The corpses of three 鈥渄ead鈥� galaxies 鈥� which to the surprise of astronomers stopped forming stars long ago 鈥� have been identified by the Spitzer Space Telescope during a survey of the distant, early universe. The find bolsters a theory that colossal black holes can starve galaxies of the gas needed to create new stars.
An infrared telescope on Earth first found the galaxies two years ago. They appeared red 鈥� a sign that most of their stars were old. But our planet鈥檚 own heat clouded the observations, making it impossible to rule out whether dust was obscuring the light from younger stars.
Now, using NASA鈥檚 Spitzer telescope, which trails behind the Earth in the coldness of space, astronomers have determined the galaxies are red because they are dead 鈥� no stars appear to have formed for 1.5 billion years. That arrested development happened early in the history of the universe 鈥� their distance means Spitzer is viewing them just 2 billion to 3 billion years after the big bang.
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鈥淲e think galaxies form over tens of billions of years,鈥� says lead researcher Ivo Labb茅, an astronomer at the Carnegie Observatories in Pasadena, California, US. For example, he notes the 13 billion-year-old Milky Way is still forming stars today. 鈥淪urprisingly, we found galaxies that are fully formed and dead when the universe was only one-fifth its present age.鈥�
No freak monsters
The researchers estimate a significant fraction of galaxies in the early universe were dead 鈥� just 10 times as many exist today. 鈥淚t means these are not freak monsters,鈥� Labb茅 told New 杏吧原创.
David Hogg, a cosmologist at New York University, who is not part of the team, agrees. 鈥淲hat is most impressive to me is that a significant fraction of the galaxies look 鈥榣ong dead鈥�, which does suggest that some kinds of galaxies could be fully in place at very early times,鈥� he says.
Labb茅 believes that black holes lurking inside the galaxies were their undoing. The galaxies appear to be as massive as the Milky Way and like our galaxy, may harbour 鈥渟upermassive鈥� black holes containing the mass of millions of Suns. Gas falling into these black holes may have spawned powerful outpourings of energy, creating what astronomers call 鈥渁ctive鈥� galaxies.
This enormous energy is thought to heat the gas remaining throughout the galaxy to tens of millions of degrees.
Turned off
Lars Hernquist, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, recently published computer simulations of this heating phenomenon.
鈥淓ventually the gas is so hot, it is no longer bound to the galaxy and simply starts to flow out to space,鈥� Hernquist told New 杏吧原创, a process that takes about 10 million years. When it is over 鈥渟tar formation is turned off鈥�, says Hernquist. 鈥淭he galaxy will just sit there and the stars will become older. I think this is a very likely explanation of what鈥檚 being seen in these galaxies.鈥�
Another possibility is that supernovae exploding within the galaxies blow gas out to space, but both Labb茅 and Hernquist say this is a less efficient mechanism.
The research will be published in a future issue of the Astrophysical Journal Letters.
