Several dozen baby galaxies have turned up in galactic retirement communities near the Milky Way, new observations reveal. The objects resemble primitive galaxies from the early universe and may shed light on what triggers galaxies to form.
Most galaxies blinked into life more than 10 billion years ago, starting as small blobs of gas and stars that gradually merged into larger structures like the Milky Way. But after that initial baby boom, the galactic birth rate dropped significantly. So astronomers must look back through 10 billion light-years of mostly older galaxies to glimpse the small, faint infants from which those galaxies grew.
But now, a space telescope has found about three dozen of these galactic 鈥渂uilding blocks鈥� just 2 billion to 4 billion light years away. These relatively nearby finds appear eerily similar to the universe鈥檚 first galaxies. They have the same mass (equivalent to about 10 billion Suns), the same elemental composition (roughly that of the Sun), and the same age of stars (approximately 1 billion years old).
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鈥淚t鈥檚 almost like looking out the window and seeing a dinosaur walking by,鈥� says Tim Heckman, an astronomer at Johns Hopkins University in Baltimore, Maryland, US.
Ultraviolet glow
Heckman and colleagues made the discoveries with NASA鈥檚 Galaxy Evolution Explorer (GALEX), a space telescope that surveys ultraviolet wavelengths. GALEX detected the galaxies by the ultraviolet glow of their newborn stars, which are being formed at rates about a hundred times higher than in mature galaxies.
Team members expect to find more such objects with the telescope, which was launched into Earth orbit in April 2003. But they say the baby galaxies are rare, appearing just once for every 3000 or so sightings of older objects.
鈥淲hat we鈥檙e seeing right now is the last few stragglers of galactic birth,鈥� Heckman said at a press briefing on Tuesday. 鈥淲e don鈥檛 know what鈥檚 controlling this decline in the formation of galaxies.鈥�
Resolving structures
Alice Shapley, an astronomer at the University of California in Berkeley, US, says the objects鈥� nearness will help answer that question and others. The Hubble Space Telescope could try to try to resolve their structures, and observations at longer wavelengths could shed light on 鈥渉ow gas is getting converted into stars鈥�.
Future studies could also target the nurseries in which the baby galaxies were born. This would help astronomers understand if an interaction with another object triggered their formation, she says.
And Heckman says infrared observations will be needed to check that the galaxies are genuine infants and are not hiding older, cooler stars.
He adds that the team鈥檚 discoveries probably resemble the first galaxies better than another nearby baby recently identified by Hubble. This is because that galaxy, I Zwicky 18, is smaller and is forming stars at a much slower rate, he says.
