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‘Hobbit’ galaxies made of dark matter
This solves the mystery behind why they often go unaccounted
Small, ultrafaint "hobbit" galaxies recently found hovering around our Milky Way are comprised almost entirely of dark matter, a new study confirms.
Dark matter is a mysterious substance scientists think accounts for most of the mass in the universe but that is invisible to current instruments.
The finding, to be detailed in the Nov. 10 issue of Astrophysical Journal, could help resolve a cosmic accounting problem that has long vexed astronomers and also explain how such small galaxies form.
Missing Dwarf Galaxies
According to the "Cold Dark Matter" model, which explains the growth and evolution of the universe, large galaxies such as our Milky Way should be surrounded by a swarm of up to several hundred smaller "dwarf" galaxies. However, until recently, only 11 such companion galaxies have been found.
To explain this so-called Missing Dwarf Galaxy problem, theorists have suggested that the majority of dwarf galaxies contain very few, if any, stars and are instead made up mostly of dark matter.
Earlier this year, members of the Sloan Digital Sky Survey team announced they had discovered eight small and extremely faint galaxies in our Local Group of galaxies that might belong to this bizarre class of "dark" galaxies.
Because the new satellites were far smaller than known dwarf galaxies such as the Large and Small Magellanic Clouds, some of the scientists dubbed them "hobbit" galaxies.
Using the Keck II telescope in Hawaii, Joshua Simon of Caltech and Marla Geha of the Herzberg Institute of Astrophysics in Canada conducted follow-up studies to calculate the mass of the hobbit galaxies based on the speeds at which stars inside them moved. The pair confirmed that each galaxy was among the smallest ever measured, more than 10,000 times smaller than the Milky Way.
They also found that the mass of the galaxies was about 100 times more than what would be expected from the mass of their stars alone. "And so the rest of it has to be in some other kind of matter," Simon told Space.com.
Most galaxies, including our own, are thought to have more dark matter in them than normal matter, but the new hobbit galaxies take this trend to an extreme.
"Our work narrows the gap between the Cold Dark Matter theory and observations by significantly increasing the number of Milky Way dwarf galaxies and telling us more about the properties of these galaxies," Simon said.
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