ARRAKIHS Science
Contents
Dark matter
ΛCDM Tensions
Ultra-low SB
Dark Matter
According to the Standard Model of Cosmology, the ΛCDM model, our Universe is a vast and dynamic structure composed of various forms of matter and energy. Ordinary matter, including stars, planets, and galaxies, makes up about 5% of the total energy density. Dark matter, a form of matter that doesn’t emit light but exerts gravitational forces, constitutes about 27%. Dark energy, a mysterious force driving the accelerated expansion of the Universe, accounts for approximately 68%.
And finally radiation, all forms of electromagnetic waves like light from the Sun and the cosmic microwave background, make up less than 1%. Understanding these proportions helps scientists determine how the Universe has evolved and predict its future behavior.
There are different theoretical models of dark matter, but observations are very challenging. Currently, we can only infer its existence through its gravitational effects on its surroundings. For example, the presence of dark matter is suggested by the rotational speeds of stars in galaxies, which do not decrease with distance from the galactic center as would be expected if only visible matter were present.
Understanding the nature and behavior of dark matter is essential for a comprehensive view of our Universe. Besides the nature of the dark matter, understanding the processes related to ordinary matter that govern the galaxy formation and evolution is also crucial. ARRAKIHS will provide deeper insights into the physics of our Universe.
The standard cosmological model describes the formation of galaxies as the result of the accretion of smaller ones, called dwarf galaxies. This continuous hierarchical process implies that around any galaxy, small objects known as satellite galaxies should be observed.
Λ-CDM tension in stellar haloes
While ΛCDM works well on large scales, it struggles to explain some of the smaller-scale structures observed in the halos of the Milky Way and the Andromeda galaxies. While improved modelling of star and gas effects may resolve some discrepancies, the number and distribution of satellite galaxies and stellar streams remain key challenges.
Current observations have not been able to fully confirm or refute these predictions due to the limits of existing ground-based telescopes, which can only detect very faint features down to a certain level of brightness. ARRAKIHS will address these challenges by delivering detailed observations that go beyond current ground-based capabilities, allowing for a more precise assessment of these discrepancies and test the ΛCDM model on smaller scales.
ARRAKIHS will assess the significance of reported tensions between predictions of the ΛCDM cosmology combined with current baryon physics (BP) models and ground-based observations of the Milky Way (MW) and Andromeda Galaxies, regarding the properties of galactic stellar halos.
Through deep visible and infrared imaging of a statistically representative sample of nearby stellar haloes of MW-type galaxies in the local Universe, obtained down to unprecedented low surface brightness levels, ARRAKIHS will provide key tests with statistical significance to probe whether the reported tensions are the result of selection effects and/or small number statistics. If the tensions are confirmed with these new observations, ARRAKIHS will demonstrate the inconsistency in the theoretical models of MW-like galaxy formation based on current implementation of CDM+BP. Consequently, such models must be refined, or alternative frameworks should be explored.
The tests will focus on comparing the latest CDM+BP models of MW-type galaxies with three sets of halo properties to be observed with ARRAKIHS:
- Identification and measurement of tidal stellar structures in these halos, including their frequency, shapes, lengths, and widths.
- Determination of the structure, shape, spatial extent, and luminosity profiles of the galaxy stellar halos.
- Measurement of the average luminosity and mass functions of the halo population of dwarf galaxy satellites, along with their variance among MW-type galaxies.
It is expected that many of these dwarf galaxies will show signs of disruption as they are accreted into their host galaxy. During their final phases, they form diffuse structures (tidal stellar structures) composed of their own stars, stripped away by gravitational tides.
Over time, these structures become more diffuse. Their accumulation around the central galaxy creates a faint and extended structure that stretches to large distances from the center, forming what is known as intra-halo light.
Ultra Low Surface Brightness
The core of the ARRAKIHS mission – observations of the unexplored ultra-low surface brightness (SB) Universe – can only be conducted from space due to the limitations of ground-based SB sensitivity imposed by the atmosphere.
Since the science goals of this mission require achieving a very low SB over a wide area with ~1.5 arcsec resolution, a large aperture camera is not necessary. Instead, the optimal payload is a small, multispectral camera with excellent optical quality over a wide field of view. These observations will be complemented by the brighter, yet higher resolution data provided by Euclid, which will map the same galaxies within its Wide Field Program.
ARRAKIHS simulations replicate real observations, as shown in the following image. Each panel displays a two-dimensional surface brightness map of a distinct galaxy and its surroundings, derived from the advanced computer simulation known as FIREbox. At the heart of each panel is the primary galaxy, carefully selected to resemble the types of galaxies that the ARRAKIHS mission will study in the future. Surrounding the central galaxy, numerous smaller satellite galaxies are visible as bright, spherical objects. These satellites orbit the central galaxy and influence its structure through gravitational interactions. Various extended features are also visible. These features result from gravitational interactions with the satellite galaxies and exhibit a wide range of shapes, sizes, and extents.
By analyzing galaxies similar to those in these simulations, the ARRAKIHS mission aims to uncover new insights into galaxy formation and the nature of dark matter.
