ARRAKIHS Science
Contents
Dark matter
ΛCDM Tensions
Ultra-low SB
ARRAKIHS Science
According to the Standard Model of Cosmology, the so called Λ-CDM model, our Universe is a vast and dynamic structure composed of various forms of matter and energy. Ordinary matter, which includes all visible and tangible objects like 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%. Radiation, encompassing all forms of electromagnetic waves like light from the Sun and the cosmic microwave background, makes up less than 1%. Understanding these proportions helps scientists determine how the Universe has evolved and predict its future behavior.
ARRAKIHS science will bring a new light on dark matter. 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 the rotational speeds of stars in galaxies that do not decrease with distance from the galactic center as expected based on visible matter alone.
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.
It is expected that many of those dwarf galaxies will display signs of disruption when getting accreted into their host galaxy (central galaxy), presenting in their final phases the formation of diffuse structures (tidal stellar structures) formed from their own stars stripped by gravitational tides.
These structures become more diffuse with time and the accumulation of them placed all around the central galaxy creates a tenuous structure that extends to large distances from the center and that is the origin of what is known as the intra-halo light.
Λ-CDM tension in stellar haloes
ARRAKIHS will assess the significance of reported tensions between predictions of the Λ-CDM cosmolgy 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 halo.
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, ARRAKIHS will provide key tests with statistical significance to probe wether the reported tensions are the result of selection effects and/or small number statistics. Should the tensions be 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, and thus such models must be changed or replaced.
The tests will center on the comparison between the latest CDM+BP models of ME-type galaxies and three sets of halo properties to be observed with ARRAKIHS:
- Identification and determination of the frequency of tidal stellar structures in these halos, together with the characterisation of their 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 and they variance among MW-type galaxies.
Ultra Low Surface Brightness
The core of the ARRAKIHS mission – observations of the unexplored ultra-low surface brightness (SB) Universe – can only be done from space owing to limitations on ground-based SB sensitivity due to the atmosphere. Since the science goals of this mission require to achieve a very low SB over a very wide area with ~1 arcsec resolution, there is no need for a large aperture camera. Instead, the optimum payload is a small, multispectral camera with excellent optical quality over a wide field of view.
In ARRAKIHS we are simulating real observations as shown in the following image. Each panel shows 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 can be seen as bright, spherical objects. These satellites orbit the central galaxy and influence its structure through gravitational interactions. There are also visible various extended features. These features result from gravitational interactions with the satellite galaxies and exhibit a wide variety of shapes, sizes, and extents.
By analyzing galaxies similar to these simulations, the ARRAKIHS mission aims to uncover new insights into galaxy formation and the nature of dark matter.
