Abstract
ABSTRACT There is a growing consensus that the vast majority of ultradiffuse galaxies (UDGs) are dwarf galaxies. However, there remain a few UDGs that seem to be special in terms of their globular cluster (GC) systems. In particular, according to some authors, certain UDGs exhibit large GC populations when compared to expectations from their stellar (or total) mass. Among these special UDGs, DF44 in the Coma cluster is one of the better-known examples. DF44 has been claimed to have a relatively high number of GCs, $N_{\mathrm{ GC}}=74^{+18}_{-18}$, for a stellar mass of only $3\times 10^8\, \mathrm{ M}_{ \odot}$ which would indicate a much larger dark halo mass than dwarfs of similar stellar mass. In this paper, we revisit this number and, contrary to previous results, find $N_{\mathrm{ GC}}=21^{+7}_{-9}$ assuming that the distribution of the GCs follows the same geometry as the galaxy. If we assume that the GCs around DF44 are distributed in a (projected) circularly symmetric way and, if we use a less strict criterion for the selection of the GCs, we find $N_{\mathrm{ GC}}=18^{+23}_{-12}$. Making use of the MGC–Mhalo relation, this number of GCs suggests a dark matter halo mass of $M_{\mathrm{ halo}}=1.1^{+0.4}_{-0.5} \times 10^{11} \mathrm{ M}_{\odot}$, a value which is consistent with the expected total mass for DF44 based on its velocity dispersion, $\sigma =33^{+3}_{-3}$ km s−1. We conclude that the number of GCs around DF44 is as expected for regular dwarf galaxies of similar stellar mass and DF44 is not extraordinary in this respect.
Highlights
The nature of very low surface brightness galaxies with large effective radii has been intensely debated in past years
We revisit the globular cluster (GC) population of Dragonfly 44 (DF44) by exploring their spatial distribution, luminosity function (LF), number count, and average colour and we find a significantly lower number of GCs, making this galaxy appear more consistent with the general dwarf galaxy population
We can use the total number of GCs to estimate the halo mass of DF44, considering that the total stellar mass contained in the GCs in a galaxy scales linearly with galaxy halo mass
Summary
The nature of very low surface brightness galaxies with large effective radii has been intensely debated in past years. One of the better-known examples of such an extreme UDG is Dragonfly 44 (DF44; van Dokkum et al 2016) This iconic galaxy, associated with the Coma galaxy cluster, has been claimed to have a dark matter halo comparable with that measured for the Milky Way (van Dokkum et al 2016). This conclusion was further supported by the assertion of an extensive number of globular clusters (GCs) in the vicinity of the galaxy (van Dokkum et al 2017, vD17).
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