An HST/COS legacy survey of intervening Si III absorption in the extended gaseous halos of low-redshift galaxies
Bibliographic record
Abstract
Aims. Doubly ionized silicon (Si <ns0:sc>iii</ns0:sc>) is a powerful tracer of diffuse ionized gas inside and outside of galaxies. It can be observed in the local Universe in ultraviolet (UV) absorption against bright extragalactic background sources. We here present an extensive study of intervening Si <ns0:sc>iii</ns0:sc>-selected absorbers and study the properties of the warm circumgalactic medium (CGM) around low-redshift (z ≤ 0.1) galaxies. Methods. We analyzed the UV absorption spectra of 303 extragalactic background sources, as obtained with the Cosmic Origins Spectrograph (COS) on-board the Hubble Space Telescope (HST). We developed a geometrical model for the absorption-cross section of the CGM around the local galaxy population and compared the observed Si <ns0:sc>iii </ns0:sc>absorption statistics with predictions provided by the model. We also compared redshifts and positions of the absorbers with those of ~64 000 galaxies using archival galaxy-survey data to investigate the relation between intervening Si <ns0:sc>iii </ns0:sc>absorbers and the CGM. Results. Along a total redshift path of Δz ≈ 24, we identify 69 intervening Si <ns0:sc>iii </ns0:sc>systems that all show associated absorption from other low and high ions (e.g., H <ns0:sc>i</ns0:sc>, Si <ns0:sc>ii</ns0:sc>, Si <ns0:sc>iv</ns0:sc>, C <ns0:sc>ii</ns0:sc>, C <ns0:sc>iv</ns0:sc>). We derive a bias-corrected number density of dN/dz(Si <ns0:sc>iii</ns0:sc>)= 2.5 ± 0.4 for absorbers with column densities log N(Si <ns0:sc>iii</ns0:sc>) > 12.2, which is ~3 times the number density of strong Mg <ns0:sc>ii </ns0:sc>systems at z = 0. This number density matches the expected cross section of a Si <ns0:sc>iii </ns0:sc>absorbing CGM around the local galaxy population with a mean covering fraction of ⟨ fc ⟩ = 0.69. For the majority (~60 percent) of the absorbers, we identify possible host galaxies within 300 km s-1 of the absorbers and derive impact parameters ρ < 200 kpc, demonstrating that the spatial distributions of Si <ns0:sc>iii </ns0:sc>absorbers and galaxies are highly correlated. Conclusions. Our study indicates that the majority of Si <ns0:sc>iii</ns0:sc>-selected absorbers in our sample trace the CGM of nearby galaxies within their virial radii at a typical covering fraction of ~70 percent. We estimate that diffuse gas in the CGM around galaxies, as traced by Si <ns0:sc>iii</ns0:sc>, contains substantially more (more than twice as much) baryonic mass than their neutral interstellar medium.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 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 | 0.001 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".