Cosmic magnetic fields

The Square Kilometre Array (SKA)

Observations at radio wavelengths play a crucial role in the study of cosmic magnetic fields. Our group is working towards science with the SKA, which is a next generation radio telescope that will transform our understanding of the Universe. In fact, the SKA is two telescopes, one operating at metre-wavelengths (called SKA-Low), and another at centimetre-wavelengths (called SKA-Mid), which will become fully operational in the 2030s. Spain is a member of the SKA, and will have full access to the data products.

Composite image of the future SKA telescopes, mixing what already exists with artist impressions / SKAO

fig_RMGrid_LoTSS_DR2 — The most recent Faraday rotation measure catalog (RM Grid) at metre-wavelengths from the ongoing LOFAR Two-metre sky survey (LoTSS).

On-going projects

Determining the origin of cosmic magnetic fields

Despite their ubiquity, we do not know the origin of the large scale magnetic fields that are observed to exist throughout the cosmic web. Recent evidence suggests they were created in the very early Universe (ie. primordial magnetic fields), and thus provide a unique probe of the physical conditions before the epoch of recombination (ie. the CMB). It is important to constrain the properties of these primordial magnetic fields because they can influence the formation of the first stars and galaxies, are linked to the stochastic gravitational wave background, and may even solve the so-called Hubble tension.

Residual RM rms (after subtraction of the Milky Way RM) vs. redshift from the LoTSS RM Grid (black points). The coloured points show the expected RM rms trends for different magnetogenesis scenarios from cosmological numerical MHD simulations (Vazza et al. 2021). Based on this data we can already ruled out uniform primordial fields of strength ~0.1 nG, as shown by the orange points (Carretti, O’Sullivan, et al. 2023).

A radio galaxy (black contours) and the location of the polarized emission from which the RM could be measured (coloured pixels), and the cosmic web filaments in the foreground, identified from the SDSS optical spectroscopic data, for which the level of magnetisation could be constrained (O’Sullivan et al. 2019).

Mapping the magnetised baryonic flows from the CGM to the WHIM

The RM Grid technique also provides an exciting opportunity to probe regions of the Universe that are challenging to study by other means, by illuminating the Warm-Hot Intergalactic Medium (WHIM) and the ionised circumgalactic medium (CGM). It may also help solve the “missing baryon” problem. In general, this is important for understanding how the evolution of galaxies in related to the gas distributed throughout the cosmic web.

Plot of the residual RM dispersion as a function of the splashback radius for a sample of 55 nearby galaxy groups, using data from the ASKAP-POSSUM survey. The green ring highlights the excess RM signal related to the hot intergroup medium (IGrM), while the blue ring highlights the excess related to the cosmic web filaments connected to the groups (Anderson et al. 2024).

Probing the magnetised intergalactic medium at the centre of the Shapley supercluster, with data from the Planck telescope (Sunyaev-Zeldovich effect y-map: colour-scale and contour) and with the RMs of background radio galaxies (coloured points) from the ASKAP-POSSUM survey (Alonso-López, O’Sullivan, et al. in prep).

Acknowledgements: This research is supported by funding from the Comunidad de Madrid Atracción de Talento program (2022-T1/TIC-23797) and from the Ministerio de Ciencia, Innovación y Universidades Proyecto de Generación de Conocimiento (PID2023-146372OB-I00).