There’s a new runt in the neighborhood. Bring out the welcome wagon for KKs3, a tiny, isolated galaxy 7 million light years away in the southern constellation Hydrus the Lesser Water Snake.
The Milky Way galaxy is one some 50 galaxies in the Local Group, a gravitationally-bound cluster of galaxies including the familiar Andromeda Galaxy on the outskirts of the much larger and grander Virgo Cluster. The new galaxy, discovered by a team of Russian-American astronomers using the Hubble Space Telescope, is incredibly anemic. With just 23 million solar masses of material, it’s 1/10,000th as massive as the Milky Way.
KKs3 is a dwarf spheroidal galaxy, a small object generally spherical in shape and lacking spiral arms and a central nucleus. Though dwarf spheroidals resemble rich star clusters, they differ by possessing two different age groups of stars (very old and intermediate) and significant amounts of dark matter like other larger galaxies.
Most of these diminutive objects are found as satellites of much larger galaxies, slavishly revolving around them in orbits lasting millions of years. Tugged on by those giants’ powerful gravity, dwarf spheroidals are stripped of their gas and dust, leaving them unable to birth new generations of stars.
The Local Group is home to some 22 dwarf spheroidal galaxies including a dozen that are satellite galaxies of the Milky Way. Oddly, KKs3 sits all alone, unbound to a larger galaxy, yet it too has been stripped of its raw, star-making materials. Being isolated, it must have formed in a different way. Perhaps it underwent only one early burst of star formation that used up what precious little gas it was allotted when the galaxy first formed.
Because they’re so dim, isolated dwarf spheroidals are tough to find with only one other known, KKR 25, discovered by the same team back in 1999.
Nature has a knack for making many more small things for every large one. Like the dim red dwarf stars that are by far the most common stars in the Milky Way and probably the universe as a whole, dwarf spheroidal galaxies are almost certainly the most abundant type of galaxy out there.
We only need “bigger eyes” with which to see them and come to realize that our neighborhood may be richer than we thought. Future large telescopes like the James Webb Telescope and European Extremely Large Telescope will help us expand our search for these “common citizens” when they come online in the next few years.
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