Astronomers have spotted one of the dimmest galaxies ever seen.聽The discovery of this super-faint galaxy 鈥� a satellite of the Milky Way 鈥� bolsters current theories of how galaxies arise.
Giant galaxies like the Milky Way are thought to form after smaller galaxies smash together. That suggests that hundreds of satellite galaxies would orbit our own, leftovers of the ones that formed our galaxy. But, so far, astronomers have found only about 50.
The new galaxy, named Virgo I, is the latest satellite to be discovered. It appeared as a team led by Daisuke Homma and Masashi Chiba of Tohoku University in Sendai, Japan, searched the sky with a new camera on the giant 8.2-metre Subaru Telescope in Hawaii.
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鈥淰irgo I might be the faintest galaxy,鈥� Chiba says. It emits about half as much light as Segue 1, another satellite of the Milky Way and the previous faint-galaxy champ. A single bright star in our galaxy outshines all of Virgo I鈥檚 stars put together.
鈥淭iny little puny things鈥�
鈥淚t continues to be interesting how faint you can go and still be a galaxy,鈥� says Erik Tollerud of the Space Telescope Science Institute in Baltimore, Maryland. 鈥淭hey鈥檙e such tiny little puny things.鈥� The new galaxy聽radiates聽so little light聽because it has so few stars, which聽give off聽only about 180 times as much light as the sun.
Virgo I is just a few hundred light years across, which nonetheless indicates it is a galaxy, Chiba says. A star cluster this dim would be even smaller.
Still, the galaxy鈥檚 luminosity is somewhat uncertain, because astronomers can only see its brightest stars, says Vasily Belokurov of the University of Cambridge, whose team discovered Segue 1 in 2006. 鈥淎ll these luminosities should be taken with a great pinch of salt,鈥� he says. 鈥淪o I would not really play this game of which one is the faintest.鈥�
Such a galaxy is so dim it can lose most of its light when just one of its red giant stars becomes a white dwarf.
Last year, a less luminous object turned up in the constellation Cetus. But that system is so compact it may be a star cluster rather than a galaxy.
No star-stealing
Virgo I definitely surpasses its dim competitors in one regard: it鈥檚 much farther, lying 280,000 light years from Earth, nearly twice as far as the Milky Way鈥檚 brightest satellite galaxy, the Large Magellanic Cloud.
If Virgo I has always been so remote, the Milky Way didn鈥檛 make it dim by stealing its stars. Instead, these galaxies likely started out as 鈥渋ncredibly faint systems鈥�, says Josh Simon of the Carnegie Observatories in Pasadena, California. 鈥淭hey didn鈥檛 become that way later on.鈥�
The great distance explains why earlier searches, which employed smaller telescopes, never found the galaxy. Many more dim and distant satellites may emerge as the astronomers continue their search, which so far has covered only 1/400th of the sky.
鈥淚鈥檓 hoping they鈥檙e going to find hundreds of these,鈥� Belokurov says. 鈥淚t鈥檚 going to be fun.鈥�
arXiv
