Structural parameters from ground-based observations of newly discovered globular clusters in NGC 5128
Bibliographic record
Abstract
We have investigated a number of globular cluster candidates from a recent wide-field study by Harris et al. (2004a, AJ, 128, 712) of the giant elliptical galaxy NGC 5128. We used the Magellan I telescope + MagIC camera under excellent seeing conditions () and obtained very high resolution images for a sample of 44 candidates. Of these, 15 appear to be bonafide globular clusters in NGC 5128 while the rest are either foreground stars or background galaxies. We also serendipitously discovered 18 new cluster candidates in the same fields. Our images allow us to study the light profiles of the likely clusters, all of which are well resolved. This is the first ground-based study of structural parameters for globular clusters outside the Local Group. We compare the psf-deconvolved profiles with King models and derive structural parameters, ellipticities and surface brightnesses. We compare the derived structural properties with those of other well-studied globular cluster systems. In general, our clusters are similar in size, ellipticity, core radius and central surface brightness to their counterparts in other galaxies, in particular those in NGC 5128 observed with HST by Harris et al. (2002, AJ, 124, 1435). However, our clusters extend to higher ellipticities and larger half-light radii than their Galactic counterparts, as do the Harris et al. sample. Combining our results with those of Harris et al. fills in the gaps previously existing in parameter space and indicates that any substantial difference between presumed distinct cluster types in this diagram, including for example the Faint Fuzzies of Larsen & Brodie (2000, AJ, 120, 2938) and the “extended, luminous” M 31 clusters of Huxor et al. (2005, MNRAS, 360, 1007) is now removed and that clusters form a continuum in this diagram. Indeed, this continuum now extends to the realm of the Ultra Compact Dwarfs. The metal-rich clusters in our sample have half-light radii that are almost twice as large in the mean as their metal-poor counterparts, at odds with the generally accepted trend. The possibility exists that this result could be due in part to contamination by background galaxies. We have carried out additional analysis to quantify this contamination. This shows that, although galaxies cannot be easily told apart from clusters in some of the structural diagrams, the combination of excellent image quality and Washington photometry should limit the contamination to roughly 10% of the population of cluster candidates. Finally, our discovery of a substantial number of new cluster candidates in the relatively distant regions of the NGC 5128 halo suggests that current values of the total number of globular clusters may be underestimates.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".