Science Objectives
Contents
ARRAKIHS Objectives
Open Questions
What we will measure?
Science Objectives
The Low Surface Brightness (LSB) Universe represents a key frontier in extragalactic astronomy. Within the LSB Universe, faint stellar components such as the diffuse stellar halo and tidal streams, trace the hierarchical assembly of galaxies, preserving the long-lived fossil record of accretion and interaction events that is largely erased in their bright central regions. These structures are shaped by both dark matter and baryonic processes acting over cosmic time, and therefore provide a direct observational link between galaxy assembly histories and the physical mechanisms that shape galaxies and their halos.
The central scientific motivation of ARRAKIHS is to understand how galaxies and their dark matter halos assemble through accretion and interaction, and how variations in these assembly histories give rise to the diversity of present-day galaxies. A second key goal is to use LSB structures as probes of baryonic physics and dark matter, linking observable faint stellar components to the underlying theoretical framework of galaxy formation.
Progress in this field has been limited by the difficulty of obtaining deep, homogeneous, and statistically representative survey datasets probing the LSB universe. Ground-based observations are affected by time-variable atmospheric emission, scattered light, and sky subtraction uncertainties, which introduce systematic effects that dominate at the lowest surface brightness levels, where many informative structures are found. In addition, existing surveys typically cover limited areas or heterogeneous samples. Even when wide coverage is achieved using mosaic detectors, variations in detector response and inter-chip gaps introduce additional systematics that hinder uniform ultra-low surface brightness measurements.
As a result, the current observational picture of stellar halos and LSBFs beyond the Local Group remains incomplete and biased toward the brightest structures. A space-based, stable imaging platform is therefore required to reach the necessary depth and uniformity over a statistically meaningful galaxy sample.
Scientific motivation and open questions
Within this context, ARRAKIHS addresses the following questions:
- How do Milky Way (MW)-mass galaxies and their dark matter halos assemble?
- How do the mass, radial structure, and shapes of accreted stellar halos constrain the amount, timing, and typical progenitor mass of past accretion events in MW-mass galaxies?
- What are the building blocks of stellar halos and progenitors of LSBFs?
- What is the distribution of halo properties for MW-mass galaxies?
- Where do the MW and Andromeda lie within these distributions when analysed to similar depth?
- Which aspects of galaxy formation models need to be updated in order to match observations?
- Do models reproduce the amount, morphology, and SB distribution of faint stellar mass distributions?
- Which elements of their baryonic or dark-matter modelling drive agreement or discrepancies?
- Do variations in stellar population ages and metallicities align with predictions from galaxy formation models and with those observed in the Local Group?
- What constraints can long, coherent tidal streams place on dark matter properties?
- How well can the three-dimensional shape and orientation of the host’s dark matter halo be inferred from the stream’s morphology?
- What constraints do these shapes place on alternative dark matter models?
Answering these questions requires moving from individual case studies to statistically robust measurements of LSB structures in a representative sample of Milky Way–mass galaxies.
ARRAKIHS scientific approach
ARRAKIHS is designed to overcome current observational limitations by providing deep, homogeneous, and simultaneous visible (VIS) and near-infrared (NIR) imaging in four different bands of a statistically representative sample of at least ~80 Milky Way–mass galaxies in the Local Universe (d ≤ 40 Mpc). The mission will characterize the faint stellar components of these galaxies and use them to constrain galaxy assembly histories, galaxy formation processes, and the role of dark matter in shaping galaxy outskirts. The key observational components are:
- Low Surface Brightness Features (LSBFs), including tidal streams and other debris structures.
- Diffuse Stellar Halos (DSH).
- Satellite galaxies (as complementary tracers of accretion history).
From these, the mission will derive statistical descriptors such as:
- Incidence, morphology, and radial distribution of LSBFs.
- Stellar mass in LSBFs.
- Accreted stellar mass halo mass and radial profiles (1D).
- Spatial shape and extent (2D) of morphology and spatial extent of diffuse halos.
- Stellar population properties from VIS–NIR colours.
These observables will be interpreted in terms of:
- Accretion history (timing and amount of events, frequency, progenitor mass spectrum).
- Assembly diversity across Milky Way–mass galaxies.
- Constraints on baryonic physics in galaxy formation models.
- Distribution of accreted progenitors and systems.
- Understand how long stellar streams can constrain the properties of dark matter.
- Others…
In particular, coherent tidal streams will provide additional constraints on the shape and orientation of dark matter halos through their orbital structure.
By combining a homogeneous dataset with a statistically significant galaxy sample, ARRAKIHS will:
- quantify the mean properties and intrinsic scatter of stellar halos and LSBFs.
- place the Milky Way and Andromeda in a broader cosmological context.
- connect faint stellar structures to galaxy assembly histories.
- provide new empirical constraints on galaxy formation physics and dark matter models.
This framework provides a direct observational link between galaxy assembly processes and the properties of theirthe observable halo properties, enabling a quantitative test of galaxy formation models in a previously unexplored regime.