Great Attractor

Great Attractor
Name	Great Attractor
Caption	Mass concentration region in the Norma Cluster vicinity
Type	Mass concentration / gravitational anomaly
Epoch	J2000
Constell	Norma, Centaurus, Hydra
Distance ly	~150 million
Distance pc	~46 million
Redshift	~0.005–0.02
Mass	≥10^16 M☉ (estimate)
Discovered	1980s
Discoverer	Lauer & Postman; Lynden-Bell et al. (surveys)

Great Attractor

The Great Attractor is a dominant gravitational anomaly in the local Universe inferred from peculiar velocities of galaxies; it represents a large-scale mass concentration that affects the motion of the Local Group and nearby clusters. First suggested in late 20th-century redshift and peculiar-velocity surveys, the feature lies behind the Zone of Avoidance near the constellations Norma, Centaurus, and Hydra and has been studied using multiwavelength surveys and cosmic microwave background analyses.

Overview and Discovery

The idea of a coherent attractor emerged from peculiar-velocity work by teams including Lynden-Bell, Dressler, Faber, Burstein, Mathewson, Dekel, Lauer, and Postman after earlier redshift catalogs such as the Shapley Supercluster and the CfA Redshift Survey raised questions about bulk flows. Observational programs like the Cosmic Microwave Background dipole measurement by the COBE team and galaxy distance indicators developed in the Hubble Space Telescope era sharpened the case that galaxies exhibit a flow toward a region not coinciding with any single rich cluster. Surveys such as the IRAS redshift survey, the 2dF Galaxy Redshift Survey, and the 2MASS project contributed evidence, while X-ray work by ROSAT and later by Chandra X-ray Observatory and XMM-Newton mapped hot gas in suspected concentrations.

Location and Structure

The region implicated lies at low Galactic latitude behind the Milky Way's Zone of Avoidance near the Norma Cluster (Abell 3627), the Centaurus Cluster (Abell 3526), and the Hydra Cluster (Abell 1060). Coordinates cluster roughly toward the constellation Norma and nearby Ara and Triangulum Australe, at an estimated distance of ~150 million light-years (≈46 Mpc). Structure in this zone includes the massive Norma Cluster, the extended Shapley Supercluster complex farther beyond, filamentary connections to the Centaurus–Pavo–Indus Supercluster, and contributions from the rich cluster Abell 3627. Galactic extinction from the Milky Way's disk complicates optical mapping, so infrared and X-ray observations are crucial.

Observational Evidence and Methods

Evidence for the attraction arises from measurements of galaxy peculiar velocities using distance indicators such as the Tully–Fisher relation, the Fundamental Plane for ellipticals, and Type Ia supernovae calibrations tied to the Hubble Space Telescope distance scale. Redshift surveys like CfA Redshift Survey, IRAS Point Source Catalog Redshift Survey, 2dFGRS, and 6dF Galaxy Survey mapped density fields; X-ray surveys by ROSAT, Chandra, and XMM-Newton detected intracluster medium in obscured clusters. Cosmic microwave background work by COBE, WMAP, and Planck provided complementary constraints via the dipole anisotropy and kinetic Sunyaev–Zel'dovich studies connected to clusters like Abell 3627 and the Shapley Concentration. Radio surveys including Parkes Radio Telescope and neutral-hydrogen mapping helped penetrate the Zone of Avoidance, while infrared surveys such as 2MASS and IRAS identified obscured galaxy concentrations.

Influence on Local Group and Cosmic Flows

Peculiar velocity measurements show the Local Group, which includes the Milky Way, Andromeda Galaxy, and satellite systems, participating in a bulk flow that points toward the attractor region. The Local Group's motion relative to the cosmic microwave background is observed as a dipole roughly aligned with flows toward the attractor and the more distant Shapley Supercluster. Analyses involving the Virgo Cluster infall, the Local Supercluster (Virgo-centric flow), and contributions from the Hydra–Centaurus Supercluster quantify competing gravitational pulls; the attractor's mass helps explain deviations from pure Hubble expansion measured by Tully–Fisher and Fundamental Plane distances. Measurements by groups using peculiar-velocity catalogs such as SFI++ and surveys tied to the H0 Key Project have refined the amplitude and scale of the bulk flow.

Relationship to Large-Scale Structure

The attractor resides within the cosmic web of filaments, walls, and voids that include the Local Void, the Perseus–Pisces Supercluster, the Shapley Supercluster, and the Great Wall. Its gravitational contribution is coupled to filamentary links between the Centaurus Cluster and the Shapley Concentration, making it part of a larger anisotropic density field rather than an isolated monopole. Cosmological analyses using ΛCDM-based simulations and surveys like Sloan Digital Sky Survey compare the observed flows with expected large-scale structure contributions, and studies of anisotropic clustering and power spectrum measurements by 2dFGRS and SDSS place the attractor in context of structure formation.

Theories and Interpretations

Interpretations range from a single dominant mass concentration centered near the Norma/Abell 3627 region to a superposition of contributions from the Shapley Concentration, the Centaurus–Pavo–Indus filament, and other obscured clusters. ΛCDM cosmology framed by Dark Matter halos, gravitational instability theory developed by Peebles, and numerical simulations by groups using codes from institutions like Princeton University, Cambridge University, and Max Planck Society support a distributed-source explanation. Alternative analyses debated by researchers such as Lauer, Postman, Dekel, and Courteau have probed whether observed bulk flows exceed ΛCDM expectations; more recent work using Planck's kinetic Sunyaev–Zel'dovich constraints and expanded redshift/distance catalogs tends to favor a combination of local and distant overdensities rather than a single supermassive attractor.

Category:Large-scale structure of the Universe