Euclid: Early Release Observations -- Interplay between dwarf galaxies and their globular clusters in the Perseus galaxy cluster

TL;DR

This study uses Euclid ERO data to analyze GC systems around a large sample of Perseus cluster dwarfs, focusing on $N_{ m GC}$ and $R_{ m GC}$ as functions of host properties like $M_*$, $\mu_{0}$, and $R_{ m e}$. The authors build GC catalogs with a dedicated GCEx pipeline, validate completeness with artificial GCs, and derive stacked radial profiles and galaxy-by-galaxy measurements across multiple dwarf categories, including UDGs. They find that $R_{ m GC}$ at fixed $M_*$ is largely independent of $\mu_{0}$ and $R_{ m e}$, while $R_{ m GC}/R_{ m e}$ tracks surface brightness and compactness, with LSB/DIFF/UDGs showing smaller ratios than HSB/COMP; $N_{ m GC}$ increases with $M_*$ and is elevated in LSB/DIFF/UDGs. Interpreting $N_{ m GC}$ in terms of total mass yields SHMRs consistent with $z\!=\!0$ relations down to $M_*\sim10^6\,M_\odot$, arguing against universally over-massive UDGs in this cluster, and providing a benchmark for galaxy-formation simulations and GC-system assembly in dense environments. The work also highlights biases in GC-based mass estimates if a single $R_{ m GC}/R_{ m e}$ is assumed across diverse dwarf-types, motivating more nuanced treatments in future analyses.

Abstract

We present an analysis of globular clusters (GCs) of dwarf galaxies in the Perseus galaxy cluster to explore the relationship between dwarf galaxy properties and their GCs. Our focus is on GC numbers ($N_{\rm GC}$) and GC half-number radii ($R_{\rm GC}$) around dwarf galaxies, and their relations with host galaxy stellar masses ($M_*$), central surface brightnesses ($μ_0$), and effective radii ($R_{\rm e}$). Interestingly, we find that at a given stellar mass, $R_{\rm GC}$ is almost independent of the host galaxy $μ_0$ and $R_{\rm e}$, while $R_{\rm GC}/R_{\rm e}$ depends on $μ_0$ and $R_{\rm e}$; lower surface brightness and diffuse dwarf galaxies show $R_{\rm GC}/R_{\rm e}\approx 1$ while higher surface brightness and compact dwarf galaxies show $R_{\rm GC}/R_{\rm e}\approx 1.5$-$2$. This means that for dwarf galaxies of similar stellar mass, the GCs have a similar median extent; however, their distribution is different from the field stars of their host. Additionally, low surface brightness and diffuse dwarf galaxies on average have a higher $N_{\rm GC}$ than high surface brightness and compact dwarf galaxies at any given stellar mass. We also find that UDGs (ultra-diffuse galaxies) and non-UDGs have similar $R_{\rm GC}$, while UDGs have smaller $R_{\rm GC}/R_{\rm e}$ (typically less than 1) and 3-4 times higher $N_{\rm GC}$ than non-UDGs. Examining nucleated and not-nucleated dwarf galaxies, we find that for $M_*>10^8M_{\odot}$, nucleated dwarf galaxies seem to have smaller $R_{\rm GC}$ and $R_{\rm GC}/R_{\rm e}$, with no significant differences between their $N_{\rm GC}$, except at $M_*<10^8M_{\odot}$ where the nucleated dwarf galaxies tend to have a higher $N_{\rm GC}$. Lastly, we explore the stellar-to-halo mass ratio (SHMR) of dwarf galaxies and conclude that the Perseus cluster dwarf galaxies follow the expected SHMR at $z=0$ extrapolated down to $M_*=10^6M_{\odot}$.

Figures (17)

• Figure 1: A view of a Perseus cluster dwarf galaxy (EDwC-0120 in EROPerseusDGs, and R84 in janssens2024) as seen by the VIS and NISP instruments onboard (in , , , and ), by HST/ACS (in F814W) and by CFHT/MegaCAM (in $r$). The galaxy seems to host several GCs, which appear as point sources on top of the galaxy, as well as a nuclear star cluster (NSC) in the centre of the diffuse component. The brightest object projected near the centre of the galaxy could be a foreground star, or a massive GC destined to merge with the already existing NSC.
• Figure 2: A zoomed view of the dwarf galaxy in Fig. \ref{['vis-vs-hst']} as seen by /VIS in and HST/ACS in F814W (PIPER survey, harris2020). The total integration times for these images are 8960 s and 2100 s respectively. The depths of both data sets are comparable while HST/ACS provides a higher spatial resolution with a better sampled PSF than Euclid/VIS.
• Figure 3: Completeness of the performed source detection in based on artificial GC tests. Our assessment shows that the GC detection is 80% complete at $\IE=26.7$ (without considering the extinction). This magnitude is about the expected location of GCLF TOM for dwarf galaxies at the distance of the Perseus cluster (72 Mpc) (taking into account the average Galactic foreground dust extinction $A_{\IE}=0.2$ mag).
• Figure 4: Photometric criteria applied for GC identification, aiming for high completeness. The three rows demonstrate these criteria for compactness index ($C_{2-4}$), ellipticity ($\varepsilon$), and $\IE-\YE$ colour (as an example). In each row, we show the real-sky sources detected in images (grey points), the detected artificial GCs (black points), and the real-sky sources selected as GC candidates (yellow points). The dashed red curves represent the selection boundaries adopted based on the distribution of artificial GCs in each parameter space. In the top row, we see that the artificial GCs follow the vertical sequence of point sources (with an average $C_{2-4}=0.7$), and the scatter becomes larger with fainter magnitude due to an increase in photometric uncertainties. In the bottom row, we see that the selection boundaries reject the majority of objects with $\IE<24$ and $\IE-\YE>1$, which are foreground stars. However, a fraction of those stars end up in the final catalogue. This contamination is unavoidable considering the expected colour range of GCs. Tightening the selection criteria removes these contaminant stars, but leads to lower completeness.
• Figure 5: Source detection and GC candidate selection for eight of the dwarf galaxies in this work. The figure shows galaxies with high and low central brightness respectively in the top and bottom half. For each dwarf galaxy, it includes a small cutout with sides of $4R_{\rm e}$ and the galaxy-subtracted version of this cutout. Galaxy-subtraction has been done by unsharp-masking of the cutouts using a small background mesh size (8 $\times$ 8 pixels), using SExtractor. Among the detected sources, GC candidates are highlighted in red; other sources (in green) did not match the selection criteria.