Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey (1506.08828v2)

Abstract: Motivated by the recent, serendipitous discovery of the densest known galaxy, M60-UCD1, we present two initial findings from a follow-up search, using the Sloan Digital Sky Survey, Subaru/Suprime-Cam and Hubble Space Telescope imaging, and SOuthern Astrophysical Research (SOAR)/Goodman spectroscopy. The first object discovered, M59-UCD3, has a similar size to M60-UCD1 (half-light radius of r_h ~ 20 pc) but is 40% more luminous (M_V ~ -14.6), making it the new densest-known galaxy. The second, M85-HCC1, has a size like a typical globular cluster (GC; r_h ~ 1.8 pc) but is much more luminous (M_V ~ -12.5). This hypercompact cluster is by far the densest confirmed free-floating stellar system, and is equivalent to the densest known nuclear star clusters. From spectroscopy, we find that both objects are relatively young (~9 Gyr and ~3 Gyr, respectively), with metal-abundances that resemble those of galaxy centers. Their host galaxies show clear signs of large-scale disturbances, and we conclude that these dense objects are the remnant nuclei of recently accreted galaxies. M59-UCD3 is an ideal target for follow-up with high-resolution imaging and spectroscopy to search for an overweight central supermassive black hole as was discovered in M60-UCD1. These findings also emphasize the potential value of ultra-compact dwarfs and massive GCs as tracers of the assembly histories of galaxies.

• The paper identifies and characterizes two record-breaking compact stellar systems, M59-UCD3 and M85-HCC1, discovered in Sloan Digital Sky Survey data.
• These systems represent the densest known galaxy (M59-UCD3) and densest free-floating stellar system (M85-HCC1), challenging existing classifications of stellar systems.
• The findings suggest these compact systems could be remnants of stripped galactic nuclei, offering insights into galaxy evolution and indicating potential for future high-resolution studies.

Insightful Analysis of "Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey"

The research paper titled "Hiding in plain sight: record-breaking compact stellar systems in the Sloan Digital Sky Survey" by Sandoval et al. investigates the identification and characterization of exceptionally dense stellar systems discovered through Sloan Digital Sky Survey (SDSS) data. The principal findings revolve around two unprecedented stellar systems: M59-UCD3 and M85-HCC1, revealing new insights into the continuum between star clusters and galaxies, and the potential remnants of galaxy evolution processes.

Core Findings

M59-UCD3 and M85-HCC1, discovered through photometric and spectroscopic analysis, represent critical additions to the taxonomy of dense stellar systems—ultra-compact dwarfs (UCDs) exhibiting unique properties:

• M59-UCD3: Identified near the giant elliptical galaxy M59, M59-UCD3 has a half-light radius around 20 pc, closely paralleling the dimensions of the previously recognized M60-UCD1 but distinguishing itself as 40% more luminous. Consequently, it currently stands as the densest known galaxy with a potential central supermassive black hole.
• M85-HCC1: Contrastingly, M85-HCC1 boasts a size comparable to conventional globular clusters but with an exceptional luminosity, suggesting a distinctive categorization as a "hypercompact cluster." Its discovery underscores its status as the densest known free-floating stellar system.

The paper meticulously delineates the environmental and structural characteristics of these objects. Spectroscopic analysis positions these systems as relatively young enclosures with high metallic abundances akin to galaxy centers, implicating them as remnants of nucleated dwarf galaxies or stripped nucleus of larger galaxies following recent galactic mergers.

Implications and Future Directions

The implications of these findings are multifaceted, impacting both theoretical astrophysics and observational strategies. Firstly, the evidence suggests a profound overlap in parameter spaces between galaxies and star clusters, compelling a reassessment of classical definitions. Secondly, the structural density and potential central black holes posited in these systems suggest that they hold critical secrets regarding the assembly and evolutionary trajectory of their host galaxies.

Practically, M59-UCD3 serves as a promising candidate for future high-resolution imaging and spectroscopic studies aimed at probing the existence of disproportionately massive central black holes, akin to M60-UCD1. This could unravel further the role of dense stellar systems as end-products of galactic evolutionary processes, providing more precise mass profiles necessary to understand their formative conditions.

Theoretical and Observational Prospectives

The discoveries provoke a reevaluation of the formation mechanisms of UCDs and related compact systems. The potential origins as remnant galactic nuclei, supported by stellar population synthesis highlighting significant metallicity and alpha-element enhancements, propose a narrative of continued galactic integration phenomena independent of environment density—a notable paradigm shift from previous high-density assumptions.

In future astronomical endeavors, there lies substantial potential in leveraging large-scale surveys like SDSS to unearth similarly obscured stellar systems. Expanding upon these methodological frameworks could enhance our understanding of the broader spectrum of stellar density and galaxy morphology, especially in light of advancements in observational technology like JWST and ground-based telescopic arrays equipped to extend the observational frontiers initiated by SDSS.

In summary, Sandoval et al.'s paper advances the field's knowledge of compact stellar systems, challenging extant perceptions, and encouraging further exploration of these enigmatic entities' roles within galactic hierarchies.