ARRAKIHS: Seeing the Unseen, Revealing the Hidden History of MW-like Galaxies
ARRAKIHS (Analysis of Resolved Remnants of Accreted galaxies as a Key Instrument for Halo Surveys) is an ESA F-class mission (F2) designed to address a central Cosmic Vision question: how galaxies assemble their mass and structure through hierarchical accretion. By targeting the extremely faint stellar outskirts of nearby galaxies, ARRAKIHS will provide the first statistically robust and homogeneous census of stellar halos and low–surface–brightness features (LSBFs) around Milky Way-mass galaxies beyond the Local Group.
The scientific motivation for ARRAKIHS is rooted in the recognition that the outer regions of galaxies preserve a fossil record of their assembly history. Stellar halos, tidal streams, shells, and diffuse envelopes encode information on the frequency, mass spectrum, and timing of past accretion events, as well as on the structure of dark matter halos. Existing surveys are fundamentally limited by systematic effects and insufficient surface-brightness depth. ARRAKIHS is specifically designed to overcome these limitations by reaching surface–brightness limits of approximately 31.5 mag/arcsec² in the visible, enabling the detection of structures significantly fainter than those accessible to current wide–field surveys.
ARRAKIHS will observe a statistically representative sample of at least 80 nearby Milky Way-mass galaxies, selected from a homogeneous, volume and mass limited target pool in the local Universe.
The survey strategy employs repeated visits, arcminute-scale pointing offsets, and arcsecond-scale dithering to optimise depth, sampling, and calibration while suppressing instrumental and environmental systematics. This approach ensures uniform data quality across the survey and enables robust comparative studies of galaxy assembly across a wide range of environments.
The payload comprises a four-band wide-field imaging system providing continuous wavelength coverage from 280–1600 nm, with two visible (VIS1, VIS2) and two near-infrared (NIR1, NIR2) channels.
The instrument is based on two binocular telescopes with a modified Maksutov-Cassegrain design and a common field of view exceeding 1.15° in diameter.
ARRAKIHS at UZH
The UZH Department of Astrophysics hosts two key leadership roles within ARRAKIHS and is responsible for the Thermal and Structural Subsystem (TSS), a mission-critical element that ensures the spacecraft can operate safely and achieve the stability required for ARRAKIHS’ scientific observations. In particular:
- Prof. Ben Moore is the Swiss Coordinator who leads the national scientific team
- Dr. Rebekka Bieri is the ARRAKIHS Mission Consortium (AMC) Science Coordinator, leading the scientific coordination of the international consortium
- Dr. Valentina Tamburello is the TSS Project Manager, leading a consortium that includes FHNW, ZHAW, HSLU and the company Koegl Space.
- Prof. Robert Feldmann is part of the science team.