Legacy & Ancillary Science
Contents
Introduction
Ultra-faint dwarf galaxies
Four-band detection
The primary ARRAKIHS science cases define the core mission objectives, focusing on the formation and evolution of stellar halos, low-surface-brightness features, and their connection to galaxy assembly. However, the survey design, depth, and multi-band imaging strategy naturally enable additional scientific return beyond the baseline goals.
These additional opportunities arise in two complementary ways: (i) legacy science, exploiting the depth, area, and multi-band nature of the ARRAKIHS dataset, including repeated or serendipitous analyses of faint structures; and (ii) ancillary science, enabled through targeted observations during survey gaps and implemented through an ESA-managed filler programme (see Chapter 10). Together, they significantly enhance the scientific impact of the mission.
Legacy science from the ARRAKIHS survey
Faint structures in the outer disks of MW-mass galaxies. At the surface-brightness depth reached by ARRAKIHS, the outer regions of galaxy disks become accessible alongside stellar halos. These regions host a wide range of faint and complex structures that trace both internal disk evolution and external interactions.
Such features include extensions and truncations of spiral arms, tidal distortions induced by past interactions, low-surface-brightness rings, and diffuse star-forming structures in the outskirts of disks. In several cases, these outer components are associated with ongoing but extremely diffuse star formation, as observed in extended ultraviolet (XUV) disks and outer star-forming rings in nearby galaxies (e.g. Thilker et al., 2007).
A systematic, multi-band characterization of these structures across the ARRAKIHS sample will enable a uniform census of outer-disk phenomena in Milky Way–mass galaxies. This will provide new constraints on disk regrowth, secular evolution, and the role of minor interactions in shaping galaxy outskirts, complementing the halo-focused science cases of the mission.
Ultra faint Galactic cirrus. Galactic cirrus is produced by interstellar dust scattering optical and near-infrared light from the Galactic radiation field, and is commonly observed as diffuse filamentary emission in deep extragalactic imaging. While cirrus is a contaminant for low-surface-brightness extragalactic science, ARRAKIHS mitigates this effect through strict field selection in regions of very low dust column density (E(B−V) < 0.03).
However, the depth and multi-band nature of ARRAKIHS also provide a unique opportunity to study cirrus in the optically thin regime. The four-filter system enables characterization of the spectral energy distribution of diffuse dust scattering across optical and near-infrared wavelengths.
These measurements can be used to constrain dust grain properties, including albedo and scattering phase function, and to investigate the size distribution of interstellar dust. When combined with far-infrared surveys, ARRAKIHS observations can also improve extinction corrections in the low E(B−V) regime, where current calibrations are systematics-limited.
Ancillary science with the ARRAKIHS mission
Measuring the splashback radius in galaxy clusters. The splashback radius marks the apocentre of recently accreted material in galaxy clusters and provides a direct tracer of halo growth and accretion history. It is expected at ~1.5–2 r200 and can be probed through the radial profile of intra-cluster light (ICL).
At the surface-brightness levels reached by ARRAKIHS (low-to-mid 30 mag/arcsec²), the ICL profile steepening associated with the splashback feature becomes observable for intermediate-redshift clusters. By measuring radial ICL profiles across a carefully selected sample, ARRAKIHS can directly constrain splashback radii and provide a physically motivated measurement of cluster assembly state.
Legacy science from the ARRAKIHS survey
Stellar streams around dwarf galaxies. Stellar streams around dwarf galaxies probe hierarchical structure formation at the lowest galaxy mass scales. While most studies have focused on more massive dwarf hosts, recent detections show that tidal substructure also exists in lower-mass systems, demonstrating that accretion processes extend well below Milky Way scales.
By targeting nearby dwarf galaxies, ARRAKIHS can search for faint stellar streams and tidal debris in regimes where deviations from standard ΛCDM predictions may be more pronounced. This provides a complementary test of hierarchical galaxy formation in the low-mass regime.
