An evolutionary continuum from nucleated dwarf galaxies to star clusters (2311.05448v1)

Abstract: Systematic studies have revealed hundreds of ultra-compact dwarf galaxies (UCDs) in the nearby Universe. With half-light radii $r_h$ of approximately 10-100 parsecs and stellar masses $M_*$ $\approx$ $10^6-10^8$ solar masses, UCDs are among the densest known stellar systems. Although similar in appearance to massive globular clusters, the detection of extended stellar envelopes, complex star formation histories, elevated mass-to-light ratio, and supermassive black holes suggest that some UCDs are remnant nuclear star clusters of tidally-stripped dwarf galaxies, or even ancient compact galaxies. However, only a few objects have been found in the transient stage of tidal stripping, and this assumed evolutionary path has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition, and fills the size gap' between star clusters and galaxies. Their spatial distribution and redder color are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies. Theultra-diffuse' tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping, and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy (UDG). These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found.

• The paper presents evidence for a continuous evolutionary path from nucleated dwarf galaxies to UCDs via tidal stripping.
• Using high-resolution imaging, it identifies two intermediate populations marked by overabundant nuclear star clusters and extended stellar envelopes.
• The analysis of size-luminosity relations supports the idea that repeated tidal stripping shapes galaxy morphology in cluster environments.

An Evolutionary Continuum from Nucleated Dwarf Galaxies to Star Clusters

This paper presents a detailed analysis of a population of 106 galaxies in the Virgo cluster, which are proposed as intermediate forms between nucleated dwarf galaxies (dE,Ns) and ultra-compact dwarf galaxies (UCDs), suggesting an evolutionary continuum through tidal stripping. Through high-resolution imaging and spectroscopy, the authors identify structural and color characteristics, along with tidal features that support the hypothesis of a continuous morphological transition driven by gravitational interactions.

Key Findings

1. Intermediate Populations: Two distinct populations bridge the gap between typical dE,Ns and UCDs: strongly nucleated dEs, exhibiting an overabundance of nuclear star clusters (NSCs), and UCDs with extended stellar envelopes (eUCDs).
2. Tidal Features: The imaging depth of the NGVS and BSDVS has revealed significant tidal features in some strongly nucleated dEs and eUCDs, indicative of ongoing tidal disruption. The presence of these features suggests that these galaxies are experiencing tidal stripping on their initial pericentric passages around larger galaxies.
3. Size-Luminosity Relation: Analysis of size-luminosity scaling relations reveals that the sizes of these intermediate systems fill the observed “size gap” between star clusters and galaxies. The strongly nucleated dEs and eUCDs form a contiguous sequence with decreasing size and luminosity, supporting the idea of repeated tidal stripping events leading to the formation of UCDs.
4. Morphological Evolution: The structural properties observed in these systems indicate a potential evolutionary sequence, wherein nucleated ultra-diffuse galaxies (UDGs), through tidal stripping, might transform into UCDs.
5. Impact on Smooth Transition Models: The results of structural decomposition and color-magnitude analyses propose that the evolutionary route from dE,Ns to UCDs is more complex than previously thought, which can be observed as a smooth transition supplemented by intermittent stripping phases.

Implications and Speculation on Future Developments

The results have several implications for both the formation and evolutionary theories of compact stellar systems and galaxy clusters. The identification of these transitional objects emphasizes the complex interplay of tidal forces in shaping galaxy morphologies. Additionally, it challenges previous purely structural classifications of galaxies and introduces nuanced views on their evolutionary paths.

In terms of theoretical implications, this paper proposes scenarios where dE,Ns, possibly on their initial orbits in cluster environments, evolve through various morphological states via tidal interactions. These findings may require updating models of dark matter distribution, particularly concerning the density profile's role in galaxy transformation through stripping.

Looking forward, with ongoing advancements in observational capabilities, future surveys, including those from the Rubin Observatory and the Nancy Grace Roman Space Telescope, are anticipated to unveil more such transitional systems. This can potentially lead to a reassessment of the population statistics of nucleated galaxies and UCDs in clusters, further refining models that aim to replicate observed distributions and kinematics of these systems.

In conclusion, this research contributes valuable insights into our understanding of galaxy morphology and evolution, demonstrating the dynamic nature of galaxies as they are influenced by larger cosmic structures. The paper effectively shows how detailed observational surveys and structural modeling can uncover previously hidden aspects of galaxy evolution, enriching our knowledge of the cosmos.