An international team of astronomers has mapped the temperature of dust entrained in one of the oldest spiral galaxies in the universe, providing new insights into how fast the galaxy is growing. Until now, researchers have only been able to measure the temperatures of the most distant galaxies in a broad sense, without showing how the temperature varies in individual regions.

This research, described in a paper published today, Monthly Notices of the Royal Astronomical Society (MNRAS) shows precise temperature fluctuations in a distant galaxy and points to two different sources of heat: the supermassive black hole at the center of the galaxy and the heat emitted by newly formed stars in the surrounding spinning disk.

“The temperature of dust in a galaxy can vary greatly depending on where it is located,” says the Australian National University in Canberra (ANU), the paper’s lead author. “But in the past, most dust temperature measurements for distant galaxies have been made for the galaxy as a whole, due to the device’s limited resolution.

“We were able to measure the temperature from region to region to determine how much heat is coming from individual sources. Previously, this type of mapping was mostly limited to nearby galaxies.”

The study shows a clear distinction between hot dust in the central region, where the heat comes from the galaxy’s supermassive black hole, and cooler dust in the outer region, which is likely to warm up during star formation.

Most galaxies have a supermassive black hole at their center, the mass of which is thought to grow with the galaxy. When gas builds up in a black hole, it is heated by the collision of fast-moving particles around the black hole and sometimes glows brighter than the galaxy itself.

“The heating energy from a black hole reflects the amount of gas fed into it and therefore the growth rate of the black hole, while the heating energy from star formation reflects the number of stars that are re-forming in the galaxy – the growth rate,” says Dr Tsukui.

“This discovery shows more clearly how galaxies and central-mass black hole formed and grew in the early universe.”

The current work was made possible by the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, operated by the European Southern Observatory (ESO) in Chile.

“This work demonstrates the detailed mapping potential of the ALMA telescope operated by ESO,” said Professor Emma Ryan-Weber, Director of Astro3D. “ALMA is the most powerful array for measuring millimeter and submillimeter radiation. It’s incredible that ALMA can look at a 12 billion-year-old galaxy and split the image into two components, one from dust heated by the central supermassive hole, and the other from dust in its host galaxy. Source