HKU Bulletin May 2020 (Vol.21 No.2)

Contrary to previous belief, globular clusters around the giant galaxy at the centre of the Perseus galaxy cluster are not all ancient objects. Instead, several thousand have been forming at a roughly steady rate out of a cool gas in the centre of the Perseus galaxy cluster, and they have been doing so over at least the past 1 billion years, and perhaps many more over the course of cosmic history. This discovery was made by a group of astrophysicists led by Dr Jeremy Lim and his research assistant, Miss Emily Wong, from the Department of Physics, using data from the NASA/ESA Hubble Space Telescope. Working in collaboration with Professor Thomas Broadhurst from the University of the Basque Country in Spain and a Visiting Research Professor at HKU’s Department of Physics, Dr Youichi Ohyama at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan, and Dr Elinor Medezinski at Princeton University in the United States, the team’s journal paper ‘Sustained Formation of Progenitor Globular Clusters in a Giant Elliptical Galaxy’ was published recently in Nature Astronomy . The oldest visible objects in the Universe, globular clusters were thought to be all relics from the first epoch of galaxy formations. These spherical groups each contain hundreds of thousands to occasionally over 10 million stars gravitationally bound into a single structure about 100 to 200 light years across. “They are most numerous around massive elliptical galaxies and typically reside in galaxies’ outer, less-crowded areas,” said Dr Lim. “Our Galaxy is surrounded by about 150 such globular clusters, some of which are visible to the naked eye from a sufficiently dark site.” Scientists have long thought that globular clusters were formed soon after the Universe began nearly 13.8 billion years ago, at the same time as – or possibly even before – the first galaxies formed. They have since remained largely unchanged, apart from the ageing and death of their constituent stars. Dr Lim explained: “Globular clusters have always been regarded as important as ancient witnesses to the formation of galaxies, and therefore a possible source of vital clues to how infant galaxies form and then grow over time. But until now we have had scant information about how globular clusters themselves form and accumulate around galaxies.” Astronomers shed new light on the origin of globular clusters surrounding giant galaxies. FINDING ANSWERS TO AN AGE-OLD QUESTION The team’s analysis of the Hubble Space Telescope data revealed that the younger globular clusters are closely associated with, and therefore formed from, a complex network of cool gas that extends to the outer reaches of the giant galaxy. Professor Broadhurst said: “This network of cool gas precipitated from the hot gas that infuses the entire Perseus galaxy cluster. In fact, the gas concentrates in the centre of the cluster, where the giant elliptical galaxy resides, allowing it to cool faster and that leads to the creation of globular clusters.” Raining inwards “Once formed, these infant globular clusters do not remain in the network of cool gas but rain inwards on to the giant galaxy like raindrops falling from the clouds,” added Dr Lim. “Those not disrupted will end up orbiting the galaxy, reaching distances as far away from the centre as the locations where they were born.”​ The team’s analysis explains several other aspects that have previously had scientists puzzled. “First is their sheer numbers – evidently, some fraction of globular clusters around giant galaxies formed over cosmic history from the gas that infuses galaxy clusters,” said Dr Lim. “Second is the especially broad range of colours exhibited by globular clusters around giant galaxies. The colours of globular clusters change progressively from blue to red as they age – because more massive and bluer stars die first, leaving less massive and redder stars. Hence, their broad range of ages results in a broad range of colours.” The globular clusters that formed from the network of cool gas at the centre of the Perseus galaxy cluster span a broad range of masses, but with a diminishing number at higher masses. “Their number dependence with mass follows the same trend as the truly ancient globular clusters, as well as less massive star clusters in our Galaxy and other spiral galaxies,” said Miss Wong. “This affirms a common formation mechanism for star clusters over all mass scales – from Young globular clusters (ages up to 1 billion years) are indicated by the blue dots. those weighing just over 10 Suns to those weighing about 10 million Suns – irrespective of the environment in which they formed, whether it be from gas compressed in the spiral arms of galaxies or dense gas at the centres of galaxy clusters; or in ways yet to be discovered as is the case for truly ancient globular clusters.” Finally, the sustained formation of globular clusters at the centres of galaxy clusters also helped explain the enormous size of giant galaxies – up to 10 times or more that of our Galaxy. “Whereas the more massive globular clusters will long endure, the less massive globular clusters are expected to be ripped apart as they orbit the galaxy,” said Dr Lim. “Their constituent stars, spread throughout the galaxy at the centre of the galaxy cluster, contribute to the growth in size of giant galaxies over time.” Old globular clusters (ages up to 10 billion years or more) are indicated by the red dots. Globular clusters have always been regarded as important as ancient witnesses to the formation of galaxies, and therefore a possible source of vital clues to how infant galaxies form and then grow over time. DR JEREMY LIM 25 RESEARCH 24 The University of Hong Kong Bulletin | May 2020

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