A dark matter halo is a theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy. The halo's mass dominates the total mass. Thought to consist of dark matter, halos have not been observed directly. 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. The dark matter halo is not fully explained by the presence of massive compact halo objects (MACHOs).
The presence of dark matter (DM) in the halo is inferred from its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass throughout the (roughly spherical) halo, the rotational velocity of the galaxy would decrease at large distances from the galactic center, just as the orbital speeds of the outer planets decrease with distance from the Sun. 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 flattens out, meaning that rotational velocities do not decrease with distance from the galactic center. The absence of any visible matter to account for these observations implies either that unobserved ("dark") matter, first proposed by Ken Freeman in 1970, exist, or that the theory of motion under gravity (General Relativity) is incomplete. Freeman noticed that the expected decline in velocity was not present in NGC 300 nor M33, and considered an undetected mass to explain it. The DM Hypothesis has been reinforced by several studies.
The formation of dark matter halos is believed to have played a major role in the early formation of galaxies. During initial galactic formation, the temperature of the baryonic matter should have still been much too high for it to form gravitationally self-bound objects, thus requiring the prior formation of dark matter structure to add additional gravitational interactions. The current hypothesis for this is based on cold dark matter (CDM) and its formation into structure early in the universe.
The hypothesis for CDM structure formation begins with density perturbations in the Universe that grow linearly until they reach a critical density, after which they would stop expanding and collapse to form gravitationally bound dark matter halos. These halos would continue to grow in mass (and size), either through accretion of material from their immediate neighborhood, or by merging with other halos. Numerical simulations of CDM structure formation have been found to proceed as follows: A small volume with small perturbations initially expands with the expansion of the Universe. As time proceeds, small-scale perturbations grow and collapse to form small halos. At a later stage, these small halos merge to form a single virialized dark matter halo with an ellipsoidal shape, which reveals some substructure in the form of dark matter sub-halos.
The use of CDM overcomes issues associated with the normal baryonic matter because it removes most of the thermal and radiative pressures that were preventing the collapse of the baryonic matter. The fact that the dark matter is cold compared to the baryonic matter allows the DM to form these initial, gravitationally bound clumps. Once these subhalos formed, their gravitational interaction with baryonic matter is enough to overcome the thermal energy, and allow it to collapse into the first stars and galaxies. Simulations of this early galaxy formation matches the structure observed by galactic surveys as well as observation of the Cosmic Microwave Background.
A commonly used model for galactic dark matter halos is the pseudo-isothermal halo:
where denotes the finite central density and the core radius. This provides a good fit to most rotation curve data. However, it cannot be a complete description, as the enclosed mass fails to converge to a finite value as the radius tends to infinity. The isothermal model is, at best, an approximation. Many effects may cause deviations from the profile predicted by this simple model. For example, (i) collapse may never reach an equilibrium state in the outer region of a dark matter halo, (ii) non-radial motion may be important, and (iii) mergers associated with the (hierarchical) formation of a halo may render the spherical-collapse model invalid.
where is a scale radius, is a characteristic (dimensionless) density, and = is the critical density for closure. The NFW profile is called 'universal' because it works for a large variety of halo masses, spanning four orders of magnitude, from individual galaxies to the halos of galaxy clusters. This profile has a finite gravitational potential even though the integrated mass still diverges logarithmically. It has become conventional to refer to the mass of a halo at a fiducial point that encloses an overdensity 200 times greater than the critical density of the universe, though mathematically the profile extends beyond this notational point. It was later deduced that the density profile depends on the environment, with the NFW appropriate only for isolated halos. NFW halos generally provide a worse description of galaxy data than does the pseudo-isothermal profile, leading to the cuspy halo problem.
where r is the spatial (i.e., not projected) radius. The term is a function of n such that is the density at the radius that defines a volume containing half of the total mass. While the addition of a third parameter provides a slightly improved description of the results from numerical simulations, it is not observationally distinguishable from the 2 parameter NFW halo, and does nothing to alleviate the cuspy halo problem.
The collapse of overdensities in the cosmic density field is generally aspherical. So, there is no reason to expect the resulting halos to be spherical. Even the earliest simulations of structure formation in a CDM universe emphasized that the halos are substantially flattened. Subsequent work has shown that halo equidensity surfaces can be described by ellipsoids characterized by the lengths of their axes.
