LoCuSS: The infall of X-ray groups on to massive clusters
Bibliographic record
Abstract
Galaxy clusters are expected to form hierarchically in a cold dark matter ( CDM) universe, growing primarily through mergers with lower mass clusters and the continual accretion of group-mass haloes.Galaxy clusters assemble late, doubling their masses since z ∼ 0.5, and so the outer regions of clusters should be replete with accreting group-mass systems.We present an XMM-Newton survey to search for X-ray groups in the infall regions of 23 massive galaxy clusters ( M 200 ∼ 10 15 M ) at z ∼ 0.2, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift.These groups have mass estimates in the range 2 × 10 13 -7 × 10 14 M , and group-to-cluster mass ratios as low as 0.02.The comoving number density of X-ray groups in the infall regions is ∼25× higher than that seen for isolated X-ray groups from the XXL survey.The average mass per cluster contained within these X-ray groups is 2.2 × 10 14 M , or 19 ± 5 per cent of the mass within the primary cluster itself.We estimate that ∼10 15 M clusters increase their masses by 16 ± 4 per cent between z = 0.223 and the present day due to the accretion of groups with M 200 ≥ 10 13.2 M .This represents about half of the expected mass growth rate of clusters at these late epochs.The other half is likely to come from smooth accretion of matter not bound within haloes.The mass function of the infalling X-ray groups appears significantly top heavy with respect to that of 'field' X-ray systems, consistent with expectations from numerical simulations, and the basic consequences of collapsed massive dark matter haloes being biased tracers of the underlying large-scale density distribution.
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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 | 0.001 | 0.000 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 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".