WASHINGTON, D.C.--The oldest galaxies ever seen and a black hole on a diet were two of the highlights here today at the meeting of the American Astronomical Society.
Sifting through images taken by the newly refurbished Hubble Space Telescope, astronomers have spotted seven galaxies that date back to a mere 600 million to 800 million years after the big bang--the earliest galaxies found so far by 200 million years. That takes researchers close to the primordial stage of cosmic evolution, when the first galaxies were coming to life.
Astronomers have already learned a few things about the newly discovered galaxies. One is that they are tiny compared with contemporary galaxies--barely 5% the size of the Milky Way and less than 1% its mass. "These are the seeds of the great galaxies of today," says Garth Illingworth of the University of California, Santa Cruz. Illingworth led the survey team that took the new images using Wide Field Camera 3, one of two new instruments mounted on the Hubble in a servicing mission last year.
The other striking fact about the galaxies is that they are populated by stars that formed 300 million years prior to the time stamp of the galaxies themselves. That pushes back the earliest star-forming days of the universe to within a few hundred million years of the big bang--a blink of an eye in astronomical time.
By comparison, our Milky Way galaxy is a fully developed modern entity--and today we learned a bit more about it as well. Black holes are monsters that supposedly consume everything that comes close to them, yet the supermassive black hole at the heart of the Milky Way has been known to eat up only a tiny fraction of the gas and dust blowing in from massive young stars nearby. Astronomers have estimated that this fraction ought to be about 1% of the total fuel that lies within reach of the black hole. But in the case of the Milky Way's central black hole--known as Sagittarius (Sgr) A*--that fraction turns out to be even smaller: a tiny 1% of 1%. Why does Sgr A* eat so little?
A possible answer comes from a new model of the black hole's feeding behavior, presented by a team led by Roman Shcherbakov, an x-ray astronomer at Harvard University. Based on data taken by NASA's Chandra X-ray Observatory, the model takes into account how energy flows between two regions around the black hole--an inner core close to the boundary beyond which light cannot escape (the event horizon) and an outer ring that extends far out and includes the massive young stars lurking near the black hole. Energy released from the collisions of particles within the hot core travels to the outer part through conduction, causing pressure that drives much of the gas in the outer region beyond the reach of the black hole.
One way to validate the model is to predict how the x-ray brightness of gas around the black hole would vary as one travels outward from the center. Shcherbakov says the model has passed this test swimmingly.
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