Large-scale and local environmental drivers of quenching: tracing H$α$ concentration in X-ray and optical galaxy groups

Abstract

To explore the environmental mechanisms causing quenching in nearby star-forming galaxies, we study the variation with local and large-scale environments of a star formation concentration index, C-index $\equiv\log{(r_{50,{\rm H}α}/r_{50,\rm cont}})$, that traces the spatially-resolved distribution of H$α$ emission. Our analysis combines (i) GAMA spectroscopic redshift survey data to optically select galaxy groups and reconstruct the cosmic web, (ii) eROSITA data to identify X-ray-emitting groups, and (iii) SAMI Galaxy Survey data to characterise spatially-resolved star formation. We find that galaxies in X-ray+optical groups exhibit the lowest median C-index and the highest fraction of centrally-concentrated star-forming galaxies relative to optical groups and the field (independently of group or stellar mass). Star-forming galaxies in more X-ray luminous groups at fixed dynamical mass show more concentrated star formation. At large scales, nodes show the lowest median C-index and the highest fraction of centrally-concentrated star-forming galaxies relative to filaments and voids, which have similar C-index distributions. C-index correlates most strongly with the distance to the closest node, leaving no significant role for other local or large-scale environment metrics. Finally, regular star-forming galaxies tend to have spins aligned parallel to filaments, consistent with smooth gas accretion, while centrally-concentrated galaxies tend have spins aligned perpendicular to filaments, likely driven by mergers and associated with bulge growth. These results suggest that multi-scale environmental processes, i.e. locally and at large-scale, act to concentrate star formation toward galaxy centres, via gas-related mechanisms in nodes and ram-pressure stripping in X-ray+optical groups.

Figures (19)

• Figure 1: X-ray+Optical groups in G09 with SAMI galaxies (gray dots) overlapping the optical group centre (red cross) and the X-ray group centre (black circle).
• Figure 2: Left: stellar mass distributions for the local environments. Right: halo mass distributions for X-ray+Optical groups (black) and Optical groups (red). Dashed lines in both panels are medians; the dash-dot line (right panel) is the $M_{200}$ above which the distributions are statistically consistent.
• Figure 3: Projected network of cosmic filaments (brown lines) for the GAMA G09 region. Red crosses are the GAMA optical group centres, black circles are the cross-matched eROSITA X-ray centres and gray points are SAMI galaxies.
• Figure 4: Left panel: galaxies of local environments within the $D_{\rm fil}$ versus $D_{\rm node}$ plane. Right panel: $D_{\rm node}$ distributions for galaxies in X-ray+Optical groups (black), Optical groups (red) and Field (blue). Dashed lines are medians. X-ray+Optical groups are found closest to nodes.
• Figure 5: BPT diagram to select star-forming galaxies (blue-based colours) for the three local environments (circles for Field, crosses for Optical groups and diamonds for X-ray+Optical groups). The dashed and solid curves are the star-forming limit of Kauffmann2003 and the theoretical maximal star-forming limit of Kewley2001, respectively. Green-based colours represent composite galaxies, while pink-based colours are AGN-like galaxies.