Lyman continuum escape fraction and mean free path of hydrogen ionizing photons for bright<i>z</i>∼ 4 QSOs from SDSS DR14
Bibliographic record
Abstract
Context.One of the main challenges in observational cosmology is related to the redshift evolution of the average hydrogen (HI) ionization in the Universe, as evidenced by the changing in ionization level of the intergalactic medium (IGM) through cosmic time. Starting from the first cosmic reionization, the rapid evolution of the IGM physical properties in particular poses severe constraints for the identification of the sources responsible for maintaining its high level of ionization up to lower redshifts. Aims.In order to probe the ionization level of the IGM and the ionization capabilities of bright quasi-stellar objects (QSOs) atz = 4, we selected a sample of 2508 QSOs drawn from the Sloan Digital Sky Survey (SDSS, DR14) in the redshift interval 3.6 ≤ z ≤ 4.6 and absolute magnitude range −29.0 ≲ M1450 ≲ −26.0. Particularly, we focus on the estimate of the escape fraction of HI-ionizing photons and their mean free path (MFP), which are fundamental for characterizing the surrounding IGM. Methods.Starting from UV/optical rest-frame spectra of the whole QSO sample from the SDSS survey, we estimated the escape fraction and free path individually for each of the QSOs. We calculated the Lyman continuum (LyC) escape fraction as the flux ratio blueward (∼900 Å rest frame) and redward (∼930 Å rest frame) of the Lyman limit. We then obtained the probability distribution function (PDF) of the individual free paths of the QSOs in the sample and studied its evolution in luminosity and redshift, comparing our results with those in literature. Results.We find a lower limit to the mean LyC escape fraction of 0.49, in agreement with the values obtained for both brighter and fainter sources at the same redshift. We show that the free paths of ionizing photons are characterized by a skewed distribution function that peaks at low values, with an average of ∼49 − 59 proper Mpc atz ∼ 4, after possible associated absorbers (AAs) were excluded. This value is higher than the one obtained at the same redshift by many authors in the literature using different techniques. Moreover, the PDF of free path gives information that is complementary to the MFP derived through the stacking technique. Finally, we also find that the redshift evolution of this parameter might be milder than previously thought. Conclusions.Our new determination of the MFP atz ∼ 4 implies that previous estimates of the HI photoionization rate ΓHIavailable in the literature should be corrected by a factor of 1.2−1.7. These results have important implications when they are extrapolated at the epoch of reionization.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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".