The SAMI Galaxy Survey: Quenching of Star Formation in Clusters III. Ram-Pressure-Affected Galaxy Populations

TL;DR

This study investigates ram-pressure stripping (RPS) as a key driver of star-formation quenching in cluster galaxies by exploiting spatially resolved spectroscopy from the SAMI Galaxy Survey. A visual classification of ionised gas morphologies identifies Unperturbed, Asymmetric, and Truncated systems, which are then analyzed in terms of non-parametric gas morphology, projected phase-space location, and resolved star-formation properties. The results show that Asymmetric systems reside near cluster centers with large velocity offsets and that Truncated systems represent a post-RPS phase, both displaying suppressed SF compared with unperturbed and field galaxies; the resolved star-formation main sequence is steeper for RPS-affected galaxies, with outskirts suppression indicating outside-in quenching. This work supports an evolutionary sequence from pre-RPS to ongoing stripping to post-RPS, emphasizes the importance of gas morphology in identifying RPS, and sets the stage for larger, multi-cluster analyses with next-generation surveys such as Hector.

Abstract

Cluster environments influence galaxy evolution by curtailing star formation activity, notably through ram-pressure stripping (RPS). In this study, using spatially resolved spectroscopic data from the SAMI Galaxy Survey, we identify galaxies undergoing or recently affected by RPS in eight nearby clusters ($0.029 < z < 0.058$), through a visual classification scheme based on the ionised gas ($\rm Hα+ [NII]λ6584$) morphologies, split into unperturbed, asymmetric, and truncated. The projected phase-space analysis shows that asymmetric galaxies are found in a narrow region in cluster-centric distance ($\rm 0.1 < R/R_{200} < 0.6$) and have a larger dispersion in line-of-sight velocity ($σ(|v_{pec}|)_\mathrm{Asym} = 0.71^{+0.09}_{-0.07}\ σ_{200}$) compared to the truncated and unperturbed samples. In terms of star formation activity, RPS candidates yield a much steeper resolved star-forming main sequence (rSFMS; $Σ_\mathrm{SFR} - Σ_\ast$) relation compared to the unperturbed counterparts, primarily emerging from having lower $Σ_\mathrm{SFR}$ values for the low mass density regime, with the steepest gradient deriving from the truncated sample. Moreover, radial star formation profiles reveal that star formation in RPS candidates is suppressed in the outskirts relative to unperturbed galaxies and is more prominent for the truncated sample. In contrast, central ($\rm r/r_{eff}<0.5$) star formation activity in RPS candidates is comparable with that in their unperturbed and field counterparts, suggesting no elevated activity. Taken together, this suggests an evolutionary trend linked to the RPS stage, where unperturbed galaxies likely represent recently accreted systems (pre-RPS), while asymmetric and truncated galaxies may correspond to populations undergoing RPS and post-RPS phases, respectively, favouring outside-in quenching.

Figures (15)

• Figure 1: A visualisation of the steps involved in producing the ionised gas map for an example galaxy, 9011900084. The two leftmost panels show the H$\alpha$ and [NII]$\lambda$6584 flux maps generated using the detection procedure described in Section \ref{['subsec:spectral classification maps']}, and the colour bar presents the flux values. The middle panel presents the final emission detection map, highlighting connected spaxels and coloured by the detected emission line, which is also shown in the colour bar.The two rightmost panels show the total emission map, derived from the final detection map using the same colour bar as the leftmost panels but applied to H$\alpha$ + [NII]$\lambda$6584 emission. The corresponding binary detection map is also shown, where "1" and "0" indicate spaxels with and without detected emission, respectively. The black and red contours displayed in all panels refer to the stellar continuum defined in Section \ref{['subsec:Visual Classification']}, and the SAMI field of view, respectively. The orientation is as North towards up, and East towards the left.
• Figure 2: Left panel: Stellar mass and redshift ($z_{tonry}$ for GAMA) distribution of the full SAMI sample Croom2021 with available spectroscopic classification from Owers2019 (N=2908). The cyan and red points represent the primary targets for GAMA and cluster regions, respectively, whereas the green pluses and black crosses show the secondary targets for the same samples. The black solid line shows the selection steps in stellar mass for primary GAMA targets as defined in Bryant2015. The black dashed line marks the lower stellar mass limit we adopted in this study --- $log(M_\ast/M_\odot)=10$. The blue hatched area encloses GAMA galaxies selected as a control sample. Middle panel: The distribution of detection fraction ($f_{det}$) of cluster galaxies, as defined in Section \ref{['subsec:Sample selection']}, with $N_{spaxel}(Emission) \neq 0$ (N=194). The red, blue, and green step histograms show the distributions for passive galaxies (PASGs), star-forming galaxies (SFGs), and H$\delta$-strong galaxies (HDSGs), respectively. The black vertical line marks the cut applied as $f_{det} > 0.2$. Right panel: The stellar mass distributions of the final cluster sample defined in Section \ref{['subsec:Results - Populations']} (N=81) and the selected star-forming GAMA sample (N=462, the cyan histogram). P-value from the KS test for the comparison between the final cluster and the GAMA samples is shown on the right.
• Figure 3: Examples of visual classes defined in Section \ref{['subsec:Visual Classification']}. Galaxy IDs above each panel are coloured based on their classification stated below: unperturbed - grey, truncated - magenta, asymmetric - blue, unclear - green, aperture - black. The cividis colour map shows the total (i.e. H$\alpha$+[NII]$\lambda$6584) emission, while the black and red contours represent the boundary of the stellar continuum at SNR=2, and the SAMI field of view (i.e. $\sim$15$^{\prime \prime}$ in diameter), respectively. The magenta and lime ellipses correspond to 0.5 and 1 $\rm R_e$, respectively. North is towards the up, and East is towards the left.
• Figure 4: The number of galaxies per visual class. The colours are the same as described in Section \ref{['subsec:Visual Classification']} --- unperturbed - grey, truncated - magenta, asymmetric - blue, unclear - green, and aperture - black. The numbers above each bin show the number of galaxies per class.
• Figure 5: Distribution of stellar mass and half light radius of H$\alpha$ + [NII]$\lambda$6584 flux, measured within the SAMI bundle, for the visual classes defined in Section \ref{['subsec:Visual Classification']} --- unperturbed - grey points, truncated - magenta squares, asymmetric - cyan stars, and aperture - black points. Here, we exclude "Unclear" cases. The majority of galaxies with larger $\mathrm{R_{e}(H}\alpha \rm + [NII])$ values are primarily aperture-affected galaxies, quantitatively supporting the reasoning behind them, which is the emission filling the bundle.