Keck Spectroscopy of 44 SMUDGes Galaxies Detects Clusters in Every Object, Revealing Formation Diversity

The study targeted galaxies spanning the red sequence to blue cloud using Keck Cosmic Web Imager integral-field spectroscopy. Researchers extracted cluster spectra from high-surface-brightness regions that appear smooth in ground-based imaging, identifying three nuclear star cluster candidates. Rotation velocities were measured in 17 galaxies via emission-line clusters and in 12 via stellar continuum, yielding dynamical-to-stellar mass ratios that vary widely.

Cluster luminosity functions were brighter in bluer galaxies even among objects lacking nebular lines, implying young or intermediate-age populations alongside old globular clusters. This mix undermines photometric globular-cluster counts as reliable halo-mass tracers. The sample shows no uniform formation pathway, consistent with multiple channels including tidal stripping and failed-galaxy scenarios.

Prior imaging surveys underestimated internal substructure; the spectroscopic detection of clusters in all 44 targets indicates that low-surface-brightness populations are more heterogeneous than color-magnitude diagrams alone suggest. Follow-up multi-wavelength data will be required to age-date the clusters and map dark-matter fractions at larger radii.

Next steps include JWST near-infrared imaging to separate age and metallicity effects and larger IFU samples to test whether the observed mass-ratio scatter persists across environments.