Implications for Galaxy Formation Models from Observations of Globular Clusters around Ultra-Diffuse Galaxies

Abstract

We present an analysis of Hubble Space Telescope observations of globular clusters (GCs) in six ultra-diffuse galaxies (UDGs) in the Coma cluster, a sample that represents UDGs with large effective radii (𝑅 e ), and use the results to evaluate competing formation models. We eliminate two significant sources of systematic uncertainty in the determination of the number of GCs, 𝑁 GC by using sufficiently deep observations that (i) reach the turnover of the GC luminosity function and (ii) provide a sufficient number of GCs with which to measure the GC number radial distribution. We find that 𝑁 GC for these galaxies is on average ∼ 20, which implies an average total mass, 𝑀 total , ∼ 10 11 𝑀 when applying the relation between 𝑁 GC and 𝑀 total . This value of 𝑁 GC lies at the upper end of the range observed for dwarf galaxies of the same stellar mass and is roughly a factor of two larger than the mean. The GC luminosity function, radial profile and average colour are more consistent with those observed for dwarf galaxies than with those observed for the more massive (𝐿 * ) galaxies, while both the radial and azimuthal GC distributions closely follow those of the stars in the host galaxy. Finally, we discuss why our observations, specifically the GC number and GC distribution around these six UDGs, pose challenges for several of the currently favoured UDG formation models.

Still at odds with conventional galaxy evolution: the star formation history of ultradiffuse galaxy Dragonfly 44

“…Points are coloured according to their stellar mass. consistent with the conclusion of Saifollahi et al (2022), who find that the elevated GC populations at a given stellar mass (𝑁 GC /𝑀 * ) of large UDGs (including DF44) are inconsistent with scenarios which explain the sizes of UDGs via redistributing the stars to larger radii (i.e., tidal interactions, stellar feedback, or high-spin). Villaume et al (2022) similarly ruled out such scenarios given DF44's 'inside-out' stellar population gradients.…”

“…We show our quenching time results for DF44 in this figure with the size measured by van Dokkum et al (2017) based on the deepest imaging available (𝑅 eff = 4.7 ± 0.2 kpc; with black outlines). However, Saifollahi et al (2022) obtained a smaller size (𝑅 eff = 3.83 ± 0.4 kpc; grey point) based on the same data. A hatched region covers the parameter space of the various quenching times from our results, and the two size measurements.…”

“…5, where an increase in dust suggests a higher stellar mass. As a point of comparison, a solid black line indicates the estimated stellar mass from van Dokkum et al ( 2016), and a dotted black line indicates that measured by Saifollahi et al (2022) with uncertainties reflecting the systematics of the model fitting. Both of our fits produce stellar masses lower than (and statistically inconsistent within their 68 per cent CRs) with the van Dokkum et al ( 2016) value, but consistent with Saifollahi et al (2022).…”

“…As a point of comparison, a solid black line indicates the estimated stellar mass from van Dokkum et al ( 2016), and a dotted black line indicates that measured by Saifollahi et al (2022) with uncertainties reflecting the systematics of the model fitting. Both of our fits produce stellar masses lower than (and statistically inconsistent within their 68 per cent CRs) with the van Dokkum et al ( 2016) value, but consistent with Saifollahi et al (2022). Given that the photometry included in our fits is measured within an aperture, and thus does not include all of the light of the galaxy, it is not unexpected that the stellar mass we recover underestimates that from the literature.…”

“…Considering that DF44 quenched 7 Gyr ago, this may suggest 18 We note that Saifollahi et al (2022) refer to this scenario as 'lack of late mergers'. 19 Less than 5 per cent of the simulated UDGs with masses 𝑀 * > 10 8 M are quenched, in the sense that they are gas poor.…”