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Spiral galaxies with flat radial abundance gradients at large radii

L. S. Pilyugin, G. Tautvaisiene

TL;DR

This study analyzes the radial oxygen abundance profiles of 20 massive MaNGA spiral galaxies and demonstrates that the outer disc exhibits a flat $O/H$ gradient beyond an outer break radius $R_{ m b,outer}$, which lies between $0.8R_{25}$ and $1.45R_{25}$ with a mean $R_{ m b,outer}/R_{25}=1.08\pm0.14$. The circumgalactic medium shows $12+\log( ext{O/H})$ values from approximately $8.0$ to $8.5$, indicating substantial metal enrichment beyond the optical disc. Inner breaks at $R_{ m b,inner}\sim0.45$–$1.15R_{25}$ separate steeper inner and flatter outer gradients, while the outer gradient is consistently steeper than the inner one. The authors argue that interactions with a small companion and a radial gas-flow evolution framework can naturally produce the observed flat CGM gradients and the outer-disc abundance features, with the outer-zone size tracking the time since the change in gas inflow rate. These findings suggest long-lived signatures of minor-merger–driven gas mixing and provide important constraints for chemical-evolution models that incorporate radial inflows and external gas capture.

Abstract

We consider the oxygen abundance distributions for a sample of massive spiral galaxies from the MaNGA survey in which the radial abundance gradient flattens to a constant value outside of the outer break radius, Rb,outer. The outer break radius can be considered as a dividing radius between the galaxy and the circumgalactic medium (CGM). The values of the Rb,outer range from 0.8R_{25} to 1.45R_{25}, where R_{25} is the optical radius of the galaxy. The oxygen abundances in the CGM range from 12+log(O/H) ~ 8.0 to ~ 8.5. The O/H distribution in each of our galaxies also shows the inner break in the radial abundance profile at the radius Rb,inner. The metallicity gradient in the outer part of the galaxy is steeper than in the inner part. The behaviour of the radial abundance distributions in these galaxies can be explained by assuming an interaction with (capture of the gas from) a small companion and adopting the model for the chemical evolution of galaxies with a radial gas flow. The interaction with a companion results in the mixing of gas and a flat metallicity gradient in the CGM. The capture of the gas from a companion increases the radial gas inflow rate and changes the slope of the radial abundance gradient in the outer part of the galaxy.

Spiral galaxies with flat radial abundance gradients at large radii

TL;DR

This study analyzes the radial oxygen abundance profiles of 20 massive MaNGA spiral galaxies and demonstrates that the outer disc exhibits a flat gradient beyond an outer break radius , which lies between and with a mean . The circumgalactic medium shows values from approximately to , indicating substantial metal enrichment beyond the optical disc. Inner breaks at – separate steeper inner and flatter outer gradients, while the outer gradient is consistently steeper than the inner one. The authors argue that interactions with a small companion and a radial gas-flow evolution framework can naturally produce the observed flat CGM gradients and the outer-disc abundance features, with the outer-zone size tracking the time since the change in gas inflow rate. These findings suggest long-lived signatures of minor-merger–driven gas mixing and provide important constraints for chemical-evolution models that incorporate radial inflows and external gas capture.

Abstract

We consider the oxygen abundance distributions for a sample of massive spiral galaxies from the MaNGA survey in which the radial abundance gradient flattens to a constant value outside of the outer break radius, Rb,outer. The outer break radius can be considered as a dividing radius between the galaxy and the circumgalactic medium (CGM). The values of the Rb,outer range from 0.8R_{25} to 1.45R_{25}, where R_{25} is the optical radius of the galaxy. The oxygen abundances in the CGM range from 12+log(O/H) ~ 8.0 to ~ 8.5. The O/H distribution in each of our galaxies also shows the inner break in the radial abundance profile at the radius Rb,inner. The metallicity gradient in the outer part of the galaxy is steeper than in the inner part. The behaviour of the radial abundance distributions in these galaxies can be explained by assuming an interaction with (capture of the gas from) a small companion and adopting the model for the chemical evolution of galaxies with a radial gas flow. The interaction with a companion results in the mixing of gas and a flat metallicity gradient in the CGM. The capture of the gas from a companion increases the radial gas inflow rate and changes the slope of the radial abundance gradient in the outer part of the galaxy.
Paper Structure (11 sections, 6 figures, 1 table)

This paper contains 11 sections, 6 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data and mapping of the galaxy properties
  3. Data
  4. Determining the geometrical parameters and the galaxy size
  5. Determining the oxygen abundances
  6. Sample of galaxies with a flat abundance distribution in the circumgalactic medium
  7. Discussion
  8. Properties of the abundance distributions
  9. Origin of the flat abundance distribution in the CGM
  10. Origin of the inner break in the metallicity gradient
  11. Conclusions

Figures (6)

  • Figure 1: Radial oxygen abundance distributions for our sample of the MaNGA galaxies. Each panel shows the radial oxygen abundance distribution for individual spaxels (grey points) and binned (0.05$R_{25}$) values (red circles). The line denotes the adopted O/H -- $R$ relation.
  • Figure 2: Characteristics of the radial oxygen abundance distributions for our sample of galaxies. The circles designate the oxygen abundance in the circumgalactic medium ( panel a), the radial positions of the outer break ( panel b), and the radial positions of the inner break ( panel c) as a function of the stellar mass of MaNGA galaxies. The line is the linear best fit to these data points. The plus signs mark the nearby galaxies.
  • Figure 3: Radial oxygen abundance distributions for nearby galaxies. The circles in each panel show the oxygen abundances for individual H ii regions as a function of radius. The line denotes the adopted O/H -- $R$ relation.
  • Figure 4: Characteristics maps for five galaxies. Panels of the left columna: Observed surface brightness distribution in the galaxy image in sky coordinates (pixels). The surface brightness value is colour-coded. The circle shows the kinematic centre of the galaxy, the line indicates the position of the major kinematic axis of the galaxy, and the ellipse is its optical radius. Panels of the middle columnb: Line-of-sight velocity field. Panels of the right columnc: Distribution of O/H in the galaxy image.
  • Figure 5: Asymmetry of a light distribution in the galaxies of our sample. The asymmetry index of residual fluxes after the model flux is subtracted, $R_{A}$, vs. the asymmetry parameter in a light distribution for the galaxy, $A$. The circles denote all the galaxies, and the galaxies shown in Fig. \ref{['figure:sequence']} are marked by symbols shown in the legend.
  • ...and 1 more figures