A galactic conspiracy disproven

A longstanding ‘conspiracy’ in astronomy – that stars and dark matter are interacting in inexplicable ways – has been overturned by an international team of astronomers, in a paper today in Monthly Notices of the Royal Astronomical Society (MNRAS). 

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

The conspiracy emerged to explain a phenomenon that had puzzled astronomers for a quarter of a century. The density of matter in different galaxies appeared to be decreasing at the same rate from their centre to outer edges. This was perplexing because galaxies are diverse, with many different ages, shapes, sizes, and numbers of stars. So why would they have the same density structure?

“This homogeneity suggested that dark matter and stars must somehow compensate for each other in order to produce such regular mass structures,” says Dr Caro Derkenne, the first author of the paper and an ASTRO 3D researcher from Macquarie University.

Like many conspiracies, no researcher could come up with a mechanism. If dark matter and stars could interact in this way, then we would need to change our understanding of how galaxies form and evolve. But they also couldn’t find an alternate reason to explain what they were seeing, until now.

Derkenne and her colleagues found that the similarity in density might not be due to the galaxies themselves but in how astronomers were measuring and modelling them.

The team observed 22 middle-aged galaxies (looking back some four billion years in the past due to their great distance) in extraordinary detail, using the European Southern Observatory’s Very Large Telescope in Chile. It enabled them to create more complex models that better captured the diversity of galaxies in the universe. 

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

“Galaxies are complicated, and we have to model them with freedom or we’re going to measure the wrong things. Our models ran on the OzStar supercomputer at Swinburne University, using the equivalent of about 8,000 hours of desktop computing time.”

Derkenne is now applying her astronomy expertise to complex data for the Australia Public Service. 

“Astronomy sets you up really well to understand big data,” she says. “The real world is messy, and we don’t always have all the data. No one is there to tell you the answers or if you’re wrong or right. You need to accumulate data and analyse until you find something that works.”

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

“The MAGPI project is great example of how 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 the ESO Very Large Telescope has not only solved a long-standing problem in Astronomy, but also enabled young scientists, such as Dr Caro Derkenne, a platform on which to launch their careers to solve real-world problems,” she says.

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

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ABOUT ASTRO 3D

The ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) is a $40m Research Centre of Excellence funded by the Australian Research Council (ARC) and nine collaborating Australian universities – The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia, Curtin University, Macquarie University, The University of New South Wales, and Monash University.

About MAGPI

MAGPI provides an in-depth resolved view of galaxy evolution. 

The Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey is a large program on the European Southern Observatory Very Large Telescope, using the Multi Unit Spectroscopic Explorer (MUSE) to obtain spatially-resolved spectroscopy of stars and ionised gas for galaxies in a range of environments 3-4 billion years ago.

Our sample amounts to more than a hundred galaxies and will reveal the mechanisms responsible for the morpho-kinematic variety of today’s massive galaxies.

Abstract

The MAGPI Survey: Evidence Against the Bulge-Halo Conspiracy

C. Derkenne,^1,2 R. M. McDermid,^1,2★ G. Santucci,^3,2 A. Poci,⁴ S. Thater^,5 S. Bellstedt,³ J. T. Mendel,^2,6 C. Foster,^2,7 K. E. Harborne,^3,2 C. D. P. Lagos,^2,3 E. Wisnioski,^2,6 S. Croom,^2,8 R-S. Remus,⁹ L. M. Valenzuela,⁹ J. van de Sande,^2,8 S. M. Sweet,^2,10 and B. Ziegler⁵.

• 1 Research Centre for Astronomy, Astrophysics, and Astrophotonics, School of Mathematical and Physical Sciences, Macquarie University, NSW 2109, Australia
• 2 ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
• 3 International Centre for Radio Astronomy Research, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
• 4 Centre for Extragalactic Astronomy, University of Durham, Stockton Road, Durham DH1 3LE, United Kingdom
• 5 Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
• 6 Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
• 7 School of Physics, University of New South Wales, Sydney, NSW 2052, Australia
• 8 Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
• 9 Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 München, Germany
• 10 School of Mathematics and Physics, University of Queensland, Brisbane, QLD 4072, Australia.

Abstract

Studies of the internal mass structure of galaxies have observed a ‘conspiracy’ between the dark matter and stellar components, with total (stars + dark) density profiles showing remarkable regularity and low intrinsic scatter across various samples of galaxies at different redshifts. Such homogeneity suggests the dark and stellar components must somehow compensate for each other in order to produce such regular mass structures. 

We test the conspiracy using a sample of 22 galaxies from the ‘Middle Ages Galaxy Properties with Integral field spectroscopy’ (MAGPI) Survey that targets massive galaxies at 𝑧 ∼ 0.3. We use resolved, 2D stellar kinematics with the Schwarzschild orbit-based modelling technique to recover intrinsic mass structures, shapes, and dark matter fractions. 

This work is the first implementation of the Schwarzschild modelling method on a sample of galaxies at a cosmologically significant redshift. We find that the variability of structure for combined mass (baryonic and dark) density profiles is greater than that of the stellar components alone. Furthermore, we find no significant correlation between enclosed dark matter fractions at the half-light radius and the stellar mass density structure. Rather, the total density profile slope, 𝛾tot, strongly correlates with the dark matter fraction within the half-light radius, as 𝛾tot = (1.3 ± 0.2) 𝑓DM − (2.44 ± 0.04). 

Our results refute the bulge-halo conspiracy and suggest that stochastic processes dominate in the assembly of structure for massive galaxies.

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