Science Cases
Contents
Tidal Stellar Structures
Intra Halo Light
Satellite membership
The ARRAKIHS mission has the following scientific objectives: Provide robust statistics of the numbers and properties of low surface brightness features (LSBFs), streams, tides, shells and tails, characterize the shape and extent of the ultra-faint Intra Halo Light and finally characterize the abundance and locations of satellite galaxies, down to MV (absolute visual magnitude) < -6 for a complete sample of Milky Way-like galaxies beyond the Local Group.
Characterisation of low surface brightness features
The detection and characterisation of these LSBFs, mainly remnants of the accretion of dwarf galaxies – including measurements of their abundance, width, and shapes/morphology – probe the recent merger activity, abundance of substructure, shape of the potential, and even the nature of dark matter, as WDM cosmological models predict fewer tidal streams around Milky Way-like galaxies than CDM models. At present, leading theoretical models predicts a higher frequency of streams than those obtained from ground-based ultra-deep observations of massive galaxies in the local Universe. The question that arises is then: are the discrepancies between observed streams and ΛCDM models the result of incompleteness in observing the faint end of the luminosity function due to their very low (>28 mag / arcsec2) or a real tension in the CDM model?
ARRAKIHS will provide the first comprehensive survey of a large population of debris of satellites. This unparalleled survey of LSBFs will benefit from reaching ×10 deeper in surface brightness, enable for the first time a statistical analysis of LSBFs populations in multiple Milky-Way like systems. By characterizing stellar streams, tides, shells and tails in these galactic halos, we can (i) learn about the recent merger history, and (ii) probe the disruption mechanism which is a product of the merger orbit and DM halo (which are both predictions of our adopted cosmology and DM model). Alternatively, identifying large populations of galaxies lacking LSBFs all-together would pose a serious challenge to ΛCDM.
Intra Halo Light
The extragalactic background light (EBL) serves as a cumulative representation of the total emission from all stars and galaxies throughout the history of the universe. By examining the diffuse light not linked to any specific astronomical source, scientists can gain insights into the combined emission from the first galaxies during the epoch of reionization. Recent analyses of cosmic infrared background (CIB) fluctuations have revealed unexpected fluctuations at scales larger than 20 arcseconds.
There are two main hypotheses regarding these fluctuations: one suggests they are due to the redshifted light from the first stars and black holes formed during the epoch of reionization, while the other proposes they result from the combined light of stars outside galaxies at a closer distance. In the latter case, this intra-halo light (IHL) is believed to predominantly come from stars that were stripped from galaxies during mergers. Studying stellar tidal streams in the local Universe can provide important constraints on cosmological simulations, which can then predict the diffuse emission associated with galaxy formation. By analyzing the characteristics of IHL, such as its brightness and color, researchers can differentiate between the contributions of low-redshift stars and early cosmic sources. This approach enables the use of near-infrared observations to study the properties of early high-redshift objects, such as the first stars and black holes, which might otherwise be challenging to observe directly even with advanced future telescopes.
Statistical evaluation of satellite membership
One of the current challenges to the Λ-CDM model is the number of satellite galaxies: cosmological simulations considering only dark matter predict a much larger number of dwarf galaxies than what is actually observed. This discrepancy is observed for the Local Group, which is the best studied galaxy system, but there are hints that satellites are also under-abundant in nearby galaxies. The understanding of how baryonic physics influences the formation of galaxies has provided several avenues to address some of the discrepancies between Λ-CDM theory and observational data. For example, when considering the luminosity function of satellite galaxies in the Milky Way (MW) or Andromeda (M31), it is possible to reconcile observations with Λ-CDM expectations by hypothesizing that most subhalos produce only a small number of stars. A a similar decrease in the number of faint satellites could also be expected if these lower mass subhalos simply do not form exist in the first place, as proposed by some warm dark matter (WDM) models.
Unfortunately, our current understanding is limited by the fact that only the MW and M31 systems have been extensively studied in the ultrafaint regime. Consequently, it is impossible to discriminate between a) an incomplete understanding of the baryonic physics shaping dwarf galaxies and b) a fundamental challenge to the ΛCDM scenario. At the heart of these tensions lies a lack of observational data. To solve the missing satellites, and the satellite planes problems, it is essential to make a complete catalog of ultra-low surface brightness satellite galaxies below 28 mag/arcsec2 for a statistically representative sample of galaxy halos. The accurate photometry of these extremely low SB systems provided by ARRAKIHS in the visible and infrared bands is crucial to obtain the stellar masses and metallicities of the detected dwarf satellites, a fundamental step to confirm if their dwarf galaxy nature and to understand their star formation history, including the quenched fraction of satellites around the host galaxy. The multi- wavelength observations are also required to distinguish the faint, unresolved satellites population from the cosmological background. This catalog will allow us to assess whether the predictions of ΛCDM are correct or if they have to be reviewed in favor of other dark matter model candidates, such as Warm Dark Matter (WDM) or Self-Interacting Dark Matter (SIDM), which predict less numbers of dwarf-galaxy satellites.
Science
Instrument
Survey
