Exploring the uncharted: filaments with NewAthena

Iacopo Bartalucci

INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano

Cosmological simulations predict that the evolution history of galaxy clusters is driven by dark matter clumps falling preferentially along filamentary structures. The baryonic matter falls anchored along these structures and accretes into the main halo sitting at the intersection of the filaments. The plasma within is undergoing merging processes where collisions heat the gas to temperatures up to several million degrees and feedback processes which regulate their physical and chemical composition. Thus, the filaments around galaxy clusters represent an extremely rich evolution laboratory where assembly and baryon interaction processes can be observed directly along and off-filaments. X-ray observations are the ideal tools to study filament emission as they give access to fundamental quantities such as the plasma density, temperature and chemical composition.

Unfortunately, the plasma within the filaments is extremely rarefied, the typical density being n < 10^-4cm^-3 with a temperature of the order of kT ~ 0.5 keV, and its X-ray emission is very faint. Furthermore, a typical X-ray observation points to the cluster center and its outskirts fall in the external part of the detector. In these regions, the degradation of the point spread function (PSF) of current X-ray telescopes may easily confuse the diffuse emission from filament or structures within with point-source emission from background Active Galactic Nuclei (AGN) which are diluted over large detector areas. Despite their scientific value, filaments remain an almost unexplored territory still.

In this context, NewAthena will be a game changer thanks to the high throughput of its mirrors that enhances by a factor of 10 the current generation X-ray telescope collecting area and the unique synergy between the Wield Field Imager (WFI) and the X-ray-Integral Field Unit (X-IFU) instruments. Furthermore, the stable PSF across the field of view will allow us to discriminate between AGN and diffuse emission also in the outer parts of the detector. The figure presents a clear example of the two instruments synergy. The left panel shows the simulated WFI observation of a massive, M ~ 1.5x10¹⁵ M_☉, and nearby cluster at z = 0.068. The bulk of the cluster emission is within the white circle encompassing where the mean cluster mass density is 500 times the critical density, namely R₅₀₀, which represents roughly the limits of current X-ray generation telescopes. WFI is capable of detecting and characterising the emission in the outskirts giving us a complete view of the complex plasma spatial distribution. This map is of paramount importance to guide deep X-IFU follow-up pointed observations on the two regions of interest shown with the white squares. The right panel of the figure shows an X-IFU spectrum extracted from the Filament 1 region. The black solid line represents the sky background model and the residuals on the bottom show the emission lines of the filaments that will be detected. Therefore, we can use WFI relatively short observations to map the plasma spatial distribution and anchor X-IFU deep observations to characterise the physical properties within and off the filaments for the first time.