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⋆ = 3×108 M⊙) 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 canonical turnover of the GC luminosity function. We find that DF44 hosts NGC=73.1±8.6 GCs in a spatially extended system with a half-number radius of Rgc=1.48+0.39−0.51Re, at the high end of previously published values. The GC system comprises ∼3.9% of the total stellar mass and, if DF44 follows the empirical GC number--halo mass relation, implies log(Mvir/M⊙)=11.5±0.3, consistent within the uncertainties with the cored halo mass inferred from stellar kinematics by van Dokkum et al. 2019. Together with a specific frequency of SN=42.1±4.9, the rich GC population strongly supports the interpretation of DF44 as a failed-galaxy candidate that assembled a massive halo and rich GC population early, but never formed the field stellar mass expected for its halo. Current formation models reproduce several individual properties of DF44, but its combination of GC richness, early quenching, and high inferred halo mass remains an important constraint on models of dwarf-galaxy formation.
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BibTeX
@misc{buzzo2026extended,
title = {{The extended globular cluster system of the archetypal "failed galaxy" Dragonfly-44 from deep white-light Hubble Space Telescope imaging}},
author = {Maria Luisa Buzzo and Pieter van Dokkum and Roberto Abraham and Shany Danieli and Aaron J. Romanowsky},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2607.26152},
eprint = {2607.26152},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.26152},
}
RIS
TY - UNPB
TI - The extended globular cluster system of the archetypal "failed galaxy" Dragonfly-44 from deep white-light Hubble Space Telescope imaging
AU - Maria Luisa Buzzo
AU - Pieter van Dokkum
AU - Roberto Abraham
AU - Shany Danieli
AU - Aaron J. Romanowsky
PY - 2026
DA - 2026/09/11
PB - arXiv
DO - 10.48550/arxiv.2607.26152
UR - https://arxiv.org/abs/2607.26152
AB - For nearly a decade, Dragonfly-44 (DF44, M_⋆ = 3×10^8 M_⊙) 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 canonical turnover of the GC luminosity function. We find that DF44 hosts N_GC=73.1±8.6 GCs in a spatially extended system with a half-number radius of R_gc=1.48^+0.39_−0.51R_e, at the high end of previously published values. The GC system comprises ∼3.9% of the total stellar mass and, if DF44 follows the empirical GC number--halo mass relation, implies log(M_vir/M_⊙)=11.5±0.3, consistent within the uncertainties with the cored halo mass inferred from stellar kinematics by van Dokkum et al. 2019. Together with a specific frequency of S_N=42.1±4.9, the rich GC population strongly supports the interpretation of DF44 as a failed-galaxy candidate that assembled a massive halo and rich GC population early, but never formed the field stellar mass expected for its halo. Current formation models reproduce several individual properties of DF44, but its combination of GC richness, early quenching, and high inferred halo mass remains an important constraint on models of dwarf-galaxy formation.
ER -