Because of uncertainties in both the data and the model predictions, it is still unclear whether the halo shapes inferred from observations are consistent with the predictions of ΛCDM cosmology.
Up until the end of the 1990s, numerical simulations of halo formation revealed little substructure. With increasing computing power and better algorithms, it became possible to use greater numbers of particles and obtain better resolution. Substantial amounts of substructure are now expected. When a small halo merges with a significantly larger halo it becomes a subhalo orbiting within the potential well of its host. As it orbits, it is subjected to strong tidal forces from the host, which cause it to lose mass. In addition the orbit itself evolves as the subhalo is subjected to dynamical friction which causes it to lose energy and angular momentum to the dark matter particles of its host. Whether a subhalo survives as a self-bound entity depends on its mass, density profile, and its orbit.
As originally pointed out by Hoyle and first demonstrated using numerical simulations by Efstathiou & Jones, asymmetric collapse in an expanding universe produces objects with significant angular momentum.
Numerical simulations have shown that the spin parameter distribution for halos formed by dissipation-less hierarchical clustering is well fit by a log-normal distribution, the median and width of which depend only weakly on halo mass, redshift, and cosmology:
with and . At all halo masses, there is a marked tendency for halos with higher spin to be in denser regions and thus to be more strongly clustered.
The visible disk of the Milky Way Galaxy is embedded in a much larger, roughly spherical halo of dark matter. The dark matter density drops off with distance from the galactic center. 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 luminous matter makes up approximately 9 x 1010 solar masses. The dark matter halo is likely to include around 6 x 1011 to 3 x 1012 solar masses of dark matter.
The 215th meeting of the American Astronomical Society (AAS) took place in Washington, D.C., Jan. 3 to Jan. 7, 2010. It is one of the largest astronomy meetings ever to take place as 3,500 astronomers and researchers were expected to attend and give more than 2,200 scientific presentations. The meeting was actually billed as the "largest Astronomy meeting in the universe". An array of discoveries were announced, along with new views of the universe that we inhabit; such as quiet planets like Earth - where life could develop are probably plentiful, even though an abundance of cosmic hurdles exist - such as experienced by our own planet in the past.Axion Dark Matter Experiment
The Axion Dark Matter Experiment (ADMX, also written as Axion Dark Matter eXperiment in the project's documentation) uses a resonant microwave cavity within a large superconducting magnet to search for cold dark matter axions in the local galactic dark matter halo. Unusually for a dark matter detector, it is not located deep underground. Sited at the Center for Experimental Nuclear Physics and Astrophysics (CENPA) at the University of Washington, ADMX is a large collaborative effort with researchers from universities and laboratories around the world.CoGeNT
The CoGeNT experiment has searched for dark matter. It uses a single germanium crystal (~100 grams) as a cryogenic detector for WIMP particles. CoGeNT has operated in the Soudan Underground Laboratory since 2009.DAMA/LIBRA
The DAMA/LIBRA experiment
is a particle detector experiment designed to detect dark matter using the direct detection approach, by using a matrix of NaI(Tl) scintillation detectors to detect dark matter particles in the galactic halo. The experiment aims to find an annual modulation of the number of detection events, caused by the variation of the velocity of the detector relative to the dark matter halo as the Earth orbits the Sun. It is located underground at the Laboratori Nazionali del Gran Sasso in Italy.
It is a follow-on to the DAMA/NaI experiment which observed an annual modulation signature over 7 annual cycles (1995-2002).Directional Recoil Identification from Tracks
The Directional Recoil Identification from Tracks (DRIFT) detector is a low pressure negative ion time projection chamber (NITPC) designed to detect weakly interacting massive particles (WIMPs) - a prime dark matter candidate.There are currently two DRIFT detectors in operation. DRIFT-IId, which is located 1100m underground in the Boulby Underground Laboratory at the Boulby Mine in North Yorkshire, England, and DRIFT-IIe, which is located on the surface at Occidental College, Los Angeles, CA, USA.
