Wavelength (angstroms)8,580 8,600 8,620 8,700 8,7209433,262 light-years10 arc sec17901968223925372726145294317901968223925372726145248 ASTRONOMY • MARCH 2020
ABOVE: Shortly after determining
galaxy DF2 is lacking dark matter,
van Dokkum and his team
identified another galaxy, DF4,
that likewise seems to be missing
the mysterious substance. The
highlighted objects are DF4’s
globular clusters. VAN DOKKUM ET AL. (2019)LEFT: To determine how quickly
the globular clusters were moving
in DF2, the researchers analyzed
the absorption lines found in each
globular cluster’s spectrum. This
revealed clues about the clusters’
motions, and researchers then
used this information to calculate
the overall mass of the galaxy.
GEMINI OBSERVATORY/NSF/AURA/W.M. KECK
OBSERVATORY/JEN MILLER/JOY POLLARDCLOCKING GLOBULAR CLUSTERS