2026/07/28 by Maria Luisa Buzzo, Pieter van Dokkum, Roberto Abraham +2
Physics and Astronomy · #astro-ph.GA
Submitted to ApJL. 10 pages, 4 figures
arxiv created 2026/07/28 · arxiv updated 2026/07/30
For nearly a decade, Dragonfly-44 (DF44, M⋆ = 3×108 \rm M_\odot) has been considered an archetypal ''failed galaxy'', a system so rich in globular clusters (GCs) and dark matter that it challenges standard dwarf galaxy formation scenarios. Yet a key measurement underpinning this classification has remained controversial, with published GC counts differing by a factor of four. Here we present new ultra-deep Hubble Space Telescope WFC3/UVIS imaging of DF44 in the F350LP filter, reaching more than one magnitude below the turnover of the GC luminosity function. We find that DF44 hosts N\rm GC=78.3±3.7 GCs in a spatially extended system with a half-number radius of R\rm gc=1.41+0.57-0.25R\rm e, at the high end of previously published values. The GC system comprises ∼5% of the total stellar mass and implies a halo mass of log(M\rm vir/M_\odot)=11.6±0.3, in agreement with the cored halo mass inferred from stellar kinematics by van Dokkum et al. 2019. This places DF44 among the most dark matter-dominated galaxies known, with a dark matter fraction exceeding 99.9%. The combination of extreme GC richness and extreme dark matter content establishes DF44 as one of the clearest examples of a failed galaxy: a system that assembled a massive halo and rich GC population early, but never formed the field stellar mass expected for its halo. Although clustered star formation in high-pressure, early-collapsing halos offers a promising starting point, no current model or simulation reproduces failed galaxies as a class, making DF44 a critical benchmark for future models of galaxy formation.