Initially known as SDSS J1049+5103,
Beth Willman and collaborators discovered Willman 1 in the 2nd data release of the
Sloan Digital Sky Survey. At the time, it was unclear whether Willman 1 was a globular cluster or a very faint dwarf spheroid galaxy. Combining
spectroscopy from the
W. M. Keck Observatory with imaging from the
Kitt Peak National Observatory, they reported the presence of stellar multi-directional tails, suggesting possible tidal interactions with the Milky Way. Despite tentative evidence for
mass segregation, a feature only observed in globular clusters, they leaned towards a dwarf galaxy classification based on a metallicity spread from three
red giant branch stars. This metallicity spread indicates multiple epochs of
star formation because stars with different metal abundances, or
metallicities, must have formed at different times. These successive generations of stars enrich the surrounding gas with heavier elements leading to an observed spread in metallicity for stars in the object. This extended chemical evolution is a property of
galaxies but not
globular clusters because the
dark matter in galaxies allows them to retain gas as a long-term reservoir for continued star formation. Further spectroscopy in 2007 on the Keck Telescope identified seven
red giant branch members, also with a significant metallicity spread. However, follow-up spectroscopy from the
Hobby-Eberly Telescope in 2008 determined that up to five of these members were likely Milky Way foreground dwarf stars whose presence was inflating the metallicity spread. In 2011,
Beth Willman and collaborators re-analyzed spectroscopy from the
Keck Telescope and carefully characterized Milky Way contaminates, identifying 45 member stars (although a later study showed that this includes four binaries and seven Milky Way stars This sample contains an additional red giant branch member and two
horizontal branch members whose metallicity spread was once again indicative of a dwarf galaxy. Photometry from the
Hubble Space Telescope confirmed this spectroscopic metallicity spread and measured an updated distance to the object. These data also show evidence for an irregular kinematic distribution and a tentative excess of stars at large half-light radii that could point to a disturbed morphology. In 2026,
Marla Geha homogeneously re-reduced over 20 years of archival spectroscopy from the
Keck Telescope for 78 Milky Way satellites, including Willman 1. From these new data, 56 member stars were identified and used to measure the kinematic and chemical properties of the object. Ultimately, Willman 1 was confirmed to be a dwarf galaxy on the basis of its
velocity dispersion and inferred
mass-to-light ratio, lack of
mass segregation, and position on the size-luminosity relation. Additionally, Willman 1 was found to be likely tidally disrupted based on detailed orbital modeling. These models predict that Willman 1 has had a sustained passage near the Milky Way over the course of its orbit, when and where
tidal forces from the Milky Way could have gravitationally disturbed its stars. This history could explain Willman 1’s elliptical shape and odd velocity distribution. As a result of this potential tidal stripping, it is cautioned that the dynamical mass inferred from the internal velocity dispersion may be inaccurate as the object is likely not in dynamical equilibrium. == Properties ==