Free Astronomy Magazine September-October 2015

SPACE CHRONICLES oratory and Kavli Institute for Cos- mology, University of Cambridge, United Kingdom) trained ALMA on galaxies that were known to be seen only about 800 mil- lion years after the Big Bang (they had redshifts ranging from 6.8 to 7.1 ). The astronomers were not looking for the light from stars, but instead for the faint glow of ionised car- bon coming from the clouds of gas from which the stars were forming. They wanted to study the interac- tion between a young generation of stars and the cold clumps that were assembling into these first galaxies. Astronomers are particularly inter- ested in ionised car- bon as this partic- ular spectral line carries away most of the energy inject- ed by stars and al- lows astronomers to trace the cold gas out of which stars form. Specifically, the team were looking for the emission from singly ionised carbon (known as [C II]). ALMA witnesses assembly of galaxies in the early Universe for the first time by Keck Observatory W hen the first galaxies start- ed to form a few hundred million years after the Big Bang, the Universe was full of a fog of hydrogen gas. But as more and more brilliant sources — both stars and quasars powered by huge black holes — started to shine they cleared away the mist and made the Uni- verse transparent to ultraviolet light. Astronomers call this the epoch of reionisation, but little is known about these first galaxies, and up to now they have just been seen as very faint blobs. But now new observa- tions using the power of ALMA are starting to change this. Neutral hy- drogen gas very efficiently absorbs all the high-energy ultraviolet light emitted by young hot stars. Consequently, these stars are almost impossible to observe in the early Universe. At the same time, the ab- sorbed ultraviolet light ionises the hydrogen, making it fully transpar- ent. The hot stars are therefore carv- ing transparent bubbles in the gas. Once all these bubbles merge to fill all of space, reionisation is complete and the Universe becomes trans- parent. A team of astronomers led by Roberto Maiolino (Cavendish Lab- T ime-lapse of a whole night at the ALMA Array Operations Site (AOS), located at 5000 metres al- titude on the Chajnantor plateau, Chile. Three anten- nas are pointing at the same target in the sky at any moment, so their movements are perfectly synchro- nised. As the sky appears to rotate clockwise around the south celestial pole (roughly on the upper left edge of the image), the Milky Way goes down slow- ly, until it is lying almost horizontal before sunrise. The centre of our galaxy becomes visible during the second half of the night as a yellowish bulge crossed by dark lanes in the centre of the image, just above the antennas. The flashes on the ground are the car lights of the guards patrolling at the AOS. ALMA, the Atacama Large Millimeter/submillimeter Array is the largest astronomical project in existence and is a truly global partnership between the scientific communities of East Asia, Europe and North America with Chile. [ESO/José Francisco Salgado]