H3 is a spectroscopic survey of ~200,000 Milky Way halo stars in the Northern hemisphere using the Hectochelle instrument on the MMT. H3 is a collaboration between the Center for Astrophysics (CfA) and the University of Arizona.
The primary goal of the H3 survey is to trace the assembly history of the Milky Way galaxy. This is acheived by measuring the 6D phase space and chemical space structure of the stellar halo. The H3 Survey is delivering radial velocities, element abundances, and spectroscopic distances, while the Gaia satellite is providing parallaxes and proper motions.
The H3 Survey will unravel the assembly history of the Galaxy by addressing the following key science questions:
Ana Bonaca (CfA)
Nelson Caldwell (CfA)
Phill Cargile (CfA)
Charlie Conroy (CfA)
Jesse Han (CfA)
Ben Johnson (CfA)
Rohan Naidu (CfA)
Josh Speagle (U. Toronto)
Yuan-Sen Ting (IAS)
Dennis Zaritsky (U. Arizona)
H3 stars are selected based solely on magnitude (r<18) and parallax (<0.5 mas). A sparse tiling strategy is adopted to cover 15,000 sq. degrees with 1000 fields. These fields have been chosen to avoid the Galactic plane (|b|>30) and must be observable from Arizona (Dec.>-20).
The Hectochelle instrument provides R~32,000 spectra across the wavelength range 515nm-530nm. This interval includes the surface gravity-sensitive MgI triplet as well as numerous additional atomic transitions. The survey delivers S/N~3 at r=18, which is sufficient for high quality radial velocity measurements and, when combined with optical-NIR SED coverage, good stellar parameter estimation (temperature, surface gravity, metallicity and alpha-abundance) as well.
Observations began in the Fall of 2017. 147,000 spectra have been collected as of Dec, 2020.