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. Diffuse stellar components such as stellar halos 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 primarily by gravitational processes acting over cosmic time, and therefore provide a direct observational link between galaxy assembly histories and the properties of their dark matter 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 datasets of galaxy outskirts. Ground-based observations are affected by time-variable atmospheric emission, scattered light, and sky subtraction uncertainties, which introduce systematic effects that dominate at the very surface brightness levels where most informative structures reside. 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-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 Milky Way-mass galaxies?
- What are the statistics of halo properties? What are the mean values and intrinsic scatter of these halo properties across a representative sample of Milky Way-like galaxies? where do the Milky Way and Andromeda lie within these distributions when analysed to similar depth?
- What is the range of stellar population parameters do faint stellar halos and LSBFs?do their variations in age and metallicity align with predictions from galaxy-formation models and with those observed in the Local Group?
- How well do current galaxy formation models work? Do they reproduce the amount, morphology, and surface-brightness distribution of faint stellar mass distributions? hich elements of their baryonic or dark-matter modelling drive agreement or discrepancies?
- What constraints can the morphologies and orientations of long, coherent tidal streams place on the three-dimensional shape of the host dark matter halo?
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 of a statistically representative sample of at least ~80 Milky Way–mass galaxies in the Local Universe (d ≤ 40 Mpc). The mission will measure the properties of faint stellar components of these galaxies and translate them into physical constraints on galaxy formation models and dark matter structure. The key observational components are:
- Diffuse Stellar Halos (DSH)
- Low Surface Brightness Features (LSBFs), including tidal streams and debris structures
- Satellite galaxies (as complementary tracers of accretion history)
From these, the mission will derive statistical descriptors such as:
- Stellar halo mass fractions and radial profiles
- 2D morphology and spatial extent of diffuse halos
- Incidence, morphology, and distribution of LSBFs
- Integrated stellar mass in accreted structures
- Stellar population properties from VIS–NIR colours
These observables will be interpreted in terms of:
- Accretion history (timing, frequency, progenitor mass spectrum)
- Assembly diversity across Milky Way–mass galaxies
- Constraints on baryonic physics in galaxy formation models
- Tests of ΛCDM and alternative dark matter scenarios
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 models and dark matter physics
This framework provides a direct observational link between galaxy assembly processes and the properties of their dark matter halos, enabling a quantitative test of galaxy formation models in a previously unexplored regime.