ner_tags: O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O
tokens: ii regions are sensitive to time since the onset of the most recent burst, driving scatter from the average relationships. Henry et al. ( 2000 ) found that chemical evolution models differing only by their assumed star formation efficiencies (SFEs) produced a range of primary N/O plateaus. We illustrate the effect of varying the SFE by over-plotting the Henry et al. ( 2000 ) constant SFR models, where efficiency has been varied by a factor of 25, on our N/O versus O/H data in Figure 14 . For low SFRs, the buildup of oxygen is slow and on the order of the lag time before intermediate-mass stars begin ejecting nitrogen. This allows a high N/O plateau to be established at low oxygen abundances (darkest purple curve). On the other hand, high SFRs early in the SFH form a large number of massive stars that produce greater levels of oxygen ahead of N enrichment, establishing a lower plateau (lightest purple curve) and shifting the entire N/O–O/H trend in Figure 14 to the right toward greater O/H. In between these scenarios, continuous star formation with roughly 250 Myr between bursts will result in N and O increasing in lockstep, dependent on the elemental yields. The coupling of the N/O plateau with galaxy SFH is also reported by cosmological hydrodynamical simulations of individual regions within spatially resolved galaxies (Vincenzo Kobayashi 2018 ). In these simulations, asymptotic giant branch (AGB) stars contribute significant N at low O/H, but the exact value of the primary N/O plateau will vary from galaxy to galaxy according to the relative contributions from SNe and AGB stars, as determined by their galaxy formation time and SFH. On the left-hand side of Figure 14 , we extend the highest N/O plateau from NGC 5194 (green) and the lowest N/O plateau from NGC 5457 (yellow). Based on the above discussion, for the low N/O plateau of NGC 5457, we can put forth an SFH scenario in which the SFE was high early in the galaxy’s evolution, allowing oxygen to build up from many bursts of star formation before nitrogen was returned from longer-lived intermediate-mass stars. Due to the higher level of nucleosynthetic products from massive stars, contributions from secondary nitrogen production may dominate over primary nitrogen production at relatively low O/H and S/H values. On the other hand, the high N/O plateau of NGC 5194 could be due to an SFH in which low SFE at early times allows nitrogen production, although delayed, to keep pace with oxygen and sulfur production and enrich the ISM. Here, we assume low SFE to mean either constant, low SFRs or long quiescent periods between bursts. In this scenario, primary nitrogen production is the dominant mechanism until the galaxy reaches relatively high O/H. Note, however, that this is a very simplistic model where N/O is changing monotonically, in a hierarchical galaxy-building scenario that may not be true. In summary, the primary N/O plateau sensitively probes the SFH of a galaxy, rather than being set by