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Spinning Halos in the Cosmic Web

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Galaxy with a big halo [made of dark matter] surrounding it
Illustration of a galaxy and its dark matter halo (shown in blue). [ESO/L. Calçada; CC BY 4.0]

Does the spin of a dark matter halo align with the spin of the galaxy it’s situated around? And what can this tell us about the early universe? Hydrodynamical simulations of galaxies in the early universe might help us answer these questions. 

Bullet cluster (two galaxy clusters colliding)

The Bullet Cluster of galaxies. X-rays are shown in pink and the gravitational lensing is shown in blue. This cluster is considered one of the smoking guns for the presence of dark matter. [X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.]

No Longer in the Dark About Dark Matter 

Though the field of dark matter is expansive and exciting these days, astronomers didn’t even know dark matter existed until the 1980s. Now, dark matter can be used to probe everything from the force of gravity to galactic structure and evolution. Studying dark matter also has implications for cosmology and can help us better understand the initial conditions of the universe. Specifically, the alignment between the spins of a galaxy and its dark matter halo can help constrain the dark matter equation of state (which can tell us about the mass of the galaxy and help with predictions of its dynamics).

Taking Dark Matter for a Spin 

Galaxy with arrows on it showing how the galaxy is spinning.

Illustration of the spin of a galaxy. The velocity stays constant with radius, which shows that dark matter is present in the halo. [Adapted from ESO/L. Calçada; CC BY 4.0]

There are two main questions: first, how well do the observable spins of galaxies align with the spins of their dark matter halos, which can’t be measured? Studies exploring this question using numerical simulations have concluded that galaxies and their surrounding dark matter halos can be substantially misaligned. The second, potentially more important question is what this misalignment implies — if the spins are not aligned, does this mean that the motions of a galaxy’s stars are decoupled from changes in the background gravitational field? If so, this implies that we can no longer use the spin alignment of visible matter to probe the background cosmology. A team led by Jounghun Lee at Seoul National University aims to address this second question using hydrodynamical simulations to probe different scenarios. 

Illustrating What’s Happening Inside Galaxies 

Galaxies placed along filaments in the cosmic web

Simulation of the cosmic web. Every point of light is a galaxy and those galaxies are placed along filaments. [Illustris Project]

The team used the IllustrisTNG suite of simulations to model galactic dynamics. This software takes into account everything from star formation rate of the galaxy to feedback from supernovae and growth of black holes to model the physics going on. When the universe was ~9 billion years old, the luminous matter and various forms of stellar feedback like supernovae occurred along filaments in the cosmic web, which coupled the galaxy and dark matter halo spins, allowing us to probe early cosmology. However, going back to when the universe was ~5 billion years old, those filaments hadn’t yet formed and the matter density was fairly uniform, so these matter processes occurred randomly and didn’t have any structure to follow. This led to the uncoupling of the galaxy spin and the dark matter halo spin.

Lee and collaborators also find that properties such as black hole-to-stellar mass ratio, specific star formation rate (rate of creation of stars per unit stellar mass), and average metallicity either correlate or anticorrelate with the angle between the galaxy stellar and dark matter spins. 

Future work will involve finding direct evidence for the scenario of decoupling between the spins of the galaxy and its dark matter halo earlier in the universe’s history, modeling it, and exploring its connection to the initial conditions of the universe.

Citation 

“How Do the Galaxy Stellar Spins Acquire a Peculiar Tidal Connection?,” Jounghun Lee et al 2022 ApJ 927 29. doi:10.3847/1538-4357/ac4bda