Title: Hunting Dark Matter Lines in the Infrared Background with the James Webb Space Telescope

Authors: Ryan Janish, Elena Pinetti

First author Institution: Fermi National Accelerator Laboratory, Theoretical Astrophysics Department, Batavia, Illinois, 60510, USA

Status: Available on ArXiv, Accepted for publication in Physical Review Letters

Dark matter refers to the invisible mass in the Universe, accounting for most of the mass (5x more than normal matter) responsible for the rotation of galaxies and clusters of galaxies in the Universe. When we measure the rotational motion, the rotation speed is found to be too large to be well explained by the visible, normal matter, alone.  There seems to be matter that has no interaction with electromagnetism. Likewise, measurements of the radiation from the early Universe also seem to support the presence of dark matter.

There are two streams of thought regarding dark matter. One is that our best model of gravity, General Relativity, is perhaps not the best model after all – this idea is explored more in this, this, this and this bite with modified Newtonian dynamics. The other is that dark matter is a new kind of particle (or perhaps many new kinds of particles) to be added to the existing Standard Model. Many particle physics experiments have attempted to search for different dark matter candidates, such as in this experiment here. If a dark matter particle interacts with or produces a Standard Model particle, we might be able to infer the presence of dark matter. This bite explores this idea with a novel approach.

Instead of exploring for a dark matter candidate with an experiment, the authors use astrophysical data. This data is simply measurements of infrared photons (light) from the background of the sky! The authors exploit measurements made by others of astrophysical objects using the JWST telescope in the infrared. They choose to use the parts of this data that have blank sky for them to look at in the background of measurements of other objects. But why would they want to do this?

Supposedly, every galaxy sits inside a halo of dark matter. By looking out into the blank sky, we also can’t avoid staring into this halo, and thus into some invisible volume filled with dark matter particles. In some theories, a dark matter particle may decay into two photons or a photon and some other particle – an example is the axion, but any dark matter candidate that decays into a photon could be detected. In such a case, the wavelength of the produced photon can also tell us about the mass of the dark matter candidate. By looking at infrared photons in the blank sky, if an excess number of photons is seen, this would be telling of dark matter. Additionally, the number of photons we see tells us how strongly the dark matter interacts with photons. This data is sensitive to dark matter particles of a mass range between 0.1-4.1 eV (particle masses are often expressed as an amount of energy in electron-volts using mass-energy equivalence).

Constraints using the total photon flux

As a first test, the authors compare the total flux of infrared photons from JWST spectra to various models of flux spectra with different rates of photon emission, for a particular dark matter mass. The excess model flux relative to the data is summed up over all different wavelengths detected. Taking these results, they then only keep the models where the emission rate results in an excess flux that surpasses a minimum significance level – they discard emission rate models where the excess flux isn’t excessive. With the leftover models they allow the dark matter mass to vary and compare more models to the data, again calculating the excess total flux for each model.

This process allows the authors to effectively rule out models where the emission rate or dark matter mass is not favoured by the data. If the model predicts more flux than the data shows, the summed excess flux will be particularly significant, suggesting the model is not favoured. The disfavoured models are shown in Figure 1 below from the paper.

Constraints using a model for the flux continuum

The authors then complete a second kind of test. In this case, a model is used for the entire flux spectrum that is compared to the data at individual wavelengths. This approach allows them to search for an emission line (a peak in the spectrum due) that could belong to a dark matter candidate. They again scan over models with varying emission rates and masses. They find three models for emission lines that are significant, but not strong detections for a dark matter signal (however one emission line model is known to be due to helium). To obtain upper bounds, they again drop models where the modelled spectra and emission lines are not significant compared to the real data.

The authors also account for the fact that absorption lines (dips in the spectra) could make an emission line look more significant than it really is. Essentially, simulations are made of the flux spectra but in which the emission rate by dark matter is fixed to be exactly zero. Then the authors apply their procedure of determining upper bounds on the dark matter emission rate and masses on a number of these simulations. This might seem strange, but it allows them to determine the upper bounds we might find in the case there is definitely no dark matter emission at all. This can be used to make the final upper bound on the emission rate from the data more rigorous.

The resulting bounds from these tests are also shown in Figure 1 by the red region. The grey regions show bounds placed by other experiments, while the dashed blue lines show forecasts for the possible area that could be constrained by JWST data in the future.

Summary

With more data from JWST, it will be possible to get more and more detections of photons from the infrared background. As is shown in Figure 1, this will allow eventually in many years for more rigorous bounds to be derived, as the measured spectra gain increased signal-to-noise. Even better, it is not necessary to get telescope time for this test, because the backgrounds from other images can be used. It will be interesting to see what bounds are made for dark matter candidates in the future.

Edited by: Kylee Carden

Image credit: Webb revisits the Phantom Galaxy, CCA by 4.0, ESA/Webb, NASA & CSA, A. Adamo (Stockholm University) and the FEAST JWST team via Wikimedia Commons.