The mysterious Globular Cluster population of MATLAS-2019

TL;DR

This study resolves the long-standing disagreement over MATLAS-2019's Globular Cluster (GC) population by leveraging high-resolution HST multi-band photometry and deep GTC OSIRIS+ imaging to construct a clean GC sample. By combining detailed GC light-profile analysis, color-color and color-magnitude diagnostics, and a rigorous completeness assessment, the authors derive a robust GC census of $N_{GC}=33\pm3$, with 80% of GCs located within the galaxy's effective radius and a GC half-number radius of $R_{e,GC}=12.0''$, giving $R_{e,GC}/R_e=0.7$. Distance estimates from the GC luminosity function and GC size distribution yield $D=20.0\pm0.9$ Mpc, coherently matching recent independent measurements and implying a halo mass of $M_h=(1.14\pm0.1)\times10^{11}\,M_\\

Abstract

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.

Figures (21)

• Figure 1: GC radial profiles in the WFC3/F606W band for the 20 spectroscopically confirmed GCs (grey). The blue line shows the S/N weighted mean profile of the GCs. We also plot the profile of the empirical PSF provided by the STScI in green, and the profile of the PSF obtained directly from stars of the images in orange. The vertical dashed red line (06, equivalent to 15 px in WFC3 imaging) indicates where the profile is truncated, the vertical dotted brown line shows the FWHM of the mean GC profile, and the light blue region indicates where the background has been estimated.
• Figure 2: Normalised histograms of the effective radius (in pc, assuming a distance of 20.7 Mpc), ellipticity and $F606W$ magnitude of different samples of sources; $N$ is the number of sources in each bin and $N_{max}$ the maximum of their respective histograms. The grey histogram shows all the sources of the catalogues in grey (446 sources), the red hatched histogram the 20 spectroscopically confirmed GCs and the blue histogram the sources which fulfil the selection criteria (128 sources). The vertical dashed lines indicate the ranges of the GC selection, in each of the parameters.
• Figure 3: The $(m_{F475W} - m_{F606W})_0$ vs $(m_{F606W} - m_{F814W})_0$ colour-colour diagram of the initial sample of GC candidates. The 20 spectroscopically confirmed GCs Muller2020Haacke2025 are shown as red squares. The orange dashed box indicates the region we have selected for our next sample of GC candidates and corresponds to $\pm3\sigma$ around the median colours of the confirmed GCs (excluding the anomalous object, see Sec. \ref{['sec:colour-colour']}). The violet star, and its error bar, shows the prediction from Bruzual2003 models for an SSP of [Fe/H] = -1.44 dex and 9.1 Gyr Muller2020. The inset shows a zoom-in into the dashed orange box for ease of viewing. The error bars shown in the upper-left corner are the errors of an object located at the peak of the GCLF ($m_{F606W}$ = 23.77 mag, assuming a distance of 20.7 Mpc to the galaxy).
• Figure 4: The $(m_{F475W} - m_{F606W})_0$ vs $(m_{F606W} - m_{F814W})_0$ colour-colour diagram of GC candidates, colour coded with the $(m_u-m_{F475W})_0$ colour. The 20 spectroscopically confirmed GCs Muller2020Haacke2025 are shown as squares and the candidates as circles. The size of the marker indicates the $m_{F606W}$ of the source, as shown in the legend. The sources in grey are objects that are too faint to be measured in the $u$-band. The blue open circles highlight those objects rejected based on their $(m_u-m_{F475W})_0$ colour.
• Figure 5: Normalised histograms of the $(m_u-m_{F475W})_0$ colour of the GCs of MATLAS2019; N is the number of sources in each bin and $N_{max}$ the maximum of their respective histograms. The red hatched histogram shows the spectroscopically confirmed GCs and the blue histogram the GC candidates. Only sources with trustable magnitude in the $u-$band are shown (i.e. $m_u$ < 25.8, see Sec. \ref{['u-band_phot']}). The dashed vertical black lines indicate the $\pm3\sigma$ cut applied for GC selection.