Spatially resolved star-formation histories of local post-starburst galaxies: Starburst and quenching spatial patterns consistent with recent mergers

TL;DR

This work uses MaNGA integral-field spectroscopy of three local post-starburst galaxies to reconstruct spatially resolved SFHs, metallicity, and dust at ~0.3 kpc resolution via a hierarchical Bayesian model. A three-stage fitting procedure (global priors, per-bin fits, and radial population modelling with azimuthal scatter) enables robust inference of radial trends in $M_*$, $t_{burst}$, $Z_{old}$, $A_V$, and $\,\sigma_{disp}$, while accounting for PSF and covariance. The results reveal a two-phase outside-in sequence: an outer, weaker, slower quenching burst followed by a central, stronger, faster quenching burst, with stronger central metallicity enrichment and non-axisymmetric dust patterns. These patterns are consistent with gas-rich major mergers and tidal interactions, with quenching largely driven by gas consumption and morphological stabilization rather than dominant AGN feedback. The methodology improves upon spatially integrated fits, provides detailed comparisons with merger simulations, and supports a merger-driven evolutionary pathway for local PSBs with LIRG-like progenitors.

Abstract

Post-starburst (PSB) galaxies, having recently experienced a starburst followed by rapid quenching, are excellent laboratories to probe physical mechanisms that drive starbursts and shutting down of star formation. Integral-field spectroscopy reveals the galaxies' spatially-resolved properties, where observed directional patterns can be linked to the galaxies' past evolution. We measure the resolved star-formation histories (SFHs), stellar metallicity evolution and dust properties of three local PSBs from the MaNGA survey, down to $0.5$" resolution ($\sim0.3\,$kpc) using a hierarchical Bayesian model. Local parameters were constrained simultaneously with parameters describing spatial trends. We found that all three galaxies first experienced an outer, weaker and slower quenching starburst, followed by a central, stronger and faster quenching starburst that peaked $\sim 1\,$Gyr after the first. The central starbursts induced a significantly stronger rise in stellar metallicity compared to the outer starbursts. These results are consistent with the effects of a recent gas-rich (wet) merger, where the first pericentre passage triggered starbursts in the outer regions, while the later coalescence triggers a stronger centralised starburst. We find non-axisymmetric features in the maps of burst mass fraction and dust attenuation in all galaxies, which could be caused by tidal effects during the recent merger. Comparisons with literature binary merger simulations suggests that the galaxies' rapid quenching was driven by gas consumption and the stabilisation against gas gravitational collapse by a growing spheroid, while AGN feedback was not necessarily a primary cause.

Figures (10)

• Figure 1: SDSS 3-colour images (left), the PSB spaxel selection (middle) and the Voronoi bin distribution (right) of our sample of three PSBs. Each galaxy's Plate-IFU is marked on the top right corner of the SDSS images. The MaNGA field of view is marked as the pink hexagon. In the middle column, we divide the spaxels into regions with no/faulty observations (not coloured), median spectral $\mathrm{SNR}<8$ too low to be classified (grey), classified as PSB (blue), and classified as non-PSB (red). Voronoi binning is performed with a thresholdnote1 $\mathrm{SNR}_{g}=10$ and accounting for spatial covariance. The colours in the right column are used to differentiate between spaxels in a given Voronoi bin. The magenta crosses in the top panels mark the location of the spaxel discussed in detail in Fig. \ref{['fig:Bayes_vs_HBayes']} and Section \ref{['sec:discussion_hbayes']}.
• Figure 2: The probabilistic graphical model of the full hierarchical model. Open, shaded and closed nodes correspond to unknown (free), observed and fixed parameters, respectively. Nodes inside the rectangle are parameters unique to each individual Voronoi bins (Section \ref{['sec:Hbayes_stage1']}). Nodes outside the rectangle are hyper-parameters of the population model (Section \ref{['sec:Hbayes_stage2']}). The stellar portion of the model is marked with green rims, where the population model describes radial dependencies for the local stellar mass ($M_*$), burst age ($t_\mathrm{burst}$), and the pre-burst metallicity ($Z_\mathrm{old}$). Parameters related to dust attenuation, velocity dispersion and redshift are marked with orange rims, where the population model describes radial dependencies for the $V$-band dust attenuation strength ($A_V$) and velocity dispersion ($\sigma_\mathrm{disp}$). The additive Gaussian Process (GP) noise component is marked with magenta rims. Each Voronoi bin's $M_*$, $t_\mathrm{burst}$, $Z_\mathrm{old}$, $A_V$ and $\sigma_\mathrm{disp}$ have dependence on the bin's radial distance from the galaxy centre ($R$).
• Figure 3: Fitting results of 7965-1902 from the non-hierarchical stage 0 and stage 1 models. The left panels plot the posterior burst age (top left) and burst mass fraction (bottom left) against radial distance from the galaxy centre for individual Voronoi bins in blue and the global fit as a gray horizontal line and shaded band. The right panel compares the SFH obtained from stacking the fitted SFH of all Voronoi bins (blue) to the fitted SFH from the global fit (black). The shaded regions denote 1 and $2\sigma$ uncertainty regions. The SFHs of 10 randomly drawn posterior samples are also shown as dashed curves. We observed the galaxy underwent two distinct starbursts, with the earlier one occurred predominantly in the outer regions, while the later, stronger one occurred predominantly in the centre. This is not captured by the global fit.
• Figure 4: A visualization of the logistic function used to model the radial dependence of the galaxies' burst age in our population model. The right hand side y-axis shows a normalized view of the logistic function, which better determines the scale width $R_w$. For this example, the burst age at the centre is $t_0=0.5\;$Gyr, the burst age at the outer edge is $t_\infty=1.2\;$Gyr, the radial distance of the curve's inflection point is $R_c=2.5\;$kpc, and the scale width of the transition region is $R_w=0.8\;$kpc.
• Figure 5: Resolved properties of 7965-1902 using the hierarchical model. Top: Radial gradients of 10 selected properties (see main text for their description). In all panels, the posterior median and $1\sigma$ uncertainty of the Voronoi bins are shown as orange dots and error bars. For model-independent properties, measurements of all spaxels individually from the MaNGA pipe3D value-added catalogue are shown as light grey dots (see main text for caveats on $A_V$). $A_{V,\mathrm{ISM}}$ measured via the Balmer decrement and assuming the MAP $\eta$ value from the posterior of the global fit is shown as dark blue dots, with the arithmetic mean of the $1\sigma$ uncertainty shown in the bottom left of the $A_V$ panel. As in Fig. \ref{['fig:justify_tburst_gradient']}, results from the global fit are shown as a horizontal grey line and shaded band. For the five parameters that we assigned population models to, the population model's posterior median and $1\sigma$ uncertainty are shown as a lime line and shaded region. The galaxy's $r$-band 2D Sérsic $R_e$ from the NSA catalogue is shown as a blue vertical dotted line. Bottom: Maps of the posterior median estimates for the same properties. The direction of the colour map is reversed and in $\log_{10}$ scaling in the half time panel compared to the others. Cyan crosses mark the central spaxel, taken as the spaxel with the lowest luminosity-weighted elliptical polar distance from the galaxy's centre.