# The radial abundance gradient of oxygen towards the Galactic anticentre

## TL;DR

This study measures the oxygen abundance gradient in the Milky Way's outer regions using deep spectroscopy of HII regions, finding no evidence of flattening and consistent temperature and metallicity gradients across the Galactic disc.

## Contribution

It provides the first detailed analysis of the radial oxygen abundance gradient extending beyond 11.5 kpc in the Milky Way using homogeneous data and methods.

## Key findings

- Radial oxygen gradient shows no flattening beyond 11.5 kpc.
- Oxygen abundance scatter is within observational uncertainties.
- Temperature gradients are positive and similar for OIII and NII.

## Abstract

We present deep optical spectroscopy of eight HII regions located in the anticentre of the Milky Way. The spectra were obtained at the 10.4m GTC and 8.2m VLT. We determined Te([NII]) for all objects and Te([OIII]) for six of them. We also included in our analysis an additional sample of 13 inner-disc Galactic Hii regions from the literature that have excellent T_e determinations. We adopted the same methodology and atomic dataset to determine the physical conditions and ionic abundances for both samples. We also detected the CII and OII optical recombination lines in Sh 2-100, which enables determination of the abundance discrepancy factor for this object. We found that the slopes of the radial oxygen gradients defined by the HII regions from R_25 (= 11.5 kpc) to 17 kpc and those within R_25 are similar within the uncertainties, indicating the absence of flattening in the radial oxygen gradient in the outer Milky Way. In general, we found that the scatter of the O/H ratios of Hii regions is not substantially larger than the observational uncertainties. The largest possible local inhomogeneities of the oxygen abundances are of the order of 0.1 dex. We also found positive radial gradients in Te([O III]) and Te([N II]) across the Galactic disc. The shapes of these temperature gradients are similar and also consistent with the absence of flattening of the metallicity distribution in the outer Galactic disc.