The leading theory of cosmology, called CDM, describes a universe in which the matter that we see and touch every day (so-called "normal" or baryonic matter), makes up just 15% of all matter. The remaining 85% is dark matter, the nature of which is still unknown. While dark matter doesn't have much of an impact on daily life today - a little dark matter probably passed through you while you read this sentence, and I'm guessing you didn't notice! - it played a leading role in the early universe, dictating when, where, and at what masses the first galaxies formed.

While galaxies today are nestled within dark matter halos and arrayed along strands of dark matter scaffolding, dark matter and luminous matter weren't always so aligned; CDM predicts that in the very early universe, just after protons and electrons came together to form atoms, there were places where normal matter moved relative to dark matter with immense speed - faster than the local speed of sound. This supersonic relative motion complicated the formation of the first stars and galaxies, potentially altering early galaxy formation in a measurable way.

Suppression of Small-Scale Structure
Claire Williams (University of California, Los Angeles) and collaborators set out to understand the impact of this supersonic relative motion on the formation of low-mass galaxies. Using high-resolution fluid dynamics simulations, Williams's team modeled the evolution of galaxies from a redshift of 200 to a redshift of 12. Their simulations tackled the cooling and condensing of molecular clouds, the ignition of the first stars, and the assembling of galaxies and star clusters.

The team found that having dark matter and normal matter moving at supersonic velocities relative to one another makes it harder for small galaxies to form; an order of magnitude fewer galaxies formed with stellar masses less than about 3 million solar masses when a velocity difference was present. Larger galaxies formed in the same numbers regardless of the velocity difference.

Clues from Ultraviolet Photons
What effect do these findings have on quantities that we could potentially measure? The velocity difference affects the simulated galaxies' star formation, which impacts their ultraviolet luminosity. This in turn determines the number of galaxies present at each ultraviolet luminosity, otherwise known as the ultraviolet luminosity function.

Williams's team found key differences in these quantities at a redshift of 12 in areas with the velocity difference and areas without. In regions without the supersonic flow, small galaxies and star clusters form readily and begin to churn out stars. In regions with the supersonic flow, small galaxies are uncommon, and the gas that would have gone toward the smallest galaxies is instead captured by and spurs star formation in slightly more massive galaxies, in the 10-million-solar-mass range. This means that the ultraviolet luminosity function is lower for the smallest, faintest galaxies but is boosted for slightly brighter and more massive galaxies at a redshift of 12.

Because ultraviolet photons emitted in the distant past are shifted to longer wavelengths when they reach us today, the ultraviolet luminosity function is potentially measurable at infrared wavelengths with JWST. Ultra-deep JWST observations such as the NGDEEP survey may reach ultraviolet magnitudes as faint as -14, potentially revealing the small galaxies that can probe early-universe cosmology.

Research Report:The Supersonic Project: Lighting Up the Faint End of the JWST UV Luminosity Function

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