Contrasting evolutionary pathways of fast- and slow-rotating galaxies in the green valley

Abstract

We investigate the evolutionary pathways of green valley (GV) galaxies drawn from the SDSS-IV/MaNGA survey. The GV sample is divided into fast- and slow-rotating galaxies based on stellar spin, and their stellar and gas-phase metallicities are compared. Fast-rotating galaxies exhibit systematically higher metallicities than slow-rotating galaxies in both gas and stars. However, the gas-phase difference is significant only at low stellar masses, while the stellar metallicity offset persists across the full mass range. Using a simple yet physically motivated chemical evolution model, optimised to jointly fit gas-phase metallicities and integrated stellar spectra, we reconstruct the star formation and chemical enrichment histories of individual galaxies and constrain gas inflow and outflow parameters. At low stellar masses, fast- and slow-rotating galaxies show similar gas-infall and star formation timescales, but the the slower population experienced stronger outflows which reduce their chemical content in both gas and stars. At high masses, the combination of reduced pristine gas inflow and more efficient gas removal in slow-rotating galaxies produce gas-phase metallicities comparable to fast-rotating galaxies but systematically lower stellar metallicities. These differences suggest distinct evolutionary pathways for GV galaxies. Slow-rotating galaxies likely experienced more mergers, usually associated with strong gas removal processes, leading to their systematically lower metallicities. At low masses, stronger supernova-driven outflows reduce their chemical content while leaving star-formation timescales similar to fast-rotating galaxies. At high masses, merger-triggered AGN feedback may rapidly deplete and suppress gas infall, producing the shorter star-formation timescales seen in slow-rotating galaxies. Alternative environmental and assembly-driven scenarios are also discussed.

Figures (10)

• Figure 1: Top: the star formation rate as a function of stellar mass for MaNGA galaxies. Grey dots are individual MaNGA galaxies, with contours enclosing 20%, 40%, 60% and 80% of the data points. The solid line shows the SFMS obtained from Sanchez2019. Bottom: distribution of $\Delta_{\rm SFMS}$ for MaNGA galaxies. Orange histogram shows the entire sample, with blue and red lines showing the possible distribution of star-forming and passive galaxies, obtained by fitting a Gaussian function to the histogram with -0.5<$\Delta_{\rm SFMS}$ and $\Delta_{\rm SFMS}$<-2.5, respectively. The green histogram shows the possible distribution of green valley galaxies by subtracting the contribution of star-forming and passive galaxies from the whole distribution. In both panels, the shaded region indicates the green valley used in this work defined as the region of -1.3<$\Delta_{\rm SFMS}$<-0.5.
• Figure 2: The spin parameter $\lambda_{Re}$ of MaNGA galaxies as a function of their ellipticity $\varepsilon$. Colours indicate the density of the sample galaxies, with contours enclosing 20%, 40%, 60% and 80% of the data points. The black dash-dotted line represents the theoretically predicted locus for an axisymmetric galaxy with $\lambda_{\rm Re, intr}=0.4$ (assuming anisotropy $\delta=0.7\times\varepsilon_{\rm intr}$) viewed over all inclinations, as proposed by Wang2023APJL. Throughout this work, galaxies above and below this curve are defined as the faster and slower populations, respectively. For comparison, the classical fast/slow-rotator division from Cappellari2016ARA is also shown as a red dashed curve.
• Figure 3: The stellar (left) and gas phase (right) metallicities as a function of stellar mass for our sample galaxies. In each panel, fast-rotating galaxies are shown in blue, while slow-rotating galaxies are shown in red. The solid lines indicate the mean relation, with error bars representing the standard deviation of 1,000 bootstrap resamplings.
• Figure 4: Predicted star formation histories (left), chemical evolution histories (middle), and cumulative metallicity distribution functions (right) for three representative models, with key model parameters indicated in the middle panel. In the right panel, the labels indicate the light-weighted average stellar metallicity for each model.
• Figure 5: Example of the spectral fitting process to a green valley galaxy in our sample. The top-left panel shows the optical image of the galaxy, with its MaNGA plateifu ID indicated. The top-right panel shows the best-fit model to the observed data. In this panel, the orange line is the observed spectrum stacked within 1 Re of the galaxy, while the blue line shows the best-fit model spectrum. At the bottom, a green line indicates the residuals, with the grey shaded region showing the emission lines that are masked during the fitting process. The bottom panels show the SFH (left), ChEH (middle) and CMDF (right) of this galaxy as calculated from the best-fit parameters. In the middle panel at the bottom, the grey dashed line indicates the observed gas-phase metallicity as obtained from emission line analysis.