How observing supernovae explosions can help resolve the tension between the CDM hypothesis and dwarf galaxy observations

An image of a dwarf galaxy.
An image of a dwarf galaxy.

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Article by: Keisha Kwok, on 01 September 2023, at 12:31 pm PDT

The Cold Dark Matter (CDM) hypothesis has been used to explain cosmic structure formation for decades. Yet, since the beginning of its role in cosmology, the hypothesis has been in tension with various observations – and one of them is that of dwarf galaxies. The research we will look at aimed to resolve the tension by studying supernovae explosions' effects on the dark matter distribution within these galaxies.

Astrophysicists have faced several challenges when trying to use the CDM hypothesis to explain the properties of these ultra-faint dwarf galaxies. One challenge is known as the cusp-core problem, which refers to the inconsistencies between predicted and observed central densities of dark matter in dwarf galaxies. The standard CDM model predicts that dark matter accumulates at the halo's centre due to gravitational interaction, leading to central densities much higher than the observed densities.

Another challenge, the too-big-to-fail problem, refers to the apparent contradiction between the large number of massive sub-halos predicted by simulations of the CDM model around galaxies like the Milky Way and the observed scarcity of bright dwarf galaxies within these sub-halos.

Two possible directions have been proposed: one requiring new physics about the dark matter itself, and the other suggesting that the gravitational impact of baryonic matter (e.g. from supernova explosions) should also be accounted for, as it can play a role in shaping the dark matter distribution.

A specific model in the latter direction, the Scalar Field Dark Matter (SFDM) model, is seen as a solution to the cusp-core problem. In the model, the ultra-light spin-0 particles that make up dark matter form wave-like structures known as solitons. These solitons have a relatively constant central density across the dark matter halo.

This is in contrast to the aforementioned predicted cusp density profile. Therefore, the solitons can potentially explain the lower central densities observed in the centre of dwarf galaxies, addressing the cusp-core issue. Due to the model's ability to explain small-scale issues, it has the potential to address the too-big-to-fail issue as well.

Thus, the purpose of the recently published research "Scalar Field Dark Matter: Impact of Supernovae-driven blowouts on the soliton structure of low mass dark matter halos" by Robles et al. was to determine whether these explanations are possible. This was done by investigating how supernovae explosions impact the structure of the solitons in the SFDM model.

The researchers began by setting the initial conditions with equations that govern an SFDM configuration, for the model for gas accretion followed by sudden gas blowouts. They then ran the simulations for both the event of a single blowout, and that of multiple blowouts (with intervals of hundreds of millions of years).

By observing the simulations, the researchers found that due to the blowouts, the soliton central density (half of which is composed of dark matter) oscillates continuously in large amplitudes without signs of damping. The consistently large amplitudes led the researchers to conclude that these oscillations "inevitably impl[y] a range of viable density profiles."

According to the researchers, "an important consequence of the constantly evolving structures is that attempting to infer the boson mass from fitting the state of the dark matter structure at a single time would generally yield a value that is at most ∼20% accurate to the true boson mass." This means that the inconsistencies in central densities could have been caused by gravitational effects of supernovae blowouts, and therefore resolving the cusp-core problem.

However, despite the model's ability to resolve the problems in the aspect of gravitational effect, in order to confirm whether the SFDM model can describe the dwarf galaxies well, future work is needed. Such research will need to investigate whether the structure of these galaxies agrees with outcomes of the simulations of dwarf galaxies that have undergone the various astrophysical effects caused by the predicted dark matter distributions.

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