The radial abundance gradient of oxygen towards the Galactic anti-centre

Abstract

We present deep optical spectroscopy of eight H ii regions located in the anticentre of the Milky Way. The spectra were obtained at the 10.4m GTC and 8.2m VLT. We determined T e ([N ii]) for all objects and T e ([O iii]) for six of them. We also included in our analysis an additional sample of 13 inner-disc Galactic H ii 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 C ii and O ii 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 H ii 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 H ii 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 T e ([O iii]) and T e ([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.

Revisiting the radial abundance gradients of nitrogen and oxygen of the Milky Way

“…We also include data for two other Galactic star-forming regions of the literature with direct determination of T e that do not show [O iii] emission (empty circles): IC 5146 and M 43. For IC 5146, we use the average abundances of apertures 2, 3 and 4 observed by García-Rojas et al (2014) and recalculated by Esteban et al (2017). For M 43 we use the average abundances of apertures A4 and A5 observed by Simón-Díaz et al (2011) and corrected for contamination from the emission of the neighboring Orion Nebula.…”

“…Considering the positive gradient of T e in the Galactic disc (e.g. Afflerbach et al 1996;Esteban et al 2017), one would expect that N abundances based on FIR observations would be more affected by systematic errors in ICF(N 2+ ) in the case of H ii regions located in the inner parts of the Galaxy. Carigi et al (2005), from optical spectra of a small sample of H ii regions and assuming the ICF(N + ) of Mathis & Rosa (1991), calculated a radial gradient of log(N/O) of −0.042 ± 0.015 dex kpc −1 .…”

“…The most recent determinations indicate that the gradient slope is between −0.040 and −0.060 dex kpc −1 . Despite some authors claimed that the radial O abundance gradient may flatten out at the outer parts of the Milky Way (e. g. Fich & Silkey 1991;Vílchez & Esteban 1996;Maciel et al 2006), the recent work by Esteban et al (2017) has confirmed the absence of such flattening, indicating that the slope of the gradient remains constant at least up to R G ∼ 17 kpc. On the other hand, some works on metallicity gradients based on Cepheids and red giants observations have found indications of a flattening of the gradients in the inner Galactic disc, at R G < 5-6 kpc (Hayden et al 2014;Martin et al 2015;Andrievsky et al 2016).…”

“…On the other hand, some works on metallicity gradients based on Cepheids and red giants observations have found indications of a flattening of the gradients in the inner Galactic disc, at R G < 5-6 kpc (Hayden et al 2014;Martin et al 2015;Andrievsky et al 2016). An indication of the presence of such feature can be noted in the H ii region abundance gradient determined by Esteban et al (2017), although they did not report the fact because of the limited number of objects.…”

First gas-phase metallicity gradients of 0.1 ≲ z ≲ 0.8 galaxies with MUSE

“…That said, while the radial metallicity profile of the Milky Way appears to be well-described simply by an exponentiallydeclining function (Esteban et al 2017), there is some debate as to whether this is really true for all galaxies. Indeed, as noted by Sánchez et al (2014) and many others (e.g.…”