Aligned and misaligned metallicity gradients in young stars and star-forming regions in the EAGLE discs

TL;DR

This study leverages the high-resolution EAGLE Recal-L025N0752 simulation to jointly analyze metallicity gradients in young stars ($\nabla^{\rm YS}$) and star-forming gas ($\nabla^{\rm SFG}$) within disc galaxies, classifying systems into NN, NP, PP, and PN to link gradient alignment with recent evolutionary histories over $\sim2$ Gyr. Gradients are measured as slopes of azimuthally averaged $12+\log(O/H)$ profiles in the range [0.5, 1.5] $R_{\rm eff}$ for both YS and SFG, using galaxies with $M_{\star}\in[10^{9},10^{11}]\,M_{\odot}$ and central, disc-dominated morphologies (239 galaxies). The analysis reveals distinct evolutionary paths: NN shows sustained inside-out growth with high star-formation efficiency; NP and PP indicate recent or ongoing feedback-driven disruption (SN feedback dominates PP); PN suggests past violent events with gradient recovery, reflecting inflows, feedback, and cooling. The degree of YS–SFG gradient alignment serves as a timing tool for major events and offers a means to test sub-grid physics by comparing with observations, with implications for interpreting metallicity distributions in disc galaxies.

Abstract

Disc galaxies exhibit radial metallicity gradients in both their stellar and gaseous components. The star-forming gas (SFG) in HII regions and young stars (YSs) trace the recent evolutionary history of the galaxy. We aim to assess the extent to which the joint analysis of metallicity gradient alignment in YSs and SFG can constrain the recent evolutionary history of galaxies. Using the high-resolution run of the EAGLE project, we derived radial, azimuthally averaged oxygen abundance profiles for YSs (age < 2 Gyr) and SFG and measured their gradients as the slopes of linear fits to these profiles. We classified galaxies into four groups based on the signs (N for negative and P for positive) of the slopes: NN, NP, PP, and PN (the first letter is for YSs and the second for SFG). We found that galaxies with NN, NP, PP, and PN combinations of metallicity profiles reflect different evolutionary paths over the past ~ 2 Gyr. NN galaxies exhibit sustained inside-out growth accompanied by high star formation efficiency, whereas NP and PP systems show evidence of recent or ongoing feedback-driven disruption, with PP galaxies likely being predominantly shaped by supernova feedback. PN galaxies, by contrast, show evidence of past violent events followed by gradient recovery, highlighting the interplay between inflows, feedback, and gas cooling in shaping metallicity distributions. The degree of alignment between the stellar and gas metallicity gradients provides a way to time the occurrence of significant events in the evolutionary history of galaxies, which contribute through a combination of gas inflows, star formation triggering, and metal mixing. They could also serve as probes of sub-grid physics when observations provide suitable comparison datasets. [Abridged]

Figures (7)

• Figure 1: Metallicity gradients of SFG ($\nabla^{\rm SFG}$) as a function of metallicity gradients of YSs ($\nabla^{\rm YS}$) in units of dex kpc$^{-1}$ selected from the Recal-L025N0752 eagle simulation. The error bars on the gradients were estimated from the standard deviation of the linear regressions. For comparison, the oxygen abundance gradients of YSs (age $\leq2$ Gyr) and H ii regions for the MW molla2019 and NGC 1365 sextl2024 are displayed (black symbols). Both the MW and NGC 1365 lie close to the 1:1 relation (dashed black line). The eagle galaxies are colour-coded according to the quadrants to which they belong. This colour-coding scheme is followed in all subsequent plots.
• Figure 2: Cumulative distributions of the SFR, the sSFR, the gas fraction ($f_{\rm{gas}}$), and the depletion time ($\tau_{\rm{dep}}$) for low-mass (upper panel) and high-mass (lower panel) galaxies. Lines indicate galaxies classified as NN (solid red), NP (dashed green), PP (dash-dotted blue), and PN (dotted green).
• Figure 3: Cumulative distributions of disc-to-total stellar mass ratio (D/T) for low-mass (upper panel) and high-mass (lower panel) galaxies. Lines indicate galaxies classified as NN (solid red), NP (dashed green), PP (dash-dotted blue), and PN (dotted orange).
• Figure 4: SFR gradient ($\nabla_{\rm SFR}$) as a function of the SFG metallicity gradient ($\nabla^{\rm SFG}$) for low-mass (upper panel) and high-mass (lower panel) galaxies. Data points represent galaxies classified by the signs of their young stellar and gas metallicity gradients: NN (red circles), NP (green diamonds), PP (blue hexagons), and PN (orange triangles). The error bars associated with $\nabla_{\rm SFR}$ denote three times the bootstrap errors.
• Figure 5: SFE as a function of gas fraction ($f_{\rm{gas}}$) for PP, NP, and PN galaxies in the low-mass (upper panel) and high-mass (lower panel) bins. All parameters are expressed relatively to those of NN galaxies at a given $M_{\star}$. The coloured map represents the normalised SFR, while green borders around the data points highlight galaxies that increased their stellar mass by more than $25\%$ within $2.5$ Gyr ($\tau_{25}<2.5$ Gyr). The percentages of each galaxy type within each quadrant are included.