Disentangling the stellar population properties of bulges and disks in cluster galaxies

The formation of S0 galaxies is still highly debated. Our aim is to shed light on the role of the environment in the evolution of S0 galaxies. Since environmental mechanisms can influence the bulge and disk components of galaxies in different ways, it is crucial to study their respective stellar populations separately.

As a first step towards understanding the bulge and disk stellar populations, we use the multiband optical imaging of eight low-redshift clusters of galaxies. The deep spectroscopy from the Sydney-AAO Multi-object Integral field (SAMI) Cluster Redshift Survey allows us to analyse cluster members with M_⁎/M_☉ > 10^9.5 and into the cluster outskirts up to the cluster-centric distance ~ 2.5 R₂₀₀. We perform 2D photometric bulge-disk decomposition in the g, r and i-bands, from which we identify 469 double-component galaxies. Most bulges are found to be redder than their surrounding disks. Bulge colours do not correlate with environment metrics. Disk colours become significantly bluer at larger cluster-centric radii. The disk colour-radius relation is driven by galaxies at 0 ≤ R/R₂₀₀ < 0.5. No significant difference is found for the disk colours of backsplash and infalling galaxies. Beyond R₂₀₀, the disk colours do not change with the local galaxy density, indicating that the colours of double-component galaxies are not affected by pre-processing. A significant colour-density relation is observed for single-component disk-dominated galaxies beyond R₂₀₀. We conclude that the formation of S0 galaxies is primarily driven by cluster core processes acting on the disks.

To obtain a more detailed understanding of bulge and disk stellar populations, we disentangle the ages and metallicities. We take advantage of the spectroscopic data from the SAMI Galaxy Survey for 192 double-component cluster galaxies with M_⁎ > 10¹⁰M_☉ and R < R₂₀₀. We measure mass-weighted ages and metallicities of bulges and disks investigating three methods for separating the stellar population properties for each component: (i) one based on galaxy stellar mass weights, (ii) one based on flux weights and (iii) one based on radial separation. The three methods agree in finding that 62% of galaxies have bulges that are 2-3 times more metal-rich than their disks, while only 7% have bulges that are more metal-poor than their disks, and 31% where the disk and bulge metallicity is not significantly different. In terms of stellar population age, we find 23% of galaxies having bulges that are older than their disks, while 34% have bulges that are younger than their disks. The remaining 43% of galaxies have bulges and disks with statistically indistinguishable ages. Redder bulges tend to be more metal-rich than the disks, suggesting that the redder color in bulges is due to their enhanced metallicity relative to the disks instead of differences in age.

To understand possible differences between S0 galaxies in clusters and in lower galaxy density environments, we extend the bulge and disk analyses to the SAMI double-component galaxies in the field and in groups. No significant differences are found for the colour, age and metallicity median values of the bulges and the disks in different environments. The finding of bulges being younger and more metal-rich than the disks also in the field and groups suggests that pre-processing or galaxy internal mechanisms act leading to recent star formation events in bulges.

In conclusion, our results indicate that the evolution of S0 cluster galaxies is driven by star formation quenching in the disks mainly caused by cluster core processes, while pre-processing plays a marginal role. Mechanisms leading to bulges being younger and more metal-rich than the disks act in clusters, groups and in the field. The redder colour observed in bulges with respect to the disks is driven by their enhanced metallicity.