles of galaxies are calculated from the luminosity profiles and mass-to-light ratios in the stellar disks, then they do not match with the masses derived from the observed rotation curves and the law of gravity This discrepancy can be accounted for by postulating 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.[3]

A. Bosma

## History and description of the galaxy rotation problem

Vera (Cooper) Rubin (born July 23, 1928) is an American astronomer who pioneered work on galaxy rotation curves. She uncovered what we call it as "Dark Matter".

In 1932, Jan Hendrik Oort became the first to report measurements that the stars in the Solar neighborhood moved faster than expected when a mass distribution based upon visible matter was assumed, but this measurement was later determined to be essentially erroneous.[4] In 1933, Fritz Zwicky postulated "missing mass" to account for the dispersion velocities of galaxies in clusters. 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.[5] 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.[6] 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 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.[7] 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,[8] which implied that their mass densities were approximately constant well beyond the location of most of the stars (the galactic bulge) Rubin presented her results in an influential paper in 1980.[9] 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. A complete vindication of Rubin (and Bosma) view and discovery was in a study of 967 Rotation curves.[10]

In 1972, Zwicky was awarded the Gold Medal of the Royal Astronomical Society for "distinguished contributions to astronomy and cosmology".

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). It is also observed that galaxies with a uniform distribution of luminous matter have a rotation curve that slopes up from the center to the edge, and most low-surface-brightness galaxies (LSB galaxies) rotate with a rotation curve that slopes up from the center, indicating little core bulge.

The rotation curves can be explained if there is a substantial amount of matter permeating the galaxy that is not emitting light in the 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 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.

## Halo density profiles

In order to accommodate a flat rotation curve, a density profile for galactic environs must be different than one that is centrally concentrated. Newton's version of Kepler's Third Law states that the radial density profile ρ(r) equals

$\rho(r) = \frac{3 v(r)^2 }{4 \pi G r^2}(1+2~ dlog~ v(r)/dlog~ r)$

where v(r) is the radial orbital velocity profile and G is the gravitational constant. This profile closely matches the expectations of a singular isothermal sphere profile where if v(r) is approximately constant then the density $\rho \sim r^{-2}$ to some inner "core radius" where the density leveled off to a constant. Observations did not comport with such a simple profile as reported by Navarro, Frenk, and White in a seminal 1996 paper.[11]

The authors then remarked, as did a few others before them, that a "gently changing logarithmic slope" for a density profile could also accommodate approximately flat rotation curves over large scales. They found the famous Navarro–Frenk–White profile which is consistent both with N-body simulations and observations given by

$\rho (r)=\frac{\rho_0}{\frac{r}{R_s}\left(1~+~\frac{r}{R_s}\right)^2}$

where the central density, ρ0, and the scale radius, Rs, are parameters that vary from halo to halo. Because the slope of the density profile diverges at the center, other alternative profiles have been proposed, for example, the Einasto profile which has exhibited better agreement with certain dark matter halo simulations.[12]

## Further investigations

The rotational dynamics of galaxies are, in fact, extremely well characterized by their position on the Tully-Fisher relation which shows that for spiral galaxies that rotational velocity is uniquely related to its total luminosity with essentially no scatter. A consistent way to predict the rotational velocity of a spiral galaxy is to measure its bolometric luminosity and then extrapolate its rotation curve from its location on the Tully-Fisher diagram. Likewise, knowing the rotational velocity of a spiral galaxy is an excellent indication of its luminosity. Thus the amplitude of the galaxy rotation curve is related to the galaxy's visible mass.[13]

While precise fitting bulge, disk, and halo density profiles is a rather complicated process, it is straightforward to model the observables of rotating galaxies through this relationship.[14] So, while state-of-the-art cosmological and galaxy formation simulations of dark matter with normal baryonic matter included can be matched to galaxy observations, there is not yet any straightforward explanation as to why the scaling relationship exists as observed.[15][16] Additionally, detailed investigations of the rotation curves of low-surface-brightness galaxies (LSB galaxies) in the 1990s[17] and of their position on the Tully-Fisher relation[18] 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.

Very importantly, the analysis of the inner parts of low and high surface brightness galaxies showed that the shape of the rotation curves in the centre of dark-matter dominated systems, indicated a profile that differed from the NFW spatial mass distribution profile.[19] This so-called cuspy halo problem is a persistent problem for the standard cold dark matter theory. Simulations involving the feedback of stellar energy into the interstellar medium in order to alter the predicted dark matter distribution in the innermost regions of galaxies are frequently invoked in this context.[20]

## Alternatives to dark matter

MOND was proposed by Mordehai Milgrom in 1983
Mariafelicia De Laurentis (Univ. of Tomsk)

There have been a number of attempts to solve the problem of galaxy rotation curves by modifying gravity without invoking dark matter. One of the most discussed is MOND (Modified Newtonian Dynamics), originally proposed by Mordehai Milgrom in 1983, which modifies the Newtonian force law at low accelerations to enhance the effective gravitational attraction. MOND has had a considerable amount of success in predicting the rotation curves of low surface brightness galaxies,[21] the so-called baryonic Tully-Fisher relation,[22] and the velocity dispersions of the small satellite galaxies of the Local Group.[23] These results are surprising in the context of dark matter, which does not predict the same things as MOND without considerable fine-tuning.[citation needed]

