PI Prof. Anastasia Fialkov, Co-I Alex Tocher (PhD student)   

Ultralight axion dark matter, commonly known as ‘Fuzzy Dark Matter’ (FDM), offers an appealing solution to many of the observed tensions with conventional CDM models. However, many of the simulations of FDM cosmologies used for computational constraints, lack the detail to capture much of the small-scale (< kpc scale) dynamics of the FDM structures within such simulations. Previous work has determined that the effects of these dynamics are most pronounced in the smallest halos and with lower axion particle masses. Therefore, it is natural to look for the effects of the wave-like dark matter profiles on the formation of the first stars, in the first small structures to form.  

This year we produced a series of simulations covering 3 axion particle masses from 1e-22 to 7e-22 eV and a CDM comparison case for 3 halo masses between 8e8 to 8e9 𝑀_⊙ for a total of (3 + 1) x 3 = 12 simulations over a broad range of the parameter space. Within these simulations, we allow the gas of primordial composition to cool and collapse within the potential well and observe the resulting collapsed structures. We use a sink particle prescription to represent the cool and collapsed gas to prevent indefinite refinement and as a good metric of the quantity and location of collapsed gas. For this first round of simulations, the gas is given no initial velocity and allowed to fall into the halo. 

We find that under certain halo mass and axion mass conditions, the dark matter can transfer significant energy to the collapsed gas which can be seen clearly both in projections of the gas (Figure 1) and through the half mass radius and mean specific energy of the sink particles within the simulations (Figure 2).  This occurs because of the fluctuating central density within FDM halos of certain mass and axion mass configurations. If such fluctuations occur on a similar timescale to the dynamical timescales of the gas (e.g. free fall and sound crossing time), then the FDM halo can significantly disturb the in-fall of gas, as is seen in Figure 1. We also find that in certain FDM cases, the amount of gas that can cool and collapse into star-forming regions is substantially reduced. Under certain conditions, this effect can reduce the amount of collapsed gas within the halo by a factor of between 2-3 times.  

These findings would indicate that within the earliest and smallest star-forming structures, star formation efficiency may be notably reduced for certain FDM cosmologies, with implications for much of the early universe and the epoch of reionisation under FDM cosmologies.