Dark matter halo

Simulated dark matter halo from a cosmological N-body simulation

A dark matter halo is a hypothetical component of a galaxy, which extends beyond the edge of the visible galaxy and dominates the total mass. Since they consist of dark matter, halos cannot be observed directly, but their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution.

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. 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 or perhaps a modification of the law of gravity.^[1][2][3]

Rotation curves as evidence of a dark matter halo

The presence of dark matter in the halo is demonstrated by its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass in the extended halo, the rotational velocity of the galaxy should decrease at large distance from the galactic core. However, observations of spiral galaxies, particularly radio observations of line emission from neutral atomic hydrogen (known, in astronomical parlance, as HI), show that the rotation curve of most spiral galaxies remains flat far beyond the visible matter. The absence of any visible matter to account for these observations implies the presence of unobserved (i.e. dark) matter. Asserting that this dark matter does not exist would mean that the accepted theory of gravitation (General Relativity) is incomplete, and while that could be possible, most scientists would require extensive amounts of compelling evidence before seriously considering it.

The Navarro-Frenk-White profile:^[4]

$\rho(r)=\frac{\rm constant}{(r/a)(1+r/a)^2}$

is often used to model the distribution of mass in dark matter halos. Theoretical dark matter halos produced in computer simulations are best described by the Einasto profile:^[5]

$\rho(r) = \rho_0 e^{-\alpha r^n}.$

Theories about the nature of dark matter

The nature of dark matter in the galactic halo of spiral galaxies is still undetermined, but there are two popular theories: either the halo is composed of weakly interacting elementary particles known as WIMPs, or it is home to large numbers of small, dark bodies known as MACHOs. It seems unlikely that the halo is composed of large quantities of gas and dust, because both ought to be detectable through observations. Searches for gravitational microlensing events in the halo of the Milky Way show that the number of MACHOs is likely not sufficient to account for the required mass.

See also: dark matter

Milky Way dark matter halo

The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity. The dark matter halo is the location of nearly all of the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy. The luminous matter makes up approximately 90,000,000,000 (9 x 10¹⁰) solar masses. The dark matter halo is likely to include around 600,000,000,000 (6 x 10¹¹) to 3,000,000,000,000 (3 x 10¹²) solar masses of dark matter.^[6] The Milky Way is a large galaxy comprising an estimated 200 billion stars (some estimates range as high as 400 billion) arrayed in the form of a disk, with a central elliptical bulge (some 12,000 light-years in diameter) of closely packed stars lying in the direction of Sagittarius. It is surrounded by a flat disk marked by six spiral arms—four major and two minor—which wind out from the nucleus like a giant pinwheel. Because of these arms, the Milky Way was classified as a spiral galaxy. However, increasing evidence indicates that the Milky Way probably has a bar or barlike structure of new, bright stars in its central region. This would modify its classification to a barred spiral or an intermediate type between barred and “normal” spiral. Our sun is situated in one of the smaller arms, called the Local or Orion Arm, that connect the more substantial next inner arm and the next outer arm. The sun lies roughly two thirds of the way from the center of the disk, which is some 28,000 light-years distant, and in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way. The diameter of the disk is c.100,000 light-years; its average thickness is 10,000 light-years, increasing to 30,000 light-years at the nucleus.

Certain features of the region near the sun suggested that our galaxy resembles the Andromeda Galaxy. In 1951 a group led by William Morgan detected evidence of spiral arms in Orion and Perseus. Another bright arm stretches from Sagittarius to Carina in the southern sky. With the development of radio astronomy, scientists have extended a nearly complete map of the spiral structure of the galaxy by tracing regions of hydrogen that dominate the spiral arms.

Surrounding the galaxy is a large spherical halo of globular star clusters that extends to a diameter of about 130,000 light-years; this is called the stellar halo. The galaxy also has a vast outer spherical region called the corona, or dark halo, which is as much as 600,000 light years in diameter and, in addition to dark matter which accounts for most of the Milky Way's mass, includes some distant globular clusters, the two nearby galaxies called the Magellanic clouds, and four smaller galaxies.

See also

• Galaxy formation and evolution
• Galactic coordinate system
• Disc (galaxy)
• Bulge (astronomy)
• Galactic halo
• Spiral arm
• Dark matter
• Dark galaxy

References

1. ^ Peter Schneider (2006). Extragalactic Astronomy and Cosmology. Springer. p. 4, Figure 1.4. ISBN 3540331743. http://books.google.com/books?id=uP1Hz-6sHaMC&pg=PA100&dq=rotation+Milky+way&lr=&as_brr=0&as_pt=ALLTYPES#PPA5,M1. 
2. ^ Theo Koupelis, Karl F Kuhn (2007). In Quest of the Universe. Jones & Bartlett Publishers. p. 492; Figure 16-13. ISBN 0763743879. http://books.google.com/books?id=6rTttN4ZdyoC&pg=PA491&dq=Milky+Way+%22rotation+curve%22&lr=&as_brr=0&as_pt=ALLTYPES#PPA492,M1. 
3. ^ Mark H. Jones, Robert J. Lambourne, David John Adams (2004). An Introduction to Galaxies and Cosmology. Cambridge University Press. p. 21; Figure 1.13. ISBN 0521546230. http://books.google.com/books?id=36K1PfetZegC&pg=PA20&dq=Milky+Way+%22rotation+curve%22&lr=&as_brr=0&as_pt=ALLTYPES#PPA21,M1. 
4. ^ Navarro, J. et al. (1997), A Universal Density Profile from Hierarchical Clustering
5. ^ Merritt, D. et al. (2006), Empirical Models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models
6. ^ Battaglia et al. (2005), The radial velocity dispersion profile of the Galactic halo: constraining the density profile of the dark halo of the Milky Way

Further reading

• Bertone, Gianfranco (2010). Particle Dark Matter: Observations, Models and Searches. Cambridge University Press. pp. 762. ISBN 13: 9780521763684. 

External links

• Rare Blob Unveiled: Evidence For Hydrogen Gas Falling Onto A Dark Matter Clump? European Southern Observatory (ScienceDaily) July 3, 2006
• Dark Matter Search Experiment , PICASSO Experiment