Galaxies die from the inside out

Observations show the end of galatic star formation begins at the core

Massive elliptical galaxies, like IC 2006, once had furious rates of star formation but this has since halted.
Credit: ESA/Hubble & NASA Image acknowledgement: Judy Schmidt and J. Blakeslee (Dominion Astrophysical Observatory).

By Elizabeth Pearson

Giant galaxies died from the inside out according to new observations by ESO’s Very Large Telescope (VLT) and NASA/ESA’s Hubble Space Telescope. Billions of years ago, star formation stopped at the centre of massive galaxies, a process known as quenching. The outer regions' star formation then tailed off at a later date.

The life cycle of giant galaxies has long been a mystery for astronomers. In the local Universe, many of the largest elliptical galaxies appear ‘dead’, meaning that no new stars are being formed within them.

Hubble and the VLT took observations of galaxies as they were 10 billion years ago.  It appears that the halting of star formation began at the centre, though stars formation continued in the outer regions before this too came to and end.

Researchers can follow the effects of quenching by looking at a galaxy's colour. Short-lived stars, which are blue, only last for a few million years, so if there is no star formation to constantly replace them then the galaxy will quickly lose its bluish hue. Instead the galaxy will be colourised by the long-lived stars that remain behind as they have lifespans of several trillion years, and these are all red in colour.

Large elliptical galaxies in the recent Universe look ‘red and dead’, meaning that star formation must have ended within them some time ago, but the reason behind this is not clear.

“Massive dead spheroids contain about half of all the stars that the Universe has produced during its entire life,” says Sandro Tacchella of ETH Zurich in Switzerland. “We cannot claim to understand how the Universe evolved and became as we see it today unless we understand how these galaxies come to be.”

Knowing how star formation is curtailed within these galaxies is an important step to learning how they formed, but the mechanism behind it is still unknown. It could be that star making materials are scattered by the galaxy’s central black hole, or that fresh gas needed to form stars stops flowing into a galaxy, and starves off its star formation.

“There are many different theoretical suggestions for the physical mechanisms that led to the death of the massive spheroids,” says Natascha Förster Schreiber, at the Max-Planck-Institut für extraterrestrische Physik in Garching, Germany. “Discovering that the quenching of star formation started from the centres and marched its way outwards is a very important step towards understanding how the Universe came to look like it does now.”


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