INTERACTING GALAXIES IN THE A901/902 SUPERCLUSTER WITH STAGES
Bibliographic record
Abstract
We present a study of galaxy mergers and the influence of environment in the Abell 901/902 supercluster at z ∼ 0.165, based on 893 bright ( R Vega ⩽ 24) intermediate-mass ( M * ⩾ 10 9 M ☉ ) galaxies. We use HST ACS F606W data from the Space Telescope A901/902 Galaxy Evolution Survey, COMBO-17, Spitzer 24 μm, and XMM-Newton X-ray data. Our analysis utilizes both a physically driven visual classification system and quantitative CAS parameters to identify systems which show evidence of a recent or ongoing merger of mass ratio >1/10 (i.e., major and minor mergers). Our results are (1) after visual classification and minimizing the contamination from false projection pairs, we find that the merger fraction f merge is 0.023 ± 0.007. The estimated fractions of likely major mergers, likely minor mergers, and ambiguous cases are 0.01 ± 0.004, 0.006 ± 0.003, and 0.007 ± 0.003, respectively. (2) All the mergers lie outside the cluster core of radius R < 0.25 Mpc: the lack of mergers in the core is likely due to the large galaxy velocity dispersion in the core. The mergers, instead, populate the region (0.25 Mpc < R ⩽ 2 Mpc) between the core and the cluster outskirt. In this region, the estimated frequency of mergers is similar to those seen at typical group overdensities in N -body simulations of accreting groups in the A901/902 clusters. This suggests the ongoing growth of the clusters via accretion of group and field galaxies. (3) We compare our observed merger fraction with those reported in other clusters and groups out to z ∼ 0.4. Existing data points on the merger fraction for L ⩽ L * galaxies in clusters allow for a wide spectrum of scenarios, ranging from no evolution to evolution by a factor of ∼5 over z ∼ 0.17–0.4. (4) In A901/902, the fraction of interacting galaxies, which lie on the blue cloud is 80% ± 18% (16/20) versus 34% ± 7% or (294/866) for non-interacting galaxies, implying that interacting galaxies are preferentially blue. (5) The average star formation rate (SFR), based on UV or a combination of UV+IR data, is enhanced by a factor of ∼1.5–2 in mergers compared to non-interacting galaxies. However, mergers in the A901/902 clusters contribute only a small fraction (between 10% and 15%) of the total SFR density, while the rest of the SFR density comes from non-interacting galaxies.
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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 | 0.000 |
| 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".