Radial Gradients and Intrinsic Scatter in MaNGA Galaxies

TL;DR

This paper analyzes spatially resolved stellar populations in 2968 MaNGA early-type galaxies to map age, [Fe/H], and light-element abundances (C, N, Na, Mg) within $0.65 R_e$, using an updated composite stellar population framework with element-sensitive isochrones (magnesium amplification) and a fixed-width abundance distribution function. By fitting index strengths with a dedicated inversion code (compfit) and employing 50 Monte Carlo realizations, the study quantifies intrinsic astrophysical scatter in addition to measurement errors, for annuli out to $0.7 R_e$. The key results show that mean stellar age rises with velocity dispersion at a rate of about $0.53$ dex per decade in $ ext{log}\sigma$, while [Fe/H] peaks near $ ext{log}\sigma \, ext{≈}\, 2.0$ and declines away from that point; light-element abundances rise with $ ext{log}\sigma$ with a notable inflection at $ ext{log}\sigma \, ext{≈}\, 2.0$, and radial gradients are generally shallow for age ($ abla_r ext{log age} \,\approx\,-0.04$ dex per decade) and Fe ($ abla_r ext{[Fe/H]} \,\approx\;-0.12$ to $-0.16$ dex per decade), while Na shows the strongest gradient. Intrinsic scatter in age and [Fe/H] decreases with increasing $ ext{log}\sigma$, whereas light-element scatter shows more complex behavior, especially for Na. Comparisons with TNG simulations reproduce the qualitative trends, including the inflection near $ ext{log}\sigma \,=\,2.0$ and generally flat gradients, yet systematic zero-point offsets in abundances remain, supporting a hierarchical merging scenario for ETG assembly. These results demonstrate the power of improved, abundance-aware stellar population models to constrain the chemical and structural growth of galaxies and provide valuable benchmarks for cosmological simulations.

Abstract

We derive stellar population parameters and radial gradients within 0.65 $R_e$ for spatially resolved spectra of 2968 early-type galaxies from MaNGA, spanning stellar velocity dispersions ($σ$) of 50--340 km s$^{-1}$. Light-weighted mean age and C, N, Na, Mg, and Fe abundances are obtained by inverting metallicity-composite stellar population models with isochrones that respond in $T_{\mathrm{eff}}$ to individual element abundance changes. Globally, $\log$(age) increases ($\sim$0.53 dex per decade), [Fe/H] declines slightly ($\sim$-0.06 dex per decade), and [X/Fe] for light elements rises (0.19--0.37 dex per decade for $\logσ< 2.0$ but steepens to nearly double slope for $\logσ> 2.0$) with $\logσ$. [Fe/H] peaks at $\logσ\sim 2.0$ and falls on either side. Light-element [X/Fe] anticorrelates with [Fe/H] ($\sim$-0.1 dex per decade). Astrophysical scatter is largest in low-$σ$ galaxies, especially for Fe and N. Internally, age gradients are nearly flat in low-$σ$ galaxies and slightly negative in high-$σ$ systems ($\sim$-0.04 dex per decade). [Fe/H] radial gradients steepen from -0.06 dex per decade to -0.15 dex per decade across $σ$, while light elements (except Na) show $\sim$-0.03 dex per decade gradients. Scatter in gradients peaks in high-$σ$ galaxies, most strongly for Fe ($\sim$0.23 dex), suggesting comparable numbers of inside-out and outside-in formation. A near-zero ($\sim$-0.03) age and light-element gradient plus mild [Fe/H] gradients supports hierarchical merging for ETG evolution. Simulations match the observed age structure, slope change at $\logσ\sim 2.0$, and flat gradients, though they overpredict absolute abundances.

Figures (35)

• Figure 1: The distribution of all galaxies from the MaNGA DR17 (blue), galaxies with $n>$2 (orange), and galaxies with $n>$2 & no H$_\alpha$ emission (green) as a function of velocity dispersion ($\sigma$).
• Figure 2: Position of the MaNGA IFU with respect to the center of the MaNGA galaxy 1-245451. The colorbar shows the ratio of mean flux within H$_\alpha$ index band and mean flux in the continuum.
• Figure 3: The map of H$_\alpha$ flux within the H$_\alpha$ index band (in absolute scale) for the MaNGA galaxy 1-245451. The orange ellipses are calculated using Eqn.\ref{['eqn:ellipse']} with parameters specific to this galaxy.
• Figure 4: Top panel: Mean flux (red/blue solid line) and emission line spectra (red/blue dashed line) from the innermost and 7$^{th}$ annuli for the MaNGA galaxy 1-245451. Bottom panel: Emission line subtracted spectra for the same galaxy (red: innermost annulus and blue: 7$^{th}$ annulus). The error shading shows the 1-$\sigma$ uncertainty in the measured fluxes.
• Figure 5: The strengths and uncertainties of selected features are displayed for MaNGA galaxy 1-25451. Values are shown for the 1$^{\rm st}$ through 7$^{\rm th}$ annulus, inclusive. For graphical scaling, CN$_1$ is multiplied by 20, and Mg$_1$ is multiplied by 15.