The nature and space density of fossil groups of galaxies
Bibliographic record
Abstract
We describe the properties of a sample of galaxy groups with very unusual distributions of galaxy luminosities. The most extreme example has an X-ray luminosity similar to that of the Virgo cluster but has a very low richness, with only one galaxy brighter than L*, compared with six in Virgo. That one galaxy, however, is optically more luminous than any galaxy in Virgo and has an optical luminosity as bright as many of the central cD galaxies in rich Abell clusters. The characteristic feature of the fossil groups we study is that most of the light arises from one dominant, central galaxy. We define a fossil system and, based on this definition, construct a small X-ray-selected, flux-limited sample of fossil groups with well-known selection criteria. We confirm that these systems are indeed groups of galaxies, but dominated by one central luminous giant elliptical galaxy and with few, or no, L* galaxies. We find that fossil systems represent 8–20 per cent of all systems of the same X-ray luminosity. Fossil groups are at least as numerous as all poor and rich clusters combined, and are thus a possible site for the formation of luminous central cluster galaxies before infall into clusters occurs. The fossil systems in our sample have significantly higher X-ray luminosities than normal groups of similar total optical luminosities (or similar X-ray temperature, where the latter can be measured). These enhanced X-ray luminosities may be due to relatively cool gas in the innermost regions or due to a low central gas entropy. We interpret fossil groups as old, undisturbed systems which have avoided infall into clusters, but where galaxy merging of most of the L* galaxies has occurred. An early formation epoch, before that of most groups, could explain low central gas entropies and high X-ray luminosities.
Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.
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 | 0.004 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.003 | 0.002 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".