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.

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.

Fig 1: Schematic view of the Local Group of galaxies. Milky Way is at the center.

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 $M/L \gtrsim 80 M_{\odot}/L_{\odot}$.

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.

Fig 2: HI in the Magellanic Clouds and the Magellanic Stream.

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.