Astronomers discover dwarf galaxy orbiting the Milky Way

Astronomers have discovered another neighbour orbiting our home galaxy, the Milky Way — and it could be the faintest satellite dwarf galaxy discovered so far.

Researchers from Tohoku University in Japan found the dwarf galaxy — a faint, diffuse type of galaxy — near the constellation Virgo, about 280,000 light years from the sun.

So far the absolute magnitude — a measure used by astronomers to determine the brightness of celestial objects — of newly discovered Virgo I has come in at a mere –0.8 in the optical waveband. 

Comparatively, the Andromeda galaxy, the brightest galaxy in the sky, is –21.77 (the scale works in the opposite direction: the brighter the galaxy, the higher the negative value). 

New satellite galaxy

The image on the right shows the density of stars in the newly discovered Virgo I galaxy, with red indicating increasing density. (Tohoku University/NAOJ)

Astronomers believe that this discovery could mean the Milky Way has many more as yet undiscovered satellite galaxies: modern cosmology predicts hundreds of these galaxies should be around the Milky Way. To date only about 50 have been identified.

The discovery was made using the 8.2-metre Subaru Telescope atop Mauna Kea, Hawaii.

A star cluster or galaxy?

At the outer reaches of most galaxies are groups of billions of stars held together by gravity, called globular clusters. The researchers determined that, due to the amount of space between the stars (globular clusters are very dense) that their discovery could only be a dwarf galaxy, not a globular cluster.

Until now, the faintest dwarf galaxy was Segue I at –1.5 magnitude. Another galaxy, Cetus II, has been measured at an even fainter magnitude of 0.0, but it has yet to be confirmed. Astronomers believe it  is too compact and may be a globular cluster.

Similar to the way the moon orbits Earth, smaller galaxies can orbit larger ones. This is due to gravitational attraction. In the case of galaxies, however, an unseen force called dark matter is also believed to be acting upon the satellite galaxies. 

The two brightest satellite galaxies include the Large and Small Magellanic Clouds, seen best from the Southern Hemisphere.

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