SPECTROSCOPIC CONFIRMATION OF THE EXISTENCE OF LARGE, DIFFUSE GALAXIES IN THE COMA CLUSTER
Bibliographic record
Abstract
We recently identified a population of low surface brightness objects in the field of the z = 0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these "ultra-diffuse galaxies" (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption features, including the Ca ii H+K lines and the G -band, and no detected emission lines. Its radial velocity of cz = 6280 ± 120 km s −1 is within the 1 σ velocity dispersion of the Coma cluster. The galaxy has an effective radius of 4.3 ± 0.3 kpc and a Sérsic index of 0.89 ± 0.06, as measured from Keck imaging. We find no indications of tidal tails or other distortions, at least out to a radius of . We show that UDGs are located in a previously sparsely populated region of the size—magnitude plane of quiescent stellar systems, as they are ∼6 mag fainter than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same luminosity or to those of galaxies with the same size.
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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 | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| 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".