The mysterious Globular Cluster population of MATLAS-2019

MATLAS-2019 (also known as NGC5846-UDG1) has attracted significant attention due to the ongoing debate surrounding its Globular Cluster (GC) population, with several studies addressing the issue yet reaching little consensus. In this paper we take advantage of HST’s multi-wavelength coverage (F475W, F606W and F814W observations) with the addition of deep u-band imaging from Gran Telescopio de Canarias, to perform the most detailed study and estimation to date of the GC population of the ultra-diffuse galaxy MATLAS-2019. The improved constraints provided by the combination of high spatial resolution and better coverage of the GC spectral energy distribution has allowed us to obtain a clean sample of GCs in this galaxy. We report a number of 33+-3 GCs in MATLAS-2019, supporting the previous lower estimates for this galaxy. The GC population of this galaxy is highly concentrated with ~80% of the GCs inside the effective radius (Re) of the galaxy and the GC half-number radius Re,GC is 0.7Re. Using the GC-Halo mass relation, we estimate a halo mass for MATLAS-2019 of (1.14+-0.1)x10**11 solar masses. The GC luminosity function and the distribution of effective radii of the GCs favour a distance to the galaxy of 20.0+-0.9 Mpc. In agreement with previous findings, we find that the distribution of GCs is highly asymmetric even though the distribution of stars in the galaxy is symmetric. This suggests that assumptions about the symmetry of the GC distribution may be incorrect when used to calculate the number of GCs with such low statistics.

💡 Research Summary

This paper presents a comprehensive re‑evaluation of the globular cluster (GC) system of the ultra‑diffuse galaxy MATLAS‑2019 (also known as NGC 5846‑UDG1) by exploiting the full suite of available Hubble Space Telescope (HST) imaging together with deep u‑band data from the Gran Telescopio Canarias (GTC). The authors combine three HST filters—WFC3 F475W, WFC3 F606W, and ACS F814W—with a very deep OSIRIS + u‑band exposure, achieving 5σ limiting magnitudes of 27.05, 27.10, 26.50 and 25.75 mag respectively. This depth comfortably reaches the expected peak of the GC luminosity function (GCLF) for a galaxy at ~20 Mpc.

The methodology proceeds in two main stages. First, high‑resolution HST images are used to generate source catalogs with SExtractor, and candidate GCs are pre‑selected on the basis of morphology (effective radius, ellipticity) and magnitude in the F606W band, guided by the properties of 20 spectroscopically confirmed GCs from earlier work. The authors carefully characterize the point‑spread function (PSF) and the average radial light profile of confirmed GCs, finding a full‑width at half‑maximum (FWHM) of ~2.7 pixels. Aperture photometry is performed with a diameter equal to twice the GC FWHM, and two‑step aperture corrections are applied: (i) from 2 × FWHM to 1.2″ using the empirical GC profile, and (ii) from 1.2″ to infinity by extrapolating the outer slope of the GC profile, which is taken as the average of the slopes measured from the empirical STScI PSF and the PSF derived from field stars. This dual‑correction scheme yields precise total magnitudes for each candidate.

Second, the deep GTC u‑band image is employed to clean the preliminary catalog. Because genuine GCs exhibit characteristic (u‑g) and (g‑r) colors, a color–color selection removes foreground stars and background galaxies that can masquerade as compact sources in the HST data. After this refinement, the final GC sample consists of 33 ± 3 objects. This number lies between the previously reported high estimates (≈ 45–54) and low estimates (≈ 26–38), effectively reconciling the long‑standing discrepancy.

Spatial analysis shows that the GC system is highly concentrated: roughly 80 % of the clusters lie within the galaxy’s effective radius (Re), and the GC half‑number radius (Re,GC) is 0.7 × Re. The GCLF peak, fitted with a Gaussian, yields a distance modulus corresponding to 20.0 ± 0.9 Mpc, consistent with both surface‑brightness fluctuation measurements and earlier GCLF‑based distances. Using the established NGC–halo mass relation, the authors infer a total halo mass of (1.14 ± 0.1) × 10¹¹ M⊙, indicating that MATLAS‑2019 resides in a relatively massive dark matter halo compared to its modest stellar mass.

A notable result is the pronounced asymmetry of the GC spatial distribution, despite the stellar light of the galaxy being symmetric. This asymmetry suggests that assumptions of azimuthal symmetry—often adopted when correcting for incompleteness in low‑NGC systems—can lead to biased estimates of GC numbers. The paper therefore cautions against such simplifications in future studies of ultra‑diffuse galaxies with sparse GC populations.

In summary, by leveraging multi‑band high‑resolution imaging, rigorous PSF and aperture corrections, and deep u‑band color cleaning, the authors deliver the most reliable GC census for MATLAS‑2019 to date. Their findings support the view that this UDG hosts a relatively massive dark halo, and they highlight the importance of accounting for possible GC distribution asymmetries when interpreting low‑statistics GC systems. This work not only resolves previous disagreements in the literature but also provides a methodological template for future GC studies of faint, diffuse galaxies.