====== The Local Group ======

The Local Group is a gravitationally bound collection of approximately 35 to 40 known galaxies, including our own Milky Way, situated within a sphere of roughly 1 megaparsec (Mpc) in radius. It represents a locally overdense region of the Universe where mutual gravitational attraction has overcome the general Hubble expansion.

==== Phenomenology and Membership ====
The group’s population is dominated by three large spiral galaxies, which together produce about 90% of its total luminosity:

**Andromeda (M31):** The most luminous member, an Sb-type spiral located approximately 770 kpc away. It is about 50% more luminous than the Milky Way and possesses a larger disk scale length of 6–7 kpc.

**The Milky Way:** An Sbc or Sc-type spiral and the second most luminous member.

**M33 (Triangulum):** An Sc or Scd spiral that is much less luminous, contributing only about 20% of the Milky Way’s light.

The remaining members are primarily **dwarf galaxies**, which are far more numerous but significantly fainter. These include the **Large and Small Magellanic Clouds** (LMC and SMC), which are irregular satellite galaxies orbiting the Milky Way at distances of ~50 kpc and ~60 kpc, respectively. Other members include various dwarf ellipticals (dE), such as M31's satellite **M32**, and numerous extremely diffuse dwarf spheroidal (dSph) galaxies like **Fornax**, **Sculptor**, and **Leo I**.

[{{ :courses:ast403:localgroup.jpg?600 | Fig 1: Schematic view of the Local Group of galaxies. Milky Way is at the center.}}]
==== Dynamics and Mass Distribution ====

The Local Group exhibits complex internal and global motions:

**Closing Velocity:** The Milky Way and Andromeda are currently approaching each other at approximately 120 km s⁻¹.

**Global Motion:** The entire Local Group moves at a peculiar velocity of approximately 600 km s⁻¹ relative to the cosmic microwave background (CMB) rest-frame.

**Mass and Dark Matter:** The total mass of the Local Group is estimated to be between $3 \times 10^{12} M_{\odot}$ and $4.8 \times 10^{12} M_{\odot}$. Because visible stars and gas account for only about 5% of this mass, the group is heavily dominated by dark matter, resulting in a high mass-to-light ratio of $M/L \gtrsim 80 M_{\odot}/L_{\odot}$.

==== Satellite Systems and Interactions ====
Many of the smaller members are organized into satellite systems around the two dominant spirals. The Milky Way’s 11 known satellites, for instance, form a highly flattened, planar distribution nearly perpendicular to the Galactic plane. These systems often show signs of tidal interaction:

**Magellanic Stream:** A long band of neutral hydrogen stripped from the Magellanic Clouds by the Milky Way's tidal forces roughly 200 million years ago.

**Sagittarius Dwarf Spheroidal:** This is the closest galaxy to Earth (24 kpc) and is currently being incorporated into the Milky Way, with its stars and globular clusters being stripped away by tidal forces.

[{{ :courses:ast403:milkyway_stream.jpg?600 | Fig 2: HI in the Magellanic Clouds and the Magellanic Stream.}}]
==== Future Evolution ====
The Local Group is a dynamic system whose components are destined to merge. The Milky Way and Andromeda are predicted to undergo a direct collision and merger in approximately 3 to 4 billion years, likely forming a single, larger elliptical or giant S0 galaxy. Additionally, smaller satellites like the LMC are expected to merge with the Milky Way even sooner, likely within the next 3 gigayears, due to the effects of dynamical friction. Over time, the Local Group will likely become a "fossil group" dominated by one massive central galaxy.