Abstract:

ARRAKIHS (Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys) is ESA's selected Fast-class space mission dedicated to exploring the ultra-low surface brightness Universe. Its primary goal is to reveal and characterise the faint stellar structures that surround nearby Milky Way-mass galaxies, including extended stellar haloes, tidal streams, shells, and other debris from past accretion events. Stellar haloes preserve the fossil record of galaxy growth. Their morphology, substructure content, and radial light profiles encode information about merger histories, satellite disruption, and the interplay between baryonic processes and dark matter. However, these features typically lie at surface-brightness levels far below what can be reliably reached from the ground due to atmospheric scattering, sky background, and instrumental systematics. ARRAKIHS is specifically designed to overcome these limitations through a purpose-built, high-contrast imaging system optimised for stability and stray-light suppression. By systematically unveiling the ultra-low surface brightness Universe, ARRAKIHS will open a new observational window on the processes that shape galaxies and their dark matter haloes across cosmic time.

The mission will fly two innovative binocular telescope assemblies, each with dual channels, covering near-UV, optical, and near-infrared bands (280-1600nm). The optical design prioritises minimisation of scattered light, flat-field stability, and accurate background control, key requirements for detecting diffuse emission at ultra-low surface brightness levels. Operating in a Sun-synchronous low Earth orbit, ARRAKIHS will conduct a homogeneous, space-based survey of a carefully defined sample of nearby galaxies over a nominal three-year observing campaign. This strategy will deliver statistically robust and internally consistent measurements of halo surface-brightness profiles, substructure frequency, and morphological diversity of LSB structures around MW-mass galaxies.

ARRAKIHS will bridge the gap between deep, targeted imaging of individual systems and large cosmological surveys that lack the surface-brightness sensitivity required to characterise extended halo structures. In synergy with facilities such as Euclid and other wide-field surveys, ARRAKIHS will combine ultra-deep diffuse-light measurements with high-resolution structural and environmental information. Together, these datasets will help to unveil the hierarchical assembly of Milky Way-like systems, provide new empirical tests for the current galaxy formation models, and potentially prove some of the small-scale predictions of the standard cold dark matter paradigm.

This presentation will outline the mission's scientific objectives, measurement strategy, and instrument concept, and will place ARRAKIHS within the broader landscape of upcoming space and ground-based facilities.