Prospects for disentangling dark matter with weak lensing

TL;DR

This paper explores how upcoming weak lensing surveys can differentiate between ultra-light axion dark matter and baryonic feedback effects, potentially constraining axion properties and breaking degeneracies in structure formation models.

Contribution

It introduces a halo model emulator for mixed dark matter cosmologies including baryonic effects, and demonstrates how LSST data can constrain axion mass and fraction in dark matter.

Findings

Galaxy shear data can limit axion mass to less than 5% of dark matter.

Weak lensing is sensitive to axions over a wide mass range from 10^{-27} to 10^{-21} eV.

External feedback constraints can help break degeneracies and detect axions at 3 sigma significance.

Abstract

We investigate the degeneracy between the effects of ultra-light axion dark matter and baryonic feedback in suppressing the matter power spectrum. We forecast that galaxy shear data from the Rubin Observatory's Legacy Survey of Space and Time (LSST) could limit an axion of mass $m = 10^{-25}\,\mathrm{eV}$ to be $\lesssim 5\%$ of the dark matter, stronger than any current bound, if the interplay between axions and feedback is accurately modelled. Using a halo model emulator to construct power spectra for mixed cold and axion dark matter cosmologies, including baryonic effects, we find that galaxy shear is sensitive to axions from $10^{-27}\,\mathrm{eV}$ to $10^{-21}\,\mathrm{eV}$, with the capacity to set competitive bounds across much of this range. For axions with $m \sim 10^{-25}\,\mathrm{eV}$, the scales at which axions and feedback impact structure formation are similar, introducing a parameter degeneracy. We find that, with an external feedback constraint, we can break the degeneracy and constrain the axion transfer function, such that LSST could detect a $10^{-25}\,\mathrm{eV}$ axion comprising 10\% of the dark matter at $\sim 3 \sigma$ significance. Direct reconstruction of the non-linear matter power spectrum provides an alternative way of analysing weak lensing surveys, with the advantage of identifying the scale-dependent features in the data that the dark matter model imposes. We advocate for dedicated cosmological hydrodynamical simulations with an axion dark matter component so that upcoming galaxy and cosmic microwave background lensing surveys can disentangle the dark matter-baryon transfer function.