Companions and the Local Group

The Milky Way does not live alone -- it has a number of "satellite" companions which orbit around the Milky Way at distance of 10 - 100 kpc, and it also lives with 2 other spiral galaxies in a galaxy group known as the Local Group.

The Magellenic Clouds

The Milky Way's biggest satellite companions are the Large and Small Magellenic Clouds, two irregular gas-rich star forming dwarf galaxies. These are naked-eye objects in the southern hemisphere:

Image courtesy Colin Legg, sourced here

The Large Magellenic Cloud (LMC)
The Small Magellenic Cloud (SMC)
  • Distance: 50 kpc
  • Size: ~ a few kpc
  • Mass: ~ 2x1010 Msun (~ few % of Mgal)
  • Distance: 60 kpc
  • Size: < kpc
  • Mass: ~ 2x109 Msun (< 1% of Mgal)

The Clouds are orbiting each other, and, as a pair, orbiting the Galaxy (like the way the Earth and Moon orbit each other, and also orbit the Sun). The Magellenic Stream is a stream of gas that can be detecting using 21cm HI observations:

Image source: Nidever et al & Mellinger, via APOD

An estimate of their orbit around the Galaxy suggests it may have a semimajor axis a=125 kpc and an orbital period of a few billion years (Gyr).

Question: Why is the orbit of the Magellenic Clouds so hard to figure out?

The Sagittarius Dwarf

In 1994, a survey of stars in the direction of the galactic bulge identified a group of stars with similar distances and kinematics -- the discovery of the Sagittarius dwarf galaxy

The Sag dwarf is on the opposite side of the Galaxy (behind the bulge), and is being ripped apart by the gravitational tidal field of the Galaxy. Each time it orbits the Galaxy, more stars are ripped off...
Simulation of tidal disruption from James Bullock and Kathryn Johnston


This picture is a schematic representation of the Sag dwarf. The black and white photo shows the starfield towards the galactic center, while the red contours trace out the structure of the Sag dwarf. Courtesy Rosemary Wyse, JHU.

Other Dwarf Galaxies

There are a good number of dwarf galaxies in total orbiting the Milky Way.

Dwarf Spheroidals
  • low mass: 107 - 108 Msun
  • low density
  • small (typically < 500 pc)
  • gas poor, no on-going star formation

Leo I, a dwarf spheroidal
Dwarf Irregulars
  • low mass and small
  • gas-rich, with star formation
  • LMC and SMC are the biggest examples

Ultrafaint Dwarfs

In the past 10 years, a number of extremely faint dwarf galaxies have been found around the Milky Way: the ultrafaint dwarfs (from Bullock 2010):

What makes a ultrafaint galaxy different from a star cluster?

The Local Group

Besides the array of dwarf companions, there are two nearby spirals, the Andromeda Galaxy (M31) and the Pinwheel Galaxy (M33). The three spirals -- the Milky Way, M31, and M33 -- and their dwarf companion galaxies make up the Local Group.

Andromeda (M31)
Pinwheel (M33)
  • Large, massive spiral galaxy: 2-3x as big as the Milky Way
  • Distance: 750 kpc
  • Radial Velocity: -200 km/s -- incoming!
  • Smaller, low mass spiral galaxy
  • Distance: 900 kpc

M31 and M33 are clearly interacting, as seen in deep star count data: (from the PAndAS Survey/Geraint Lewis):


courtesy Andrew Colvin