The oxygen abundance gradient in M81 and the robustness of abundance determinations in H ii regions

Abstract

We study the sensitivity of the methods available for abundance determinations in H ii regions to potential observational problems. We compare the dispersions they introduce around the oxygen and nitrogen abundance gradients when applied to five different sets of spectra of H ii regions in the galaxy M81. Our sample contains 116 H ii regions with galactocentric distances of 3 to 33 kpc, including 48 regions observed by us with the OSIRIS long-slit spectrograph at the 10.4-m Gran Telescopio Canarias telescope. The direct method can be applied to 31 regions, where we can get estimates of the electron temperature. The different methods imply oxygen abundance gradients with slopes of −0.010 to −0.002 dex kpc −1 , and dispersions in the range 0.06-0.25 dex. The direct method produces the shallowest slope and the largest dispersion, illustrating the difficulty of obtaining good estimates of the electron temperature. Three of the strong-line methods, C, ONS, and N2, are remarkably robust, with dispersions of ∼ 0.06 dex, and slopes in the range −0.008 to −0.006 dex kpc −1 . The robustness of each method can be directly related to its sensitivity to the line intensity ratios that are more difficult to measure properly. Since the results of the N2 method depend strongly on the N/O abundance ratio and on the ionization parameter, we recommend the use of the C and ONS methods when no temperature estimates are available or when they have poor quality, although the behaviour of these methods when confronted with regions that have different properties and different values of N/O should be explored.

Young Stars and Ionized Nebulae in M83: Comparing Chemical Abundances at High Metallicity

“…5.2 the O3N2 diagnostic calibrated by Pettini & Pagel (2004) stands out as the only one providing H II region abundances that are consistent with our stellar metallicities in M83, which are all but one above the solar value. In the similarly high metallicity ( (O) > 8.6) environment of the galaxy M81 we reach the same conclusion, analyzing the supergiant data from Kudritzki et al (2012) and the nebular emission fluxes from Patterson et al (2012) and Arellano-Córdova et al (2016). Keeping in mind the statistical nature of strong-line diagnostics (i.e.…”

Mapping Metallicity Variations across Nearby Galaxy Disks

“…This scatter is generally smaller than the amplitude of the radial variation, which typically ranges from 0.1 to 0.3 dex over the radial range we observe. It has been previously shown that strong line methods systematically produce smaller scatter in their radial gradients than direct temperature methods (Arellano-Córdova et al 2016), but this very small systematic scatter with respect to the radial gradient is similar to what has been found using direct temperature methods to measure the metallicity in the Milky Way (Esteban & García-Rojas 2018). It is, however, much smaller than what has been found with direct temperature methods for three nearby galaxies by the CHAOS project (∼0.1 dex; Croxall et al 2015;Berg et al 2015;Croxall et al 2016) and in M33 (0.11 dex; Rosolowsky & Simon 2008).…”

Metallicities in the Outer Regions of Spiral Galaxies

“…Table 1 is NGC 3031 (M81), for which a flat outer gradient has been suggested, but this result relies on a very small sample of outlying H II regions (Patterson et al 2012;Stanghellini et al 2014). A more recent work by Arellano-Córdova et al (2016) does not find evidence for a flat gradient out to a galactocentric distance of 33 kpc (2.3 R 25 ). This seems to be consistent with the shallow overall abundance gradient, both in dex kpc −1 and normalized to the isophotal radius, which is possibly the consequence of galaxy interactions.…”