GALAXY PAIRS IN THE SLOAN DIGITAL SKY SURVEY. I. STAR FORMATION, ACTIVE GALACTIC NUCLEUS FRACTION, AND THE LUMINOSITY/MASS-METALLICITY RELATION
Bibliographic record
Abstract
We present a sample of 1716 galaxies with companions within Δ v <500 km s −1 , r p < 80 h −1 70 kpc and stellar mass ratio 0.1 < M 1 / M 2 < 10 from the Sloan Digital Sky Survey Data Release 4. The galaxy pairs are selected from the Main Galaxy Sample using stringent and well-understood criteria for redshift, spectral quality, available stellar masses, and metallicities. In agreement with previous studies, we find an enhancement in the star-formation rate (SFR) of galaxy pairs at projected separations <30–40 h −1 70 kpc. In addition, we find that this enhancement is highest (and extends to the greatest separations) for galaxies of approximately equal mass, the so-called major pairs. However, SFR enhancement can still be detected for a sample of galaxy pairs whose masses are within a factor of 10 of each other. Based on these results, we define a sample of close pairs (Δ v <500 km s −1 , r p < 30 h −1 70 kpc, and 0.1 < M 1 / M 2 < 10) which we use to investigate interaction-induced effects in the luminosity–metallicity (LZ) relation. In agreement with the one previous study of the LZ relation in paired galaxies, we find an offset to lower metallicities (by ∼0.1 dex) for a given luminosity for galaxies in pairs compared to the control sample. We also present the first mass–metallicity (MZ) relation comparison between paired galaxies and the field and again find an offset to lower metallicities (by ∼0.05 dex) for a given mass. The smaller offset in the MZ relation indicates that both higher luminosities and lower metallicities may contribute to the shift of pairs relative to the control in the LZ relation. We show that the offset in the LZ relation depends on galaxy half-light radius, r h . Galaxies with r h ≲ 3 h −1 70 kpc and with a close companion show a 0.05–0.1 dex downward offset in metallicity compared to control galaxies of the same size. Larger galaxies do not show this offset and have LZ and MZ relations consistent with the control sample. We investigate the physical impetus behind this empirical dependence on r h and consider the galaxy's dynamical time and bulge fractions as possible causes. We conclude that the former is unlikely to be a fundamental driver of the offset in the LZ relation for paired galaxies, but that bulge fraction may play a role. Finally, we study the active galactic nucleus (AGN) fraction in both the pair and control sample and find that whilst selecting galaxies in different cuts of color and asymmetry yields different AGN fractions, the fraction for pairs and the control sample are consistent for a given set of selection criteria. This indicates that if AGNs are ignited as a result of interactions, this activity begins later than the close pairs stage (i.e. once the merger is complete).
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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 | 0.002 | 0.003 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.004 | 0.001 |
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".