The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies (2006.14630v4)

Abstract: There is a growing consensus that the vast majority of ultra-diffuse 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, $N_{GC}=74^{+18}_{-18}$, for a stellar mass of only $3\times 10^8$ $M_{ \odot }$ 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 $N_{GC}=21^{+7}_{-9}$ 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 $N_{GC}=18^{+23}_{-12}$. Making use of the $M_{\rm GC} - M_{\rm halo}$ relation, this number of GCs suggests a dark matter halo mass of $M_{halo}=1.1^{+0.4}_{-0.5} \times 10^{11} M_{\odot}$, 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.

• The paper re-evaluates the globular cluster count around UDG DF44 using Hubble data, finding a lower count ($N_{GC}=21^{+7}_{-9}$) than previous claims ($N_{GC}=74^{+18}_{-18}$) by revising methodology.
• This revised globular cluster count for DF44 indicates a dark matter halo mass consistent with expectations for a normal dwarf galaxy ($1.1 \times 10^{11} M_{\odot}$), aligning with stellar velocity dispersion estimates.
• The findings suggest UDG DF44 is a "normal" dwarf galaxy rather than an anomalous high-mass UDG, highlighting the need for careful methodological reassessment in evaluating other ultra-diffuse galaxies.

The Number of Globular Clusters around the Iconic UDG DF44

The paper "The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies" by Teymoor Saifollahi et al. presents a comprehensive re-evaluation of the globular cluster (GC) population associated with the ultra-diffuse galaxy (UDG) DF44, located in the Coma Cluster. The paper scrutinizes prior claims surrounding the GC system of DF44, which have suggested a remarkably high GC population inconsistent with the expectations for a galaxy of its stellar mass. The authors adopt a methodical approach to reassess the GC count and explore its implications on the mass and classification of DF44.

The analysis leverages Hubble Space Telescope data to re-examine the distribution and luminosity function of GCs around DF44, challenging previously reported figures by adopting different geometric assumptions and selection criteria. The findings yield a GC count significantly lower than initially reported in previous studies. Specifically, the authors determine a GC count of $N_{GC}=21^{+7}_{-9}$, opposing prior estimates of $N_{GC}=74^{+18}_{-18}$. This lower estimate aligns with typical expectations for dwarf galaxies based on stellar mass and challenges assertions of a massive dark matter halo inferred from earlier high GC counts.

The authors highlight methodological differences in prior assessments, notably with assumptions regarding the spatial distribution of GCs and discrepancies in the treatment of background contamination. The paper's recalibration of the GC count, when applied to the $M_{\rm GC} - M_{\rm halo}$ relation, suggests a dark matter halo mass of $$M_{halo}=1.1^{+0.4}_{-0.5} \times 10^{11} M_{\odot}$$, consistent with mass estimates derived from the stellar velocity dispersion of DF44 ($\sigma=33^{+3}_{-3}$ km/s).

This work contextualizes the findings within broader discussions on the nature of UDGs, proposing that DF44 does not require classification as an anomalously massive UDG. The results support the notion that DF44 is a "normal" dwarf galaxy with characteristics fitting into known relations between stellar and halo mass for low surface brightness galaxies.

The implications of these findings suggest that, while intriguing, DF44 does not constitute evidence for a fundamentally distinct class of high-mass UDGs based on its GC system. Instead, its properties align with those of typical dwarf galaxies, settling claims of it having an extraordinary dark matter content. This reframing of DF44's characteristics invites reconsideration of selection biases and methodological assumptions in evaluating UDGs and advises caution in ascribing atypical properties to other such galaxies without rigorous reassessment.

Future developments in this area should involve further examination of other UDGs' GC systems using similar methodologies to ascertain if other UDGs share DF44's revised characteristics or represent true outliers. This approach will refine our comprehension of UDGs' diversity and the factors governing their formation and evolution. Through systematic evaluation, researchers can better elucidate the spectrum of galaxy types residing within clusters like Coma, ultimately enhancing our understanding of galaxy evolution in the low-surface-brightness regime.