Metallicity gradients in small and nearby spiral galaxies

Abstract

Spectra of H ii regions obtained with Gemini/GMOS are used to derive the radial metallicity gradients of four small, low-mass spiral galaxies. The analysis of the outer disk of one of them, NGC 1058, uncovers the characteristic flattening found in similar extended disk galaxies. After combining these data with published long-slit observations of nearby spiral galaxies, no evidence for a dependence of the disk scale lengthnormalized metallicity gradients with stellar mass is found, down to log(M /M ) ∼ 8.5. The abundance gradients derived from these observations are compared to predictions from recent cosmological simulations of galaxy evolution, finding that in several cases the simulations fail to reproduce the mean steepening of the gradients, expressed in dex kpc −1 , with decreasing stellar mass for present-day galaxies, or do not extend to sufficiently small stellar masses for a meaningful comparison. The mean steepening of the abundance gradients (in dex kpc −1 ) with decreasing disk scale length is in qualitative agreement with predictions from the inside-out model of Boissier & Prantzos, although the predicted slopes are systematically steeper than observed. This indicates the necessity of including processes such as outflows and radial mixing in similar models of galactic chemical evolution. Published spatially resolved metallicity and photometric data of dwarf irregular galaxies suggest that significant, but transitory, metallicity gradients can develop for systems that have experienced recent (t < 100 Myr) enhanced star formation in their inner disks.

Exploring chemical homogeneity in dwarf galaxies: a VLT-MUSE study of JKB 18

“…This suggests that one possible explanation for large-scale variations in O/H and N/H is a recent increase in SFR. As discussed above, a similar hypothesis is also used by Bresolin (2019) to explain abundance gradients in dwarf irregular galaxies. It is possible that JKB 18 too is undergoing a period of increased star formation activity, resulting in a broadened distribution of metal content between H II regions.…”

“…(vi) As discussed in Bresolin (2019), there are a handful of dwarfirregular galaxies which actually have well-ordered gradients that are comparable to spiral galaxies. For example, NGC 6822 (Lee et al 2006), NGC 1705 (Annibali et al 2015), NGC 4449 (Annibali et al 2017), and DDO 68 (Annibali et al 2019) each show chemical abundance gradients, despite the absence of spiral structures, which were deemed necessary for gradients below M B 17 by Edmunds & Roy (1993).…”

“…For example, NGC 6822 (Lee et al 2006), NGC 1705 (Annibali et al 2015), NGC 4449 (Annibali et al 2017), and DDO 68 (Annibali et al 2019) each show chemical abundance gradients, despite the absence of spiral structures, which were deemed necessary for gradients below M B 17 by Edmunds & Roy (1993). Bresolin (2019) link the existence of these gradients to recent enhancements in their star formation activity, where metal mixing time-scales are longer than the time between bursts of star formation.…”

H ii regions in the CALIFA survey: I. catalogue presentation

“…Unfortunately, these techniques provided a limited sample of galaxies in detail, including a low number of H ii regions per galaxy, in general. Therefore, there was a limited coverage of these regions along the whole galaxies extension (e.g., Bresolin 2019). Conversely, the classical slit-spectroscopy surveys introduce a bias by aperture effects: long-slit observations do not integrate over the full extension of an H ii region.…”

Luminous Blue Variables

“…The observed metallicity gradient [173] therefore implies a low metallicity of log (O/H) = 8.2 (roughly between LMC and SMC [174]) for the star. Note in that context that metallicity gradients are a common feature in spiral galaxies, e.g., [175,176], so large spiral galaxies do not have one fixed metallicity.…”