Free Astronomy Magazine July-August 2015

42 JULY-AUGUST 2015 SPACE CHRONICLES Astronomers refer to these big gal- axies as red and dead as they exhibit an ample abundance of ancient red stars, but lack young blue stars and show no evidence of new star for- mation. The estimated ages of the red stars suggest that their host galaxies ceased to make new stars about ten billion years ago. This shutdown began right at the peak of star formation in the Universe, when many galaxies were still giving birth to stars at a pace about twenty times faster than nowadays. "Massive dead spheroids contain about half of all the stars that the Universe has produced during its en- tire life," said Sandro Tacchella of ETH Zurich in Switzerland, lead au- thor of the article. "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." Tacchella and colleagues observed a total of 22 galaxies, spanning a range of masses, from an era about three billion years after the Big Bang. They used the NASA/ESA Hubble Space Telescope's Wide Field Camera 3 (WFC3) to peer at the galaxies from above our planet's distorting atmosphere — WFC3 snapped detailed images in the near- infrared, revealing the spatial distri- bution of older stars within the actively star-forming galaxies. The researchers also used the SINFO- NI instrument on ESO's Very Large Telescope to collect light from the galaxies, showing precisely where they were churning out new stars. SINFONI could make these detailed measurements of distant galaxies thanks to its adaptive optics system, which largely cancels out the blur- ring effects of Earth's atmosphere. "Hubble was able to show us how the stars are distributed within these galaxies in amazing detail," Death of giant galaxies spreads from the core by NASA A stronomers have shown for the first time how star for- mation in "dead" galaxies sputtered out billions of years ago. A major astrophysical mystery has centred on how the massive, quies- cent elliptical galaxies, common in the modern Universe, quenched their once furious rates of star for- mation. Such colossal galaxies, of- ten also called spheroids because of their shape, typically pack in stars ten times as densely in the central regions as in our home galaxy, the Milky Way, and have about ten times its mass. T his NASA/ESA Hubble Space Telescope image shows an el- liptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quench- ed their once furious rates of star formation is an astrophysical mystery. Now, the NASA/ESA Hubble Space Telescope and ESO's Very Large Telescope (VLT) have revealed that three billion years after the Big Bang, these types of galaxies still made stars on their outskirts, but no longer in their interiors. The quenching of star formation seems to have started in the cores of the gal- axies and then spread to the out- er parts. [ESA/Hubble & NASA]