Telescopes reveal how star formation stopped in galaxies

The giant elliptical galaxies were still generating new stars three billion years after the Big Bang, but in their centers, star formation had stopped.
By Andrew McDonald | Apr 20, 2015
The gigantic elliptical galaxies that populate the modern Universe can be ten times as massive as our Milky Way galaxy and cram ten times as many stars into their central regions. This suggests that, earlier in their evolution, these vast galaxies supported extremely rapid star formation.

However, now they are quiet, their star formation extinguished. The elliptical galaxies contain huge numbers of very old red stars, but lack young blue stars. The ages of the red stars indicate that star formation in the elliptical galaxies ceased around ten billion years ago, ironically when the rest of the Universe was witnessing accelerated star formation, with other galaxies spawning new stars up to 20 times their modern rates.

According to a European Southern Observatory (ESO) statement, observations from ESO's Very Large Telescope and NASA's Hubble Space Telescope have shown how and when the giant elliptical galaxies stopped forming new stars. The researchers, led by Sandro Tacchella of ETH Zurich, studied 22 elliptical galaxies as they appeared around three billion years after the Big Bang. The telescopes showed where in these galaxies new stars were forming.

The observations reveal that, by three billion years after the Big Bang, new star formation had ceased in the central regions of the galaxies, even as new stars continued to form along the galaxies' outer edges. Thus, star formation in the giant elliptical galaxies died away from the centers out to the peripheries. Possible explanations include dispersal of the gas necessary to form new stars by energy emitted by a galaxy's central supermassive black hole as matter falls into it, and a cessation of new gas input into a galaxy.

The new findings were published in the April 17 edition of the journal Science.

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