Professor Justin Read, head of the Department of Physics at the University of Surrey, speaks to SciTech Europa about the results of a new study on how dark matter can be ‘heated up’.
In new research, scientists from the University of Surrey, UK, Carnegie Mellon University, USA, and ETH Zürich, Switzerland, have found evidence that dark matter can be heated up and moved around, as a result of star formation in galaxies. The findings provide the first observational evidence for the effect known as ‘dark matter heating’, and give new clues as to what makes up dark matter.
SciTech Europa spoke to Professor Justin Read, lead author of the study and head of the Department of Physics at the University of Surrey, about what the research involved and how the results could help to gain a better understanding of what dark matter – which is thought to make up most of the mass of the universe but, because it doesn’t interact with light in the same way as normal matter, can only be observed through its gravitational effect – actually is.
Why were you looking for signs of dark matter in dwarf galaxies?
Dwarf galaxies are the smallest galaxies in the Universe. They are also almost entirely made up of dark matter. This makes them beautiful natural ‘dark matter laboratories’. By mapping out the distribution of dark matter inside dwarfs and comparing this with numerical models, we can learn about the nature of dark matter.
What is ‘dark matter heating’ and how can it be measured? Why is it important to know more about this phenomenon?
The idea of ‘dark matter heating’ dates back to a paper by Julio Navarro in 1996. Dark matter does not have a temperature or pressure so it cannot be ‘heated up’ in the same way that you can heat up air. However, when air gets hot, the molecules of air all move at a higher relative speed. Our latest models for dark matter suggest that it is made up of some sort of weakly interacting massive particle (WIMP). This dark matter forms an extended shroud around galaxies that we call the dark matter ‘halo’. The dark matter particles in this halo move in orbits, constantly whizzing past one another. If we can raise the relative velocity of these dark matter particles, causing the halo to expand, then this is analogous to heating molecules of air in a room. This is what we mean by ‘dark matter heating’.
Since dark matter interacts with normal matter only via gravity, ‘heating it up’ is quite challenging. However, nature seems to have found a way to do this. At the centres of dwarf galaxies, gas is constantly in a cycle of inflow and outflow. The inflow is caused by cooling. The gas cools, reaches high density and then forms stars. The outflow occurs because the most massive of these stars then produce powerful winds and supernovae explosions that expel gas from the galaxy in a galaxy-wide ‘wind’. The repeated inflow and outflow of gas causes the gravitational potential at the centre of the dwarf galaxy to fluctuate. The dark matter particles feel this constantly changing gravitational force, and their kinetic energy is raised as a result, causing the dark matter particles to shift to higher energy orbits and leading to a global expansion of the dark matter halo.
To help understand this ‘dark matter heating’, imagine that I change the mass of the Sun up and down every two years or so. As the Earth moves along its orbit, and the Sun increases in mass, the Earth would suddenly feel more gravitational force, pulling it closer to the Sun. But then as the Sun reduces in mass, the Earth would find that it is moving too fast. The pull from the Sun is now too weak and the Earth would be flung outwards. The net result is that the Earth would move onto an elliptical orbit like that of Pluto, spending more of its time further away from the Sun. The Earth’s orbit will have been ‘heated up’ and pushed out. This is exactly what happens to the orbits of dark matter particles in dwarf galaxies as a result of repeated gas inflow/outflow over many billions of years.
What relationship did you find between the amount of dark matter at the centres of the dwarfs galaxies and the amount of star formation they have experienced over their lives? Why is this important?
We wanted to use observations of dwarf galaxies to test the above idea that dark matter can be ‘heated up’. A key prediction of the ‘dark matter heating’ model is that dwarf galaxies that experienced very little star formation (i.e. in which star formation ceased over 10 billion years ago) should have undergone little dark matter heating because they experienced only a short period of gas inflow and outflow. These are called ‘quenched’ dwarfs. By contrast, dwarf galaxies that are still forming stars today will have had the full age of the Universe (13.8 billion years) of repeated gas inflow and outflows due to star formation. These star forming dwarfs will have maximised their ability to ‘heat up’ their dark matter, and they should have lower central dark matter density than the quenched dwarfs.
We set out to test the above prediction by measuring the central dark matter density in a sample of 16 nearby dwarf galaxies with excellent quality data and – crucially – a wide range of different star formation histories. Some of our dwarfs stopped forming stars over 10 billion years ago (quenched dwarfs); others are still forming stars today (star forming dwarfs). We found that the quenched dwarfs have a systematically higher dark matter density than the star forming dwarfs, exactly as the dark matter heating models predicted.
This result is important because if dark matter can be ‘heated up’ in the above way, then it suggests that it does indeed comprise some sort of weakly interacting massive particle that can have its orbit altered by a fluctuating gravitational potential. This takes us one step closer to understanding what dark matter is.
Do you have any plans to expand on these discoveries? Will you be relying on (European) infrastructure (ESO instruments, JWST etc.) for future research?
We are following up on this research in several directions. We have a paper submitted and under review measuring the inner dark matter density for a quenched dwarf that lies much further from the Milky Way than any of the quenched dwarfs studied in our current work. This is important to test whether the distance from the Milky Way is somehow a confounding factor in our analysis (we find that it isn’t). We would also like to apply our analysis to data for more dwarfs in the Milky Way and in orbit around our nearby large spiral companion galaxy, Andromeda.
Finally, we can try to measure the inner dark matter density in dwarf galaxies at high redshift to see if these are denser than similar dwarfs in the nearby Universe – another key prediction of ‘dark matter heating’ models. All of this work will be facilitated by new data from the ELT, LSST, Gaia, JWST and other up-coming large surveys.
Professor Justin Read
Head, Department of Physics
+44 (0)1483 683479