The DRIFT collaboration ultimately aims to develop and operate an underground array of DRIFT detectors for observing and reconstructing WIMP-induced nuclear recoil tracks with enough precision to provide a signature of the dark matter halo.Galactic halo
A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of galaxies comprise the halo:
the stellar halo
the galactic corona (hot gas, i.e. a plasma)
the dark matter haloThe distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape of the halo contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the other components of the galaxy and the halo.Galaxy And Mass Assembly survey
The Galaxy And Mass Assembly (GAMA) survey is a project to exploit the latest generation of ground-based wide-field survey facilities to study cosmology and galaxy formation and evolution. GAMA will bring together data from a number of world class instruments:
The Anglo-Australian Telescope (AAT),
The VLT Survey Telescope (VST)
The Visible and Infrared Survey Telescope for Astronomy (VISTA)
The Australian Square Kilometre Array Pathfinder (ASKAP)
The Herschel Space Observatory
The Galaxy Evolution Explorer (GALEX)Data from these instruments will be used to construct a state-of-the-art multi-wavelength database of ~375,000 galaxies in the local Universe over a 360 deg2 region of sky,
based on a spectroscopic redshift survey on the AAT's AAOmega spectrograph.
The main objective of GAMA is to study structure on scales of 1 kpc to 1 Mpc. This includes galaxy clusters, groups, mergers and coarse measurements of galaxy structure (i.e., bulges and discs). It is on these scales where baryons play a critical role in the galaxy formation and subsequent evolutionary processes and where our understanding of structure in the Universe breaks down.
GAMA's primary goal is to test the CDM paradigm of structure formation. In particular, the key scientific objectives are:
A measurement of the dark matter halo mass function of groups and clusters using group velocity dispersion measurements.
A comprehensive determination of the galaxy stellar mass function to Magellanic Cloud masses to constrain baryonic feedback processes.
A direct measurement of the recent galaxy merger rates as a function of mass, mass ratio, local environment and galaxy type.In August 2012 GAMA received worldwide attention for its announcement of a galaxy system very similar to our own Milky-Way Magellanic Cloud system, centred on GAMA202627.Galaxy rotation curve
The rotation curve of a disc galaxy (also called a velocity curve) is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived from theory. The theory of dark matter is currently postulated to account for the variance.Halo occupation distribution
The halo occupation distribution (HOD) is a parameter of the halo model of galaxy clustering. The halo model provides one view of the large scale structure of the universe as clumps of dark matter, while the HOD provides a view of how galactic matter is distributed within each of the dark matter clumps. The HOD is used to describe three related properties of the halo model: the probability distribution relating the mass of a dark matter halo to the number of galaxies that form within that halo; the distribution in space of galactic matter within a dark matter halo; the distribution of velocities of galactic matter relative to dark matter within a dark matter halo.Messier 63
Messier 63 or M63, also known as NGC 5055 or the seldom-used Sunflower Galaxy, is a spiral galaxy in the northern constellation of Canes Venatici. M63 was first discovered by the French astronomer Pierre Méchain then later verified by his colleague Charles Messier on June 14, 1779. The galaxy became listed as object 63 in the Messier Catalogue. In the mid-19th century, Anglo-Irish astronomer Lord Rosse identified spiral structures within the galaxy, making this one of the first galaxies in which such structure was identified.This galaxy has a morphological classification of SAbc, indicating a spiral shape with no central bar feature and moderate to loosely wound arms. There is a general lack of large scale continuous spiral structure in visible light, a galaxy form known as flocculent. However, when observed in the near infrared a symmetric, two-arm structure becomes apparent. Each arm wraps 150° around the galaxy and extends out to 13 kly (4 kpc) from the nucleus.M63 is an active galaxy with a LINER nucleus. This displays as an unresolved nuclear source wrapped in a diffuse emission. The latter is extended along a position angle of 110° and soft X-rays and H-alpha emission can be observed coming from along nearly the same direction. The existence of a super massive black hole (SMBH) at the nucleus is uncertain; if it does exist, then the mass is estimated as (8.5±1.9)×108 M☉.Radio observations at 21-cm show the gaseous disk of M63 extending outward to a radius of 40 kpc (130 kly), well past the bright optical disk. This gas shows a symmetrical form that is warped in a pronounced manner, starting at a radius of 10 kpc (33 kly). The form suggests the dark matter halo of the galaxy is offset with respect to the inner region. The reason for the warp is unclear, but the position