A MaNGA about the Legacy I: Connecting the Assembly of Stellar Halo with the Average Star Formation History in Low-Redshift Massive Galaxies

Abstract

We investigate the connection between stellar mass distribution, assembly history, and star formation timescales in low-redshift massive early-type galaxies (ETGs) by combining deep LegacySurvey imaging with MaNGA's spatially resolved spectroscopy. Focusing on stellar population properties, especially the [Mg/Fe] abundance ratio, we analyze stacked spectra using both absorption line indices and full-spectrum fitting. We find that, among massive ETGs with identical average stellar mass distributions beyond 20 kpc, those with higher central velocity dispersion ($σ_{cen}$) are older and more $α$-enhanced, suggesting a connection between the in-situ star formation in the past and the central gravitational potential today for massive ETGs with a similar stellar accretion history. Conversely, at fixed $σ_{cen}$ and total stellar mass, galaxies with more extended stellar halos show lower [Fe/H], higher [Mg/Fe], and older ages, indicating an intriguing link between early starburst and quenching and later ex-situ assembly. These results demonstrate that the evolution of massive galaxies cannot be fully described by simple scaling relations alone, as the interplay between in-situ star formation and ex-situ accretion leaves distinct imprints in both their inner and outer stellar populations. Our findings highlight the importance of extending stellar population studies to large radii and underscore the scientific potential of next-generation IFU surveys and deep, high-resolution spectroscopy for probing the galaxy-halo connection.

Figures (19)

• Figure 1: A flowchart representing the sample selection procedures used in this work. Each box represents a selection stage, and the exact criteria are listed inside.
• Figure 2: An example galaxy (PGC153919) in the final sample. Top left: SGA g-band image. The title is the SGA id of this galaxy. Top right: MaNGA g-band flux map with its plate-ifu as the title. The gray region represents the original data, and the colored pixels are the ones used in the stacking after the $\mathrm{S}/\mathrm{N}$ cut and multi-source masking. Bottom left: the Voronoi-binned stellar velocity dispersion map available in MaNGA DAP maps. Similarly, the gray color map covers the entire data cube, while the colored region represents the selected region. The similar ellipses in these three plots shown in blue dash-dot-dot, green dashed and pink solid ellipses have semi-major-axis of $\mathrm{0.5\ R_e)}$, $\mathrm{1.0\ R_e)}$ and $\mathrm{1.5\ R_e}$ respectively. Bottom right: the radial profile of the g-band flux in MaNGA data. We can see that the additional masking results in a smoother profile, implying that our masks have successfully excluded contamination from other sources. The pink circles, green triangles, and blue diamonds stand for pixels with major axis range in $\mathrm{Inner(}R\mathrm{\le 0.5R_e)}$, $\mathrm{Middle(0.5R_e\le}R\mathrm{\le1.0R_e}$, and $\mathrm{Outer(}R\mathrm{\ge 1.0R_e)}$ respectively.
• Figure 3: The two sub-samples are split by position on the $\mathrm{log_{10}} (M_{\star,R>20\ \rm kpc}/M_{\odot})$ vs. $\sigma_{\mathrm{\star,cen}}$ plane (top left). For galaxies with similar outskirt stellar mass, orange pentagons represent the galaxies with larger inner velocity dispersion('$\mathrm{High}\ \sigma_{*}$' ), and blue stars represent galaxies with smaller inner velocity dispersion ('$\mathrm{Low}\ \sigma_{*}$'). The background gray circles stand for the total sample. The top right plot displays the radial profiles of stellar mass density ($\rm M_\odot{/kpc^2}$), with the thick lines representing the median profiles. Using the same color-coding convention, the histograms show the distributions of their physical parameters, including $M_{*}$ (bottom left), $\sigma_{\mathrm{\star,cen}}$ (bottom middle), and redshift (bottom right). Orange solid lines and blue dashed lines correspond to '$\mathrm{High}\ \sigma_{*}$' and '$\mathrm{Low}\ \sigma_{*}$' samples, respectively, and the gray-filled bar is the total sample. The Jupyter notebook for reproducing this figure can be found here: https://github.com/xyzhangwork/mdensity_v_stellarpop/blob/main/plot_scripts/plot_split.ipynb. This repository is also available on https://doi.org/10.5281/zenodo.17979404.
• Figure 4: The top-left plot shows the sample-split scheme, where galaxies are categorized by position on the $\mathrm{log_{10}} (M_{\star,R>10\ \rm kpc}/M_{\odot})$ vs. $\mathrm{log_{10}} (M_{\star,R>20\ \rm kpc}/M_{\odot})$ plane. The two samples have similar inner stellar masses ($M_{\star,R<10\ \rm kpc}$); the green circles represent galaxies with more extended outskirts ('Extended'), and the purple diamonds represent more compact galaxies ('Compact'). As before, the total sample is in gray circles. The top right plot displays the radial profiles of stellar mass density ($\rm M_\odot{/kpc^2}$), with the thick lines representing the median profiles. The green solid lines and purple dashed lines correspond to the Extended and Compact samples, respectively. The gray-filled bar represents the total sample. Similar to Figure \ref{['fig:split_sigma']}, the histograms show the $M_{*}$ (bottom left), $\sigma_{\mathrm{\star,cen}}$ (bottom middle)， and redshift (bottom right). We also show the p-values (with errors from resampling the original sample in their respective uncertainties. For $M_{\star,R>20\ \rm kpc}$, we assume a 0.1 dex error) of the Kolmogorov–Smirnov tests (K-S test) on the $M_{*}$ (bottom left) and $\sigma_{\mathrm{\star,cen}}$ (bottom middle) distributions between the two sub-samples on the first two histograms. The Jupyter notebook for reproducing this figure can be found here: https://github.com/xyzhangwork/mdensity_v_stellarpop/blob/main/plot_scripts/plot_split.ipynb. This repository is also available on https://doi.org/10.5281/zenodo.17979404.
• Figure 5: Normalized spectra in the three absorption index regions of the extended (solid green line) and compact (dashed purple line) sub-samples. From left to right: $\mathrm{Mg_b,\ Fe5270,\ Fe5335}$