Heat spots reveal growth rate of a galaxy 12 billion years ago
Precise mapping of temperature variation gives clue to its evolution
An international team of astronomers has drawn a temperature map of the dust drifting within one of the oldest spiral galaxies of the Universe which provides new insights into how fast the galaxy is growing. Until now researchers have only been able to measure the temperature of most distant galaxies in broad terms, without showing how temperatures vary in individual areas.
This research, described in a paper published today in Monthly Notices of the Royal Astronomical Society (MNRAS) shows unambiguous temperature variation within the distant galaxy indicating two distinct heat sources – a supermassive black hole at the centre of the galaxy, and the heat generated by newly-formed stars in the surrounding rotating disk.
“The temperature of a galaxy's dust can vary greatly according to which region it is in,” says of the Australian National University (ANU) in Canberra, lead author of the paper. “But most of the measurements of dust temperature for distant galaxies in the past have been for the galaxy as a whole, due to limited instrument resolution.
“We were able to measure the temperature by region to region that we could determine how much heat is coming from individual sources. Previously, such mapping has mostly been limited to nearby galaxies.”
The research reveals a clear distinction between warm dust in the central region – where the heat is derived from the galaxy’s supermassive black hole – and colder dust in the outer region, which is likely being heated by star formation.
Most galaxies have a supermassive black hole in the centre, which are thought to grow in mass with the galaxy. When the gas accretes to the black hole, it is heated up by collisions of the fast-moving particles in the vicinity of the black hole and sometimes shines brighter than the stellar body of the galaxy itself.
“The heating energy from the black hole reflects the amount of the gas being fed into it and so the black hole growth rate, while the heating energy from star formation reflects the number of stars newly forming in the galaxy – the galaxy growth rate,” Dr Tsukui says.
“This discovery provides a clearer picture of how galaxies and central massive black hole form and grow in the early Universe.”
The current research was made possible thanks to the Atacama Large Millimeter/submillimeter Array (ALMA) telescope operated by the European Southern Observatory (ESO) in Chile.
“This study demonstrates the detailed mapping ability of the ALMA telescope, operated by ESO,” Astro3D Director Professor Emma Ryan-Weber said. “ALMA is the most powerful array for measuring millimetre and submillimetre radiation. It’s incredible that ALMA can look at a 12-billion year old galaxy and separate the image into two components – one of dust heated from the central super massive hole, and the other from the dust in underlying host galaxy.”
ALMA is a global collaboration and comprises 66 high-precision antennas, spread over distances of up to 16 kilometres making it the world’s largest ground-based astronomical project. It is designed to detect faint light from some of the coldest objects in the Universe which have wavelengths of around a millimetre, somewhere between infrared light and radio waves.
The $40 million ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) is funded by the Australian Research Council (ARC) and six collaborating Australian universities: The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia, and Curtin University.
Bill Condie (Media Contact for ASTRO 3D)
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Takefumi Tsukui (ANU)
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Figure caption: An optical image, left, of the galaxy captured by the Hubble Space Telescope with overlaid temperature contours as detected by ALMA. The image on the right shows the dust temperature map detailed in the study.