angle points toward the smaller companion galaxy, UGC 8313.The distance to M63, based upon the luminosity-distance measurement is 8.99 Mpc (29.3 Mly). The radial velocity relative to the Local Group yields an estimate of 4.65 Mpc (15.2 Mly). Estimates based on the Tully-Fisher relation range over 5.0–10.3 Mpc (16–34 Mly). The tip of the red-giant branch technique gives a distance of 8.87 ± 0.29 Mpc (28.93 ± 0.95 Mly). M63 is part of the M51 Group, a group of galaxies that also includes M51 (the 'Whirlpool Galaxy').In 1971, a supernova with a magnitude of 11.8 appeared in one of the arms of M63. It was discovered May 24, 1971 and reached peak light around May 26. The spectrum of SN 1971 I is consistent with a supernova of type I. However, the spectroscopic behavior appeared anomalous.Messier 84
Messier 84 or M84, also known as NGC 4374, is an elliptical or lenticular galaxy in the constellation Virgo. Charles Messier discovered Messier 84 on 18 March 1781 in a systematic search for "nebulous objects" in the night sky. The object is the 84th in the Messier Catalogue. M84 is situated in the heavily populated inner core of the Virgo Cluster of galaxies.This is a giant elliptical galaxy with a morphological classification of E1, indicating a flattening of 10%. The half-light radius is 72.5″ and the extinction-corrected total luminosity in the visual band is 7.64×1010 L☉. The central mass-to-light ratio is 6.5, which steadily increases away from the core. The visible galaxy is surrounded by a massive dark matter halo.Radio observations and Hubble Space Telescope images of M84 have revealed two jets of matter shooting out from the galaxy's center as well as a disk of rapidly rotating gas and stars indicating the presence of a 1.5 ×109 M☉ supermassive black hole. It also has a few young stars and star clusters, indicating star formation at a very low rate. The number of globular clusters is 1,775±150, which is much lower than expected for an elliptical galaxy.Two supernovae have been observed in M84: SN 1957
and SN 1991bg. Possibly, a third, SN 1980I is part of M84 or, alternatively, one of its neighboring galaxies, NGC 4387 and M86. This high rate of supernova events is rare for elliptical galaxies, which may indicate there is a population of stars of intermediate age in M84.NGC 1288
NGC 1288 is an intermediate barred spiral galaxy located about 196 million light years away in the constellation Fornax. In the nineteenth century, English astronomer John Herschel described it as "very faint, large, round, very gradually little brighter middle." The morphological classification of SABc(rs) indicates weak bar structure across the nucleus (SAB), an incomplete inner ring orbiting outside the bar (rs), and the multiple spiral arms are moderately wound (c). The spiral arms branch at intervals of 120° at a radius of 30″ from the nucleus. The galaxy is most likely surrounded by a dark matter halo, giving it a mass-to-light ratio of 14 M☉/L☉.On July 17, 2006, a supernova with a magnitude of 16.1 was imaged in this galaxy from Pretoria, South Africa, at 12″ east and 2″ of the galactic core. Designated SN 2006dr, it was determined to be a type Ia supernova.NGC 3311
NGC 3311 is a supergiant elliptical galaxy (a type-cD galaxy) located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.NGC 3311 is surrounded by a rich and extensive globular cluster system rivaling that of Messier 87 in the Virgo Cluster.NGC 4555
NGC 4555 is a solitary elliptical galaxy about 40,000 parsecs (125,000 light-years) across, and about 310 million light years distant. Observations by the Chandra X-ray Observatory have shown it to be surrounded by a halo of hot gas about 120,000 parsecs across. The hot gas has a temperature of around 10,000,000 kelvins. The galaxy is one of the few elliptical galaxies proven to have significant amounts of dark matter. Large amounts of dark matter are necessary to prevent the gas from escaping the galaxy; the visible mass clearly is not large enough to hold such an extensive gas halo. The dark matter halo is estimated to have 10 times the mass of the stars in the galaxy.
NGC 4555 is important because of its isolation. Most elliptical galaxies are found in the cores of groups and clusters of galaxies, and almost all those for which dark matter estimates are available are located in the centres of these larger systems. In these circumstances it impossible to know whether the dark matter is associated with the galaxy or the surrounding cluster. NGC 4555, as a field galaxy is not part of any group or cluster, and therefore provides strong evidence that dark matter can be associated with individual ellipticals.
Despite being isolated, NGC 4555 is part of the Coma Supercluster.NGC 660
NGC 660 is a peculiar and unique polar-ring galaxy located approximately 45 million light years from Earth in the Pisces constellation. It is the only such galaxy having, as its host, a "late-type lenticular galaxy". It was probably formed when two galaxies collided a billion years ago. However, it may have first started as a disk galaxy that captured matter from a passing galaxy. This material could have, over time, become "strung out" to form a rotating ring.
