Age bimodality in pseudo-bulges of barred spiral galaxies: bar-driven evolution across cosmic time

Abstract

We investigate the stellar population properties of pseudo-bulges in barred galaxies drawn from the Sloan Digital Sky Survey Data Release 7 to assess how bars regulate central star formation and secular evolution. Our sample comprises barred spiral and barred lenticular (S0) galaxies with reliable spectroscopic indices obtained from multicomponent structural decompositions. Stellar ages and recent star formation are traced using the 4000Å break strength (Dn(4000)) and the Balmer absorption index (HδA), complemented by bulge, bar, and disc colours. Barred spirals show a clear bimodality in Dn(4000), with peaks at Dn(4000) ∼ 1.3 and ∼ 1.8. Low-Dn(4000) pseudo-bulges exhibit strong HδA absorption, blue colours, and high specific star-formation rates (sSFRs), indicating young, actively growing centres. High-Dn(4000) systems instead show weak HδA, red colours, and low sSFR, consistent with older, quenched pseudo-bulges. Barred S0s display an old-bulge dominated distribution, suggesting that gas-poor barred spirals transition into S0s following discwide quenching. We also find elevated active galactic nucleus incidence among old pseudo-bulges. These trends support a scenario in which bars funnel gas inward to build pseudo-bulges and later suppress central star formation by depleting or stabilizing the inflow. Integral Field Unit Spectroscopy (IFU) observations show that bars assemble cold nuclear discs that age and quench over time, while high-redshift imaging confirms that bars are already present at z ∼ 4, implying that this evolutionary cycle operates across cosmic time. The strong correspondence between stellar age, colour, and structure indicates that bar-driven secular evolution governs both the growth and quenching of central components, linking blue barred spirals to red S0 galaxies.