The Distribution of HCO<sup>+</sup><i>J</i> = 4–3 and HCN<i>J</i> = 4–3 in the Nuclear Region of M82
Bibliographic record
Abstract
We present the first detection of HCN J = 4-3 in the starburst galaxy M82 and a fully sampled grid of the distribution of its emission in the nuclear region. The angular resolution is 14'' and the velocity resolution 6.3 km s -1 . The distribution of HCN emission is compared with that of HCO + J = 4-3 made with the same resolution, and the conclusion is that the HCO + /HCN brightness ratio is higher at J = 4-3 than at lower transitions, with a median ratio of 3.5. There is evidence for a variation in this ratio across the nuclear region, with higher values toward the edge of the nuclear disk. This variation is consistent with a variation in gas density, with higher densities associated with regions closer to the nucleus of M82. However, some of this variation could also be produced by a variation in the HCO + /HCN abundance ratio. A large-scale velocity gradient (LVG) analysis of three transitions in each molecule centered at the nucleus shows that the line ratios within each species and between the two species are consistent with a common set of physical conditions for the emitting regions, namely, a kinetic temperature of 50 K, gas density 10 5 cm -3 , and gas column density 10 22 cm -2 . Although it has been suggested that HCO + may be subject to mechanisms of excitation other than collisions with H 2 , such as electron collisions, our analysis shows that there is no reason to invoke any such mechanisms to account for the line ratios present. The HCO + /HCN intensity ratio is known to vary significantly from one galaxy to another, with M82 at the high end of the spectrum of variation. This effect is briefly discussed in the context of a bistable chemical equilibrium model in which the HCO + abundance is a sensitive function of the cosmic-ray ionization rate and gas density in the cloud cores. Within the framework of this picture, a high cosmic-ray flux could be inversely related to HCO + abundance, contrary to what is generally assumed.
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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 | 0.000 |
| 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".