Star Formation at Very Low Metallicity. II. On the Insignificance of Metal‐Line Cooling During the Early Stages of Gravitational Collapse
Bibliographic record
Abstract
We study the influence of low levels of metal enrichment on the cooling and collapse of ionized gas in small protogalactic halos using three-dimensional, smoothed particle hydrodynamics simulations. Our initial conditions represent protogalaxies forming within a "fossil" H II region, a previously ionized H II region which has not yet had time to cool and recombine. Prior to cosmological reionization, such regions should be relatively common, since the characteristic lifetimes of the likely ionizing sources are significantly shorter than a Hubble time. We show that in these regions, H 2 is the dominant and most effective coolant, and that it is the amount of H 2 formed that determines whether or not the gas can collapse and form stars. At the low metallicities ( Z < 10 -3 Z ☉ ) thought to be associated with the transition from Population III to early Population II star formation, metal-line cooling has an almost negligible effect on the evolution of low-density gas, altering the density and temperature evolution of the gas by less than 1% compared to the metal-free case at densities below 1 cm -3 and temperatures above 2000 K. Although there is evidence that metal-line cooling becomes more effective at higher density, we find no significant differences in behavior from the metal-free case at any density below our sink particle creation threshold at n = 500 cm -3 . Increasing the metallicity also increases the importance of metal-line cooling, but it does not significantly affect the dynamical evolution of the low-density gas until Z ~ 0.1 Z ☉ . This result holds regardless of whether or not an ultraviolet background is present.
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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 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| 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".