Via

The Via Project is an all-sky spectroscopic survey operating on twin 6.5-meter telescopes—the MMT in Arizona and Magellan Clay in Chile. Via will observe over 2 million stars across five years to address fundamental questions about dark matter, galaxy formation, and the structure of our Galaxy.

The Via instrument is composed of two parts, Viaspec and Boombox. With 576 robotically-positioned fibers covering a 1-degree field of view, Viaspec combines unprecedented multiplexing capability with high spectral resolution (R ≈ 15,000) and radial velocity precision (~100 m/s). This unique combination enables Via to probe the smallest scales of dark matter structure and map cold gas throughout the Milky Way with exquisite detail.  Boombox is a 36-fiber R ≈ 1,000 spectrograph optimized for transient classification and characterization. 

Via will conduct a five-year, 500-night all-sky survey, with first light expected in 2027. The Via Project is a collaboration between Center for Astrophysics | Harvard & Smithsonian, Carnegie Observatories, Stanford University / KIPAC, and Yale University.  KIPAC is leading the simulation program, key aspects of the dark matter inference, and the cold gas tomography program, and will engage in transient science with LSST. Via's key science goals are as follows:

Stellar Streams and Dark Matter Substructure

Via will measure velocity disturbances within cold stellar streams caused by dark matter subhalos, providing a definitive test of dark matter models. By achieving ~100 m/s radial velocity precision for faint stars (G < 21), Via can detect the characteristic velocity signatures imparted by low-mass (10⁶–10⁷ M☉) subhalos—objects expected to be entirely devoid of stars. These measurements will distinguish between cold dark matter and alternative models like warm or self-interacting dark matter, and separate dark subhalos from baryonic perturbers like giant molecular clouds. 

The survey will conduct detailed observations of 15-20 stellar streams, obtaining complete coverage with multi-epoch measurements to identify binary stars and characterize subhalo interactions. This represents a unique opportunity to detect truly dark subhalos and constrain the matter power spectrum on the smallest scales.

Cold Gas Tomography

Using hundreds of thousands of Milky Way stars as backlights, Via will create the first comprehensive three-dimensional map of cold gas in the circumgalactic medium (CGM). Via's high spectral resolution enables clean separation of interstellar Na I and Ca II absorption from stellar features at most relative velocities, allowing us to trace cold gas density and velocity structure from sub-parsec to 100 kpc scales.

This tomographic mapping will reveal the origin and fate of cold CGM gas—whether it arises from thermal instabilities in the hot halo, galactic fountain outflows, direct accretion from the intergalactic medium, or gas stripped from infalling satellites. By probing gas at distances beyond 10 kpc, Via will test competing models that predict vastly different amounts of cold gas in the outer halo, directly constraining theories of gas accretion and recycling in galaxy halos. 

The Threshold of Galaxy Formation

Via's Dwarf Galaxy Survey will spectroscopically characterize Milky Way satellite galaxies down to the critical mass threshold where galaxy formation becomes suppressed. With sub-km/s velocity precision, Via will resolve the velocity dispersions of ultra-faint dwarfs (UFDs) to determine their dynamical masses and separate dark matter-dominated galaxies from morphologically similar star clusters.

Following new dwarf galaxy discoveries from the Legacy Survey of Space and Time (LSST), Euclid, and Roman, Via will provide essential spectroscopic follow-up to confirm candidates and measure their properties. These observations will constrain the halo mass function at the lowest masses, test models of reionization and feedback, and probe the nature of dark matter through the abundance and properties of the faintest galaxies. 

Characterizing the Transient Universe

Via's Boombox spectrograph (R ≈ 1,000, covering 360–960 nm) will provide rapid spectroscopic follow-up of thousands of transients discovered by LSST. With sensitivity to m ~ 24 and frequent on-sky presence, Via will capture rare and exotic phenomena including gravitational wave counterparts, superluminous supernovae, and tidal disruption events.

The survey will build an unbiased statistical sample of supernovae and their host galaxies reaching the LSST detection limit, enabling population studies across cosmic time. Real-time, data-driven anomaly detection will identify the most interesting events for immediate target-of-opportunity observations, exploring the "unknown unknowns" in the transient universe.