[Paper Review] A Catalog of Extended Clusters and Ultra-Compact Dwarf Galaxies - An Analysis of their Parameters in Early- and Late-Type Galaxies

R. C. Bruens, Pavel Kroupa|arXiv (Cornell University)|Sep 19, 2012

Galaxies: Formation, Evolution, Phenomena117 references20 citations

TL;DR

This paper compiles the largest catalog to date of extended stellar objects (EOs), including extended clusters (ECs) and ultra-compact dwarf galaxies (UCDs), analyzing their structural parameters across early- and late-type galaxies. It finds that EOs form a continuous sequence with globular clusters in the $r_{\rm eff}$ vs. $M_{\rm V}$ plane, with luminosity-dependent size limits and a peak luminosity function at $M_{\rm V} \approx -6.5$ mag, strongly supporting a formation origin via merged star cluster complexes.

ABSTRACT

Extended stellar clusters with effective radii larger than 10 pc have been found in various environments. Objects with masses comparable to globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). The paper analyses the observational parameters luminosity, effective radius, and projected distance to the host galaxy, of all known ECs and UCDs and the dependence of these parameters on the type and the luminosity of their host galaxy. We searched the available literature to compile a catalog of star clusters larger than 10 pc. As there is no clear distinction between ECs and UCDs, both types of objects will be called extended stellar objects (EOs). In total, we found 813 EOs of which 171 are associated with late-type and 642 with early-type galaxies. EOs cover a luminosity range from about MV = -4 to -14 mag. However, the vast majority of EOs brighter than -10 mag are associated with elliptical galaxies. At each magnitude EOs are found with effective radii between 10 pc and an upper size limit, which shows a clear trend: the more luminous the object the larger is the upper size limit. For EOs associated with early-and late-type galaxies, the luminosity functions peak at -6.40 and -6.47 mag, respectively, which is about one magnitude fainter than the peak of the GC luminosity function. EOs and GCs form a coherent structure in the reff vs. MV parameter space, while there is a clear gap between EOs and early type dwarf galaxies. However, there is a small potential overlap at the high-mass end, where the most extended EOs are close to the parameters of compact elliptical galaxies. We compare the EO sample with numerical models and conclude that the parameters of the EO sample as a whole can be very well explained by a star cluster origin, where EOs are the results of merged cluster complexes.

Motivation & Objective

• To compile a comprehensive, up-to-date catalog of extended stellar objects (EOs) with effective radii >10 pc, including ECs and UCDs.
• To analyze the dependence of EO parameters—luminosity ($M_{\rm V}$), effective radius ($r_{\rm eff}$), and projected distance ($R_{\rm proj}$)—on host galaxy type and luminosity.
• To test whether the observed structural parameters of EOs can be explained by a star cluster origin, particularly via the merger of cluster complexes (CCs).
• To assess the completeness and detection biases in existing surveys, especially for faint and extended EOs.

Proposed method

• Systematic literature search to compile a catalog of 813 EOs with $r_{\rm eff} > 10$ pc from diverse galactic environments.
• Classification of EOs into those associated with early-type and late-type host galaxies based on host morphology and luminosity.
• Statistical analysis of luminosity functions, $r_{\rm eff}$ vs. $M_{\rm V}$ distributions, and trends with host galaxy luminosity.
• Comparison of observed EO parameters with predictions from numerical simulations of merged star cluster complexes (CCs) to test formation scenarios.
• Use of reference catalog entries with standardized parameters (e.g., $r_{\rm eff}$, $M_{\rm V}$, $R_{\rm proj}$) from multiple sources, with quality control for consistency.
• Separate analysis of confirmed EOs (175) and candidate EOs (638) to assess robustness of structural trends.

Experimental results

Research questions

• RQ1 What is the distribution of effective radius and absolute magnitude among EOs in early- and late-type galaxies?
• RQ2 How do the luminosity functions of EOs compare to those of globular clusters and dwarf galaxies?
• RQ3 Is there a systematic trend between host galaxy luminosity and the luminosity or size of associated EOs?
• RQ4 Can the observed structural parameters of EOs be explained by a formation scenario involving the merger of star cluster complexes?
• RQ5 What is the significance of the gap between EOs and early-type dwarf galaxies in the $r_{\rm eff}$ vs. $M_{\rm V}$ plane?

Key findings

• The catalog contains 813 EOs, with 642 associated with early-type galaxies and 171 with late-type galaxies, spanning absolute V-band magnitudes from $M_{\rm V} = -4$ to $-14$ mag.
• The vast majority of EOs brighter than $M_{\rm V} = -10$ mag are associated with giant elliptical galaxies, indicating a strong host luminosity dependence at high luminosities.
• The upper size limit of EOs increases with luminosity, rising from ~30 pc at $M_{\rm V} = -5$ mag to ~100 pc at $M_{\rm V} = -14$ mag.
• The EO luminosity function peaks at $M_{\rm V} \approx -6.40$ mag for early-type hosts and $-6.47$ mag for late-type hosts, about one magnitude fainter than the peak of the globular cluster luminosity function.
• EOs and globular clusters form a coherent sequence in the $r_{\rm eff}$ vs. $M_{\rm V}$ plane, clearly separated from early-type dwarf galaxies, except for a potential overlap at the high-mass end with compact elliptical galaxies.
• The observed structural parameters of EOs, including the core-halo surface brightness profiles of extreme objects like VUCD7, are well reproduced by numerical models of merged cluster complexes, supporting a star cluster origin for EOs.