The Extended Globular Cluster System of the archetypal "failed galaxy" Dragonfly-44 from deep white-light Hubble Space Telescope imaging

Abstract

For nearly a decade, Dragonfly-44 (DF44, $M_{\star} = 3\times10^8\,{\rm M}_\odot$) 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 $N_{\rm GC}=78.3\pm3.7$ GCs in a spatially extended system with a half-number radius of $R_{\rm gc}=1.41^{+0.57}_{-0.25}R_{\rm e}$, at the high end of previously published values. The GC system comprises ${\sim}5\%$ of the total stellar mass and implies a halo mass of $\log(M_{\rm vir}/M_\odot)=11.6\pm0.3$, 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.