A Keck/DEIMOS spectroscopic survey of the faint Mâ31 satellites AndâXV and AndâXVI
Bibliographic record
Abstract
We present the results of a spectroscopic survey of the recently discovered M31 satellites And XV and And XVI, lying at projected distances from the centre of M31 of 93 and 130 kpc, respectively. These satellites lie to the south of M31, in regions of the stellar halo which wide-field imaging has revealed as relative voids (compared to the ∼degree-scale coherent stream-like structures). Using the Deep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have defined probable members of these satellites, for which we derive radial velocities as precise as ∼6 km s−1 down to i∼ 21.5. While the distance to And XVI remains the same as previously reported (525 ± 50 kpc), we have demonstrated that the brightest three stars previously used to define the tip of the red giant branch in And XV are in fact Galactic, and And XV is actually likely to be much more distant at 770 ± 70 kpc (compared to the previous 630 kpc), increasing the luminosity from MV≈−9.4 to −9.8. The And XV velocity dispersion is resolved with vr=−339+7−6 km s−1 and σv= 11+7−5 km s−1. The And XVI dispersion is not quite resolved at 1σ with vr=−385+5−6 km s−1 and σ= 0+10−indef km s−1. Using the photometry of the confirmed member stars, we find metallicities of And XV (median [Fe/H]=−1.58, interquartile range ±0.08) and And XVI (median [Fe/H]=−2.23, interquartile range ±0.12). Stacking the spectra of the member stars, we find spectroscopic [Fe/H]=−1.8 (−2.1) for And XV (And XVI), with an uncertainty of ∼0.2 dex in both cases. Our measurements of And XV reasonably resolve its mass (∼108 M⊙) and suggest a polar orbit, while the velocity of And XVI suggests it is approaching the M31 escape velocity given its large M31 centric distance.
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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 | 0.001 |
| 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".