Free Astronomy Magazine July-August 2015

43 JULY-AUGUST 2015 SPACE CHRONICLES commented Marcella Carollo, also of ETH Zurich and co-author of the study. "We were able to match this accuracy with SINFONI to find patches of star formation. Using the two telescope together, we were able to explore this population of galaxies in more detail than ever before." According to the new data, the most massive galaxies in the sample kept up a steady production of new stars in their peripheries. In their bulging, densely packed cen- tres, however, star formation had already stopped. "The newly demonstrated inside-out nature of star formation shutdown in massive galaxies should shed light on the underlying mechanisms in- volved, which astronomers have long debated," says Alvio Renzini, Padova Observatory, of the Italian National Institute of Astrophysics. A leading theory is that star-making materials are scattered by torrents of energy released by a galaxy's cen- tral supermassive black hole as it sloppily devours matter. Another idea is that fresh gas stops flowing into a galaxy, starving it of fuel for new stars and transforming it into a red and dead spheroid. "There are many different theoretical sugges- tions for the physical mechanisms that led to the death of the massive spheroids," said co-author Natascha Förster Schreiber of the Max-Planck- Institut für extraterrestrische Phy- sik in Garching, Germany. "Discovering that the quenching of star formation started from the cen- tres and marched its way outwards is a very important step towards un- derstanding how the Universe came to look like it does now." S tar formation in what are now "dead" galaxies sputtered out billions of years ago. The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. This diagram illustrates this process. Galaxies in the early Universe appear at the left. The blue regions are where star formation is in progress and the red regions are the "dead" regions where only older redder stars remain and there are no more young blue stars being formed. The resulting giant spheroidal galaxies in the modern Universe appear on the right. n