The Photometric Properties of Extragalactic Globular Cluster Systems
Bibliographic record
Abstract
Globular Clusters (GCs) are powerful tools for understanding the formation of galaxies. GCs are located in the halos of galaxies and, due to their age and density, have borne witness to the major formation events of a galaxy's lifetime. One may study these objects using a wide array of techniques and datasets, including wide-field ground-based imaging, deep space-based imaging, and spectroscopy. All approaches involve tradeoffs, and in this work we consider a variety of ways to study GC systems in imaging data. We examine a wide-field HST/ACS mosaic of the nearby lenticular galaxy NGC 3115, selecting a high-quality GC sample using the superior resolution of the ACS data. We find strong color bimodality in the GC system of NGC 3115 and examine a number of trends in the properties of the GC system. Next, we consider the situation where one is limited to ground-based imaging, where contaminants to the GC population are a major concern. We detail a novel statistical methodology in which we treat the GC population and the contaminant population as a mixture model, and evaluate the model in a Bayesian context. We demonstrate the performance of the model on mock data, and note some areas where current analysis of GC systems may be missing information using traditional selection techniques. We also apply this Bayesian methodology to a subset of SLUGGS survey galaxies with high-quality photometry from either the MegaCam instrument on the Canada France-Hawaii Telescope or the SuprimeCam instrument on the Subaru Telescope. In most cases, the mixture model recovers the GC system well, often finding the traditional bimodality and providing well-calibrated statistical uncertainties for the global parameters of the GC system. Finally, we examine the object NGC 3628 UCD1, a star cluster slightly more massive than the largest GCs. We identify that UCD1 is located in a stellar stream around the galaxy NGC 3628, and therefore is in the process of being accreted. We characterize UCD1 both in wide-field SuprimeCam imaging and in Keck/ESI spectroscopy, and identify a number of interesting parallels between UCD1 and omegaCen, the largest Milky Way GC.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.002 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".