The Outer Regions of the Nearby Sc Galaxies NGC 2403 and M33: Evidence for an Intermediate-Age Population at Large Radii
Bibliographic record
Abstract
Deep g ' r ' i ' z ' images obtained with the Gemini Multiobject Spectrograph (GMOS) on Gemini North are used to investigate the stellar content in the outer regions of the nearby Sc galaxies NGC 2403 and M33. The field observed in NGC 2403 covers galactocentric distances between 5 and 11 kpc perpendicular to the line of sight ( R ) and 7 and 19 kpc along the plane of the disk ( R ). The red giant branch (RGB) tip occurs at i ' = 23.6 ± 0.1, and the Cepheid and RGB-tip distance scales for NGC 2403 are in good agreement. The number density of bright main-sequence stars in this field experiences a steep cutoff at R ∼ 10 kpc, which is consistent with the expected truncation radius of the disk predicted from studies of edge-on spiral galaxies. While very young stars are restricted to R < 10 kpc, a population of bright asymptotic giant branch (AGB) stars is present throughout the entire GMOS field, indicating that star formation occurred outside of the present-day star-forming disk of NGC 2403 during intermediate epochs. The AGB stars are not in a tidal stream; in fact, the ratio of AGB stars above the RGB tip to those below the RGB tip does not change with radius, indicating that the bright AGB stars are uniformly mixed with the fainter stellar content throughout the field. The AGB luminosity function (LF) scales with r -band surface brightness over a wide range of radii throughout the main body of NGC 2403, indicating that the age distribution of stars in the outer regions of the present-day star-forming disk is not skewed to younger values than in the inner disk. Based on the color of stars on the upper portions of the RGB, it is concluded that metallicity changes across the field, with [Fe/H] = -0.8 ± 0.1(random) ± 0.3(systematic) at R = 5 kpc, and [Fe/H] = -2.2 ± 0.2(random) ± 0.8(systematic) at R = 11 kpc. The M33 field samples R between 8 and 10 kpc and R between 14 and 17 kpc. Bright AGB stars are detected in this field, and the ratio of bright AGB stars to stars on the upper RGB is at least as large as that measured in the outer regions of NGC 2403; thus, an intermediate-age population occurs well outside of the young star-forming disk of M33. The color of stars on the upper RGB suggests that [Fe/H] = -1.0 ± 0.3(random) ± 0.3(systematic) in this field. The globular cluster systems of both NGC 2403 and M33 contain objects that formed during intermediate epochs, and it is suggested that the luminous AGB stars detected in the current study are the field counterparts of these clusters. The detection of intermediate-age stars in the outer regions of these galaxies is consistent with models in which late-type spiral galaxies formed more slowly than earlier-type systems.
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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".