The ring is not actually polar, but rather has an inclination from the plane of the host disk of approximately 45 degrees. The extreme number of pinkish star-forming areas that occurs along the galaxy's ring could be the result of the gravitation interaction caused by this collision. The ring is 50,000 light-years across - much broader than the disk itself - and has a greater amount of gas and star formation than the host ring. This likely indicates a very violent formation. The polar ring contains objects numbering in the hundreds. Many of these are red and blue supergiant stars. The most recently created stars in the ring were just formed approximately 7 million years ago. This indicates that the formation of these stars has been a long process and is still occurring.
Data about the dark matter halo of NGC 660 can be extracted by observing the gravitational effects of the dark matter on the disk and ring's rotation. From the core of the disk, radio waves are being emitted. The source of these waves is an area only 21 light years across. This may indicate the presence of a super-cluster of stars located within an area of cloud of gas.
The region in the centre has a vast amount of star formation, so luminous that it is considered to be a starburst galaxy.Late in 2012, this polar-ring galaxy produced an enormous outburst having a magnitude of approximately ten times brighter than a supernova explosion. The cause is not certain, but this event may have resulted from a tremendous jet being emenating from galaxy's central black hole.NGC 660 is a member of the M74 Group.NGC 720
NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of circa 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.Tully–Fisher relation
In astronomy, the Tully–Fisher relation (TFR) is an empirical relationship between the mass or intrinsic luminosity of a spiral galaxy and its asymptotic rotation velocity or emission line width. It was first published in 1977 by astronomers R. Brent Tully and J. Richard Fisher. The luminosity is calculated by multiplying the galaxy's apparent brightness by 4πd2, where d is its distance from us, and the spectral-line width is measured using long-slit spectroscopy.
Several different forms of the TFR exist, depending on which precise measures of mass, luminosity or rotation velocity one takes it to relate. Tully and Fisher used optical luminosity, but subsequent work showed the relation to be tighter when defined using microwave to infrared (K band) radiation (a good proxy for stellar mass), and even tighter when luminosity is replaced by the galaxy's total baryonic mass (the sum of its mass in stars and gas). This latter form of the relation is known as the Baryonic Tully–Fisher relation (BTFR), and states that baryonic mass is proportional to velocity to the power of roughly 3.5–4.The TFR can be used to estimate the distance to spiral galaxies by allowing the luminosity of a galaxy to be derived from its directly measurable line width. The distance can then be found by comparing the luminosity to the apparent brightness. Thus the TFR constitutes a rung of the cosmic distance ladder, where it is calibrated using more direct distance measurement techniques and used in turn to calibrate methods extending to larger distance.
In the dark matter paradigm, a galaxy's rotation velocity (and hence line width) is primarily determined by the mass of the dark matter halo in which it lives, making the TFR a manifestation of the connection between visible and dark matter mass. In Modified Newtonian dynamics (MOND), the BTFR (with power-law index exactly 4) is a direct consequence of the gravitational force law effective at low acceleration.The analogues of the TFR for non-rotationally-supported galaxies, such as ellipticals, are known as the Faber–Jackson relation and the fundamental plane.VIRGOHI21
VIRGOHI21 is an extended region of neutral hydrogen (HI) in the Virgo cluster discovered in 2005. Analysis of its internal motion indicates that it may contain a large amount of dark matter, as much as a small galaxy. Since VIRGOHI21 apparently contains no stars, this would make it one of the first detected dark galaxies. Skeptics of this interpretation argue that VIRGOHI21 is simply a tidal tail of the nearby galaxy NGC 4254.Virial mass
In astrophysics, the virial mass is the mass of a gravitationally bound astrophysical system, assuming the virial theorem applies. In the context of galaxy formation and dark matter halos, the virial mass is defined as the mass enclosed within the virial radius of a gravitationally bound system, a radius within which the system obeys the virial theorem. The virial radius is determined using a "top-hat" model. A spherical "top hat" density perturbation destined to become a galaxy begins to expand, but the expansion is halted and reversed due to the mass collapsing under gravity until the sphere reaches virial equilibrium–it is said to be virialized. Within this radius, the sphere obeys the virial theorem which says that the average kinetic energy is equal to minus one half times the average potential energy, , and this radius defines the virial radius.
|Forms of dark matter|
|Theories and objects|
|Potential dark galaxies|