NGC 2579 and the carbon and oxygen abundance gradients beyond the solar circle

TL;DR

This study uses deep spectrophotometry of the HII region NGC 2579 to analyze its chemical composition and extend the understanding of radial abundance gradients of elements like carbon and oxygen in the Milky Way's outer disc.

Contribution

It provides new measurements of chemical abundances in NGC 2579 and extends the Galactic abundance gradient studies to larger galactocentric distances.

Findings

NGC 2579 shows flattened abundance gradients at its galactocentric distance.

The chemical evolution model explains gradient flattening with a decrease in star formation efficiency.

The nebula's composition aligns with the trend of gradient flattening in the outer Galaxy.

Abstract

We present deep echelle spectrophotometry of the Galactic HII region NGC 2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3550--10400 \AA\ range. This object, which has been largely neglected, shows however a rather high surface brightness, a high ionization degree and is located at a galactocentric distance of 12.4 $\pm$ 0.7 kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of the gas phase radial abundance gradients in the outer disc of the Milky Way. We derive the physical conditions of the nebula using several emission line-intensity ratios as well as the abundances of several ionic species from the intensity of collisionally excited lines. We also determine the ionic abundances of C$^{2+}$, O$^+$ and O$^{2+}$ -- and therefore the total O abundance -- from faint pure recombination lines. The results for NGC 2579 permit to extend our previous determinations of the C, O and C/O gas phase radial gradients of the inner Galactic disc (Esteban etal. 2005) to larger galactocentric distances. We find that the chemical composition of NGC 2579 is consistent with flatten gradients at its galactocentric distance. In addition, we have built a tailored chemical evolution model that reproduces the observed radial abundance gradients of O, C and N and other observational constraints. We find that a levelling out of the star formation efficiency about and beyond the isophotal radius can explain the flattening of chemical gradients observed in the outer Galactic disc.