On the radial abundance gradients of nitrogen and oxygen in the inner Galactic disc

TL;DR

This study measures nitrogen and oxygen abundance gradients in the inner Galactic disc using optical spectra of H II regions, finding linear, constant gradients with minimal azimuthal variation and a flat N/O versus O/H relation, contrasting with other galaxies.

Contribution

It provides a revised, detailed analysis of chemical abundance gradients in the inner Galactic disc using Gaia parallaxes and a combined optical and FIR data set, revealing new insights into their linearity and azimuthal uniformity.

Findings

Radial gradients of O/H and N/H are linear and constant between 4 and 17 kpc.

Minimal azimuthal variation in chemical abundances in the observed quadrant.

Flat N/O versus O/H relation, unlike other spiral galaxies.

Abstract

We present optical spectra of nine Galactic H II regions observed with the 10.4 m Gran Telescopio Canarias telescope and located at Galactocentric distances (RG) from 4 to 8 kpc. The distances of the objects have been revised using Gaia DR2 parallaxes. We determine the electron temperature for all the nebulae, which allows a precise computation of their ionic abundances. We have included published data of an additional sample of Galactic H II regions, providing a final data set of 42 objects. The shape of the radial gradients of O/H and N/H is linear and constant, discarding any substantial change of the slope, at least for RG between 4 and 17 kpc. The small dispersion of the O/H and N/H values with respect to the computed gradients implies the absence of significant azimuthal variations of the chemical abundances, at least in the quadrant covered by our observations. We find an almost flat N/O versus O/H diagram relation. This result is not observed in other nearby spiral galaxy except M31. Finally, we compare our computed gradients with those obtained using far infrared (FIR) spectra. We confirm the significant offset in the N/O distribution between the optical and FIR observations. Possible explanations involve ionization correction factors and the strong dependence on density of the abundance determinations based on FIR lines.