Local Sheet

Local Sheet
Name	Local Sheet
Type	galaxy structure
Members	Milky Way, Andromeda Galaxy, Triangulum Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, M33, M31
Major components	Milky Way, Andromeda Galaxy, Maffei 1, Centaurus A, Sculptor Group
Distance	~0–10 Mpc (approx.)
Epoch	J2000

Local Sheet The Local Sheet is a flattened arrangement of nearby galaxies in which the Milky Way, Andromeda Galaxy, Triangulum Galaxy, and numerous dwarf galaxies lie. It forms a coherent planar structure distinct from the wider distribution of galaxies in the Virgo Supercluster and the Local Supercluster, and it influences the peculiar velocities of galaxies including members of the Local Group and nearby groups like the M81 Group and Sculptor Group. Studies of the Local Sheet inform models of galaxy formation, cosmic flow, and the anisotropic distribution of matter in the nearby universe.

Overview

The Local Sheet is identified as a low-dispersion, thin sheet of galaxies roughly centered on the position of the Milky Way and oriented within a few degrees of the supergalactic plane defined by de Vaucouleurs and collaborators. Observationally it is characterized by a small velocity dispersion compared with the surrounding field and by an overdensity of bright spirals including M31 and M33. The sheet spans several megaparsecs and contains substructures such as the Local Group, the Maffei Group, and the nearby NGC 253-dominated Sculptor complex. Its geometry has been used to define reference frames for peculiar velocity studies like those employing the Cosmicflows project.

Structure and Composition

The Local Sheet comprises a mixture of massive spirals, lenticulars, ellipticals, and numerous dwarfs and irregulars. Prominent massive members include the Milky Way, Andromeda Galaxy, Triangulum Galaxy, Maffei 1, and Centaurus A where projection permits inclusion, while the population of satellites encompasses systems such as the Large Magellanic Cloud, Small Magellanic Cloud, and dwarf spheroidals like Sculptor Dwarf Galaxy and Fornax Dwarf Galaxy. The sheet’s stellar mass budget is dominated by large spirals, whereas its number counts are dominated by faint dwarfs discovered in surveys from instruments and teams such as the Sloan Digital Sky Survey, Pan-STARRS, and the Dark Energy Survey. Gas-rich members like NGC 300 and IC 342 contribute to the neutral hydrogen census mapped by facilities including the Arecibo Observatory and the Very Large Array.

Dynamics and Kinematics

Kinematically, galaxies in the Local Sheet exhibit coherent motions with small velocity dispersion relative to the sheet’s mean motion, a property used to identify the plane against the larger scale flows toward the Virgo Cluster and the dipole induced by the Great Attractor. The peculiar velocities of sheet members have been analyzed via distance indicators such as the Cepheid variables, Tip of the Red Giant Branch, and Type Ia supernova calibrations, yielding constraints on bulk flow and shear. The relative motion between the Milky Way and Andromeda Galaxy (approach velocity ~110 km/s) and the orbital kinematics of satellite systems such as the Magellanic Clouds are interpreted in the context of the sheet’s tidal field and angular momentum transfer. Numerical studies using cosmological simulations from groups working with codes like Gadget-2 probe stability, thickness, and velocity anisotropy of analogous sheets.

Formation and Evolution

The Local Sheet is thought to arise from anisotropic collapse and the cosmic web evolution driven by initial density perturbations traced by the Cosmic Microwave Background anisotropies measured by experiments such as WMAP and Planck. Sheets form at the intersection of filaments in Lambda Cold Dark Matter models explored by teams using simulations like the Millennium Simulation and Illustris projects, and the Local Sheet likely represents a locally coherent outcome of such processes. Its evolution involves accretion of dwarfs, minor mergers (events similar in character to those inferred for the Andromeda Galaxy), and feedback processes tied to star formation episodes documented in systems like M82. Over cosmic time the sheet’s flattening, rotational properties, and galaxy population have been modulated by interactions with structures such as the Virgo Cluster and the nearby Local Void.

Relationship to Local Group and Nearby Structures

The Local Sheet contains the Local Group as a prominent substructure; the Local Group’s dynamics are influenced by the sheet’s gravitational potential and bulk motion. Nearby galaxy associations such as the M81 Group, Sculptor Group, and the Centaurus A Group relate to the sheet through spatial adjacency and shared large-scale flows toward attractors like the Virgo Cluster and the Shapley Supercluster. The Local Void, identified in redshift surveys by observers using instruments like the Two Micron All Sky Survey, borders the sheet and contributes to anisotropic expansion in the local volume. Comparative studies contrast the Local Sheet with other nearby planar arrangements such as the planes of satellites observed around Centaurus A and M31.

Observational History and Methods

Historical recognition of a flattened local galaxy distribution dates to work by de Vaucouleurs and subsequent redshift surveys using instruments at facilities like the Palomar Observatory and the Cerro Tololo Inter-American Observatory. Modern mapping employs distance ladders based on Cepheid variables, RR Lyrae, Surface Brightness Fluctuations, and Tip of the Red Giant Branch measurements from teams using the Hubble Space Telescope, Gaia, and ground-based telescopes. Large redshift and photometric surveys conducted by collaborations such as 2dF Galaxy Redshift Survey, Sloan Digital Sky Survey, and Cosmicflows provide three-dimensional positions and velocities used to delineate the sheet and quantify flow patterns. Radio observations of neutral hydrogen with arrays like the Australian Square Kilometre Array Pathfinder and the Karl G. Jansky Very Large Array complement optical work by revealing gas-rich dwarfs and kinematic signatures.

Category:Local Universe structures