Dark matter and stars do not interact as previously thought

A long-standing “conspiracy” in astronomy – that stars and dark matter interact in inexplicable ways – has been debunked by an international team of astronomers, in a paper published in Monthly Notices of the Royal Astronomical Society.

The authors are based in Australia, the UK, Austria and Germany and used the Very Large Telescope in Chile.

This conspiracy was invented to explain a phenomenon that had puzzled astronomers for a quarter of a century. The density of matter in different galaxies seemed to decrease at the same rate from their centers to their outer edges. This phenomenon was puzzling because galaxies are diverse, with very different ages, shapes, sizes, and numbers of stars. So why would they have the same density structure?

“This homogeneity suggests that dark matter and stars must somehow compensate each other to produce such regular mass structures,” says Dr Caro Derkenne, first author of the study and ASTRO 3D researcher at Macquarie University.

As with many conspiracy theories, no researchers have been able to find a mechanism. If dark matter and stars could interact in this way, then it would have to change our understanding of how galaxies form and evolve. But so far, they have also failed to find another reason to explain what they were observing.

Derkenne and his colleagues discovered that the similarity in density may not be due to the galaxies themselves, but to the way astronomers measured and modeled them.

The team observed 22 middle-aged galaxies (dating back about four billion years due to their great distance) in extraordinary detail, using the European Southern Observatory’s Very Large Telescope in Chile. This allowed them to create more complex models that better account for the diversity of galaxies in the universe.

“In the past, people built simple models that had too many simplifications and assumptions,” Derkenne says.

“Galaxies are complex and we need to model them freely, otherwise we’ll measure the wrong things. Our models were run on Swinburne University’s OzStar supercomputer, which is the equivalent of about 8,000 hours of computing time on a desktop computer.”

Derkenne now applies his astronomy expertise to complex data for the Australian public service.

“Astronomy is a great way to understand big data,” she says. “The real world is messy and we don’t always have all the data. There’s no one to give you the answers or tell you if you’re right or wrong. You have to accumulate data and analyze it until you find something that works.”

The project used MUSE (Multi Unit Spectroscopic Explorer) on the VLT to analyse galaxies from the MAGPI (Middle Ages Galaxy Properties with Integral field spectroscopy) survey. MUSE collects spectral data cubes in which each pixel is actually a spectrum.

“The MAGPI project is a great example of how the training workshops and collaborative space within ASTRO 3D have utilised Australia’s strategic partnership with the European Southern Observatory,” says ASTRO 3D Director Professor Emma Ryan-Weber.

“The complex data from ESO’s Very Large Telescope have not only solved a long-standing problem in astronomy, but have also provided young scientists, like Dr Caro Derkenne, with a platform on which to launch their careers solving real-world problems,” she says.

Co-authors are from the International Centre for Radio Astronomy Research (ICRAR) Western Australia, Durham University, University of Vienna, Australian National University, University of New South Wales Sydney, University of Sydney, Ludwig-Maximilians-Universität and the University of Queensland.