MOND is not a relativistic theory, although relativistic theories which reduce to MOND have been proposed, such as tensor–vector–scalar gravity,[3][24] and scalar–tensor–vector gravity (STVG), of John Moffat and the f(R) theory of Capozziello and De Laurentis.[25]

## Footnotes

1. ^ Data are from: E. Corbelli, P. Salucci (2000). "The extended rotation curve and the dark matter halo of M33". Monthly Notices of the Royal Astronomical Society 311 (2): 441–447.. The explanation of the mass discrepancy in spiral galaxies by means of massive and extensive dark component was first put forward by: 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 DatabaseandV. 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)". The Astrophysical Journal.. See also: 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..
2. ^ Jog, C. J., 2002, “Large-scale asymmetry of rotation curves in lopsided spiral galaxies”, A&A, 391,471.
3. ^ 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]..
4. ^ Kuijken K., Gilmore G., 1989a, MNRAS, 239, 651.
5. ^ Babcock, H, 1939, “The rotation of the Andromeda Nebula”, Lick Observatory bulletin; no. 498
6. ^ L. Volders. "Neutral hydrogen in M 33 and M 101". Bulletin of the Astronomical Institutes of the Netherlands 14 (492): 323.
7. ^ V. Rubin, W. K. Ford, Jr (1970). "Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions". Astrophysical Journal 159: 379.
8. ^ Rubin, V. C., Thonnard, N., and Ford, W. K., Jr., 1978, ApJ 225 L107-L111, http://adsabs.harvard.edu/abs/1978ApJ...225L.107R
9. ^ 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.
10. ^ M. Persic, P. Salucci, F. Stel (1996). "The universal rotation curve of spiral galaxies - I. The dark matter connection". Monthly Notices of the Royal Astronomical Society 281 (1): 27–47. arXiv:astro-ph/9506004.
11. ^ Navarro, Julio F.; Frenk, Carlos S.; White, Simon D. M. (1996). "The Structure of Cold Dark Matter Halos". The Astrophysical Journal 463: 563. arXiv:astro-ph/9508025. doi:10.1086/177173.
12. ^ Merritt, David; Graham, Alister; Moore, Benjamin; Diemand, Jurg; Terzić, Balsa (20 December 2006). "Empirical Models for Dark Matter Halos". The Astronomical Journal 132: 2685. arXiv:astro-ph/0509417.
Merritt, David; et al. (May 2005). "A Universal Density Profile for Dark and Luminous Matter?". The Astrophysical Journal 624: L85–L88. arXiv:astro-ph/0502515.
13. ^ I.A. Yegorova, P. Salucci. "The radial Tully-Fisher relation for spiral galaxies - I". Monthly Notices of the Royal Astronomical Society 377 (2): 507–515.
14. ^ Reliance on Indirect Evidence Fuels Dark Matter Doubts: Scientific American
15. ^ Weinberg, David H.; et, al. (2008). "Baryon Dynamics, Dark Matter Substructure, and Galaxies". The Astrophysical Journal 678 (1): 6–21. Bibcode:2008ApJ...678....6W.
16. ^ 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. arXiv:1001.3447. Bibcode:2010MNRAS.405.2161D.
17. ^ 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. arXiv:astro-ph/9704274. available online at the Smithsonian/NASA Astrophysics Data System
18. ^ 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. available online at the Smithsonian/NASA Astrophysics Data System
19. ^ G. Gentile, P. Salucci, U. Klein, D. Vergani, P. Kalberla. "The cored distribution of dark matter in spiral galaxies". Monthly Notices of the Royal Astronomical Society 351 (3): 903–922. arXiv:astro-ph/0403154. Bibcode:2004MNRAS.351..903G..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. available online at the Smithsonian/NASA Astrophysics Data System
20. ^ P. Salucci, M. De Laurentis (2013). Dark Matter in galaxies: leads to its Nature. arXiv:1302.2268. Bibcode:2013arXiv1302.2268S.. de Blok, W. G. The Core Cusp Problem. "Dwarf Galaxy Cosmology" special issue of Advances in Astrophysics. 2009. [1].
21. ^ S. S. McGaugh, W. J. G. de Blok (1998). "Testing the Hypothesis of Modified Dynamics with Low Surface Brightness Galaxies and Other Evidence". Astrophysical Journal 499 (1): 66. arXiv:astro-ph/9801102.
22. ^ S. S. McGaugh (2011). "Novel Test of Modified Newtonian Dynamics with Gas Rich Galaxies". Physical Review Letters 106 (12): 121303. arXiv:1102.3913.
23. ^ S. S. McGaugh, M. Milgrom (2013). Andromeda Dwarfs in Light of Modified Newtonian Dynamics 766 (1). p. 22. arXiv:1301.0822.
24. ^ J. D. Bekenstein (2004). "Relativistic gravitation theory for the modified Newtonian dynamics paradigm". he FPhysical Review D 70 (8): 083509. arXiv:astro-ph/0403694.
25. ^ J. W. Moffat (2006). "Scalar tensor vector gravity theory". Journal of Cosmology and Astroparticle Physics 3 (03): 4. arXiv:gr-qc/0506021..S. Capozziello, M. De Laurentis (2012). "The dark matter problem from f(R) gravity viewpoint". Annalen der Physik 524 (9-10): 545–578. Bibcode:2012AnP...524..545C. doi:10.1002/andp.201200109..