The VIMOS Ultra Deep Survey: Nature, ISM properties, and ionizing spectra of CIII]<i>λ</i>1909 emitters at z = 2–4
Bibliographic record
Abstract
Context. Ultraviolet (UV) emission-line spectra are used to spectroscopically confirm high- z galaxies and increasingly also to determine their physical properties. Aims. We construct photoionization models to interpret the observed UV spectra of distant galaxies in terms of the dominant radiation field and the physical condition of the interstellar medium (ISM). These models are applied to new spectroscopic observations from the VIMOS Ultra Deep Survey (VUDS). Methods. We construct a large grid of photoionization models, which use several incident radiation fields (stellar populations, active galactic nuclei (AGNs), mix of stars and AGNs, blackbodies, and others), and cover a wide range of metallicities and ionization parameters. From these models we derive new spectral UV line diagnostics using equivalent widths (EWs) of [C III ] λ 1909 doublet, C IV λ 1549 doublet and the line ratios of [C III ], C IV , and He II λ 1640 recombination lines. We apply these diagnostics to a sample of 450 [C III ]-emitting galaxies at redshifts z = 2–4 previously identified in VUDS. Results. We demonstrate that our photoionization models successfully reproduce observations of nearby and high-redshift sources with known radiation field and/or metallicity. For star-forming galaxies our models predict that [C III ] EW peaks at sub-solar metallicities, whereas C IV EW peaks at even lower metallicity. Using the UV diagnostics, we show that the average star-forming galaxy (EW([C III ]) ~ 2 Å) based on the composite of the 450 UV-selected galaxies’ spectra The inferred metallicity and ionization parameter is typically Z = 0.3–0.5 Z ⊙ and log U = −2.7 to − 3, in agreement with earlier works at similar redshifts. The models also indicate an average age of 50–200 Myr since the beginning of the current star-formation, and an ionizing photon production rate, ξ ion , of log ξ ion /erg −1 Hz = 25.3–25.4. Among the sources with EW([C III ]) >= 10 Å, approximately 30% are likely dominated by AGNs. The metallicity derived for galaxies with EW(CIII) = 10–20 Å is low, Z = 0.02–0.2 Z ⊙ , and the ionization parameter higher (log U ~−1.7) than the average star-forming galaxy. To explain the average UV observations of the strongest but rarest [C III ] emitters (EW([C III ]) > 20 Å), we find that stellar photoionization is clearly insufficient. A radiation field consisting of a mix of a young stellar population (log ξ ion /erg −1 Hz ~ 25.7) plus an AGN component is required. Furthermore an enhanced C/O abundance ratio (up to the solar value) is needed for metallicities Z = 0.1–0.2 Z ⊙ and log U = −1.7 to − 1.5. Conclusions. A large grid of photoionization models has allowed us to propose new diagnostic diagrams to classify the nature of the ionizing radiation field (star formation or AGN) of distant galaxies using UV emission lines, and to constrain their ISM properties. We have applied this grid to a sample of [C III ]-emitting galaxies at z = 2–4 detected in VUDS, finding a range of physical properties and clear evidence for significant AGN contribution in rare sources with very strong [C III ] emission. The UV diagnostics we propose should also serve as an important basis for the interpretation of upcoming observations of high-redshift 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 | 0.000 | 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".