Direct observational evidence for a large transient galaxy population in groups at 0.85 < z < 1
Bibliographic record
Abstract
We introduce our survey of galaxy groups at 0.85 < z < 1, as an extension of the Group Environment and Evolution Collaboration. Here we present the first results, based on Gemini GMOS-S nod-and-shuffle spectroscopy of seven galaxy groups selected from spectroscopically confirmed, extended XMM detections in COSMOS. We use photometric redshifts to select potential group members for spectroscopy, and target galaxies with r < 24.75. In total, we have over 100 confirmed group members, and four of the groups have >15 members. The dynamical mass estimates are in good agreement with the masses estimated from the X-ray luminosity, with most of the groups having 13 < log Mdyn/M⊙ < 14. We compute stellar masses by template-fitting the spectral energy distributions; our spectroscopic sample is statistically complete for all galaxies with Mstar≳ 1010.1 M⊙, and for blue galaxies we sample masses as low as Mstar∼ 108.8 M⊙. The fraction of total mass in galaxy starlight spans a range of 0.25–3 per cent, for the six groups with reliable mass determinations. Like lower redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar > 1010.1 M⊙. A few group galaxies inhabit the ‘blue cloud’ that dominates the surrounding field; instead, we find a large and possibly distinct population of galaxies with intermediate colours. The ‘green valley’ that exists at low redshift is instead well populated in these groups, containing ∼30 per cent of the galaxies. These do not appear to be exceptionally dusty galaxies, and about half show prominent Balmer absorption lines. Furthermore, their Hubble Space Telescope morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. Unlike red-sequence galaxies, most of the green galaxies have a disc component, but one that is smaller and less structured than discs found in the blue cloud. We postulate that these are a transient population, migrating from the blue cloud to the red sequence, with a star formation rate that declines with an exponential time-scale 0.6 < τ < 2 Gyr. Such galaxies may not be exclusive to the group environment, as we find examples also amongst the non-members. However, their prominence among the group galaxy population, and the marked lack of blue, star-forming galaxies, provides evidence that the group environment either directly reduces star formation in member galaxies or at least prevents its rejuvenation during the normal cycle of galaxy evolution.
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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 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.001 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
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".