RX J0142.0+2131. I. The Galaxy Content of an X-Ray-luminous Galaxy Cluster at<i>z</i>= 0.28
Bibliographic record
Abstract
We present a photometric and spectroscopic study of stellar populations in the X-ray–luminous cluster of galaxies RX J0142.0+2131 at z = 0.280. This paper analyzes the results of high signal-to-noise ratio spectroscopy, as well as g '-, r '-, and i '-band imaging, using the Gemini Multi-Object Spectrograph on Gemini North. Of 43 spectroscopic targets, we find 30 cluster members over a range in color. Central velocity dispersions and absorption-line strengths for lines in the range 3700 Å ≲ λ rest ≲ 5800 Å are derived for cluster members and are compared with a low-redshift sample of cluster galaxies and single stellar population (SSP) models. We use a combination of these indicators to estimate luminosity-weighted mean ages, metallicities ([M/H]), and α-element abundance ratios ([α/Fe]). RX J0142.0+2131 is a relatively poor cluster and lacks galaxies with high central velocity dispersions. Although the red sequence and the Faber-Jackson relation are consistent with pure passive evolution of the early-type population with a formation redshift of z form ≃ 2, the strengths of the 4000 Å break and scaling relations between metal line indices and velocity dispersion reject this model with high significance. By inverting SSP models for the Hβ G , Mg b , and ⟨Fe⟩ line indices, we calculate that, at a given velocity dispersion and metallicity, galaxies in RX J0142.0+2131 have luminosity-weighted mean ages 0.14 ± 0.07 dex older than the low-redshift sample. We also find that [α/Fe] in stellar populations in RX J0142.0+2131 is 0.14 ± 0.03 greater than at low redshift. All scaling relations are consistent with these estimated offsets. We speculate that the older luminosity-weighted mean ages and [α/Fe] enhancement can be brought about by a rapidly curtailed burst of star formation in RX J0142.0+2131, such as may be experienced in a cluster-cluster merger. We note that the cluster's velocity dispersion, 1278 ± 134 km s -1 , is larger than expected from both its X-ray luminosity and richness. However, the velocity distribution of galaxies in RX J0142.0+2131 is consistent with being drawn from a Gaussian distribution, and no sign of substructure is found. We conclude that stellar populations in RX J0142.0+2131 cannot evolve into stellar populations similar to those seen in our low-redshift sample through passive evolution. This study provides further evidence that a more complex model, possibly involving ongoing or intermittent star formation and galaxy mergers, is required to describe the evolution of cluster 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 | 0.002 | 0.002 |
| Science and technology studies | 0.000 | 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) | 0.014 | 0.008 |
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".