A detailed study of Andromeda XIX, an extreme local analogue of ultradiffuse galaxies
Bibliographic record
Abstract
ABSTRACT With a central surface brightness of μ0 = 29.3 mag arcsec−2, and half-light radius of $r_{\rm half}=3.1^{+0.9}_{-1.1}$ kpc, Andromeda XIX (And XIX) is an extremely diffuse satellite of Andromeda. We present spectra for ∼100 red giant branch stars in this galaxy, plus 16 stars in a nearby stellar stream. With this exquisite data set, we re-derive the properties of And XIX, measuring a systemic velocity of 〈vr〉 = −109.0 ± 1.6 km s−1 and a velocity dispersion of $\sigma _{vr} = 7.8^{+1.7}_{-1.5}$ km s−1 (higher than derived in our previous work). We marginally detect a velocity gradient along the major axis of $\frac{{\rm d} v}{{\rm d}\chi }= -2.1\pm 1.8~{\rm km\, s}^{-1}$ kpc−1. We find its mass-to-light ratio is higher than galaxies of comparable stellar mass (${}[M/L]_{\rm half} = 278^{+146}_{-198}\, \mathrm{M}_\odot /\mathrm{L}_\odot$), but its dynamics place it in a halo with a similar total mass to these galaxies. This could suggest that And XIX is a ‘puffed up’ dwarf galaxy, whose properties have been altered by tidal processes, similar to its Milky Way counterpart, Antlia II. For the nearby stream, we measure $v_r=-279.2\pm 3.7~{\rm km\, s}^{-1}$, and $\sigma _v=13.8^{+3.5}_{-2.6}~{\rm km\, s}^{-1}$. We measure its metallicity, and find it to be more metal rich than And XIX, implying that the two features are unrelated. Finally, And XIX’s dynamical and structural properties imply it is a local analogue to ultradiffuse galaxies (UDGs). Its complex dynamics suggest that the masses of distant UDGs measured from velocity dispersions alone should be carefully interpreted.
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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".