BUDDI-MaNGA II: The Star-Formation Histories of Bulges and Discs of S0s

Abstract

Many processes have been proposed to explain the quenching of star formation in spiral galaxies and their transformation into S0s. These processes affect the bulge and disc in different ways, and so by isolating the bulge and disc spectra, we can look for these characteristic signatures. In this work, we used to cleanly extract the spectra of the bulges and discs of 78 S0 galaxies in the MaNGA Survey. We compared the luminosity and mass weighted stellar populations of the bulges and discs, finding that bulges are generally older and more metal rich than their discs. When considering the mass and environment of each galaxy, we found that the galaxy stellar mass plays a more significant role on the formation of the bulges. Bulges in galaxies with masses 10 10 𝑀 built up the majority of their mass rapidly early in their lifetimes, while those in lower mass galaxies formed over more extended timescales and more recently. No clear difference was found in the formation or quenching processes of the discs as a function of galaxy environment. We conclude that more massive S0 galaxies formed through an inside-out scenario, where the bulge formed first and evolved passively while the disc underwent a more extended period of star formation. In lower mass S0s, the bulges and discs either formed together from the same material, or through an outside-in scenario. Our results therefore imply multiple formation mechanisms for S0 galaxies, the pathway of which is chiefly determined by a galaxy's current stellar mass.

Comparison of the stellar populations of bulges and discs using the MaNGA survey

“…We find in general that the bulges of galaxies are slightly older than their discs for both light-weighted and mass-weighted measurements. This is consistent with the measurements made by Sánchez-Blázquez et al (2014), González Delgado et al (2015, Goddard et al (2017), Zheng et al (2017), Fraser-McKelvie et al (2018, Pak et al (2021), Johnston et al (2022). The age difference is caused by ongoing star formation in the disc after star formation has ended in the bulge.…”

“…The result for the full galaxy sample, that bulges have more metals than discs, agrees well with what is generally found in the literature (Johnston et al 2012(Johnston et al , 2014Tabor et al 2019;Moorthy & Holtzman 2006;MacArthur et al 2009;Sánchez-Blázquez et al 2011, 2014González Delgado et al 2015;Goddard et al 2017;Zheng et al 2017;Zibetti et al 2020;Barsanti et al 2021;Parikh et al 2021;Johnston et al 2022). Although most literature studies measure the gradient of metallicity with respect to radius, the sign of this gradient can broadly be interpreted in terms of comparing bulge-dominated and disc-dominated regions.…”

“…There is no trend in bulge-disc age with environment as determined by the projected distance to the 5th nearest neighbour for the sample, as was found by Johnston et al (2022). Pak et al (2021) found that lenticular galaxies have larger bulge-disc age differences in higher-density environments.…”

“…Studies finding negative metallicity gradients in galaxies include Moorthy & Holtzman (2006), MacArthur, González, & Courteau (2009), Sánchez-Blázquez et al (2011, 2014, González Delgado et al (2015), Goddard et al (2017), Zheng et al (2017), Santucci et al (2020) and Zibetti et al (2020). Studies reporting specifically that bulges are more metal-rich than discs include Johnston et al (2012), Johnston, Aragón-Salamanca, & Merrifield (2014), Ganda et al (2007), Fraser-McKelvie et al (2018), Tabor et al (2019), Barsanti et al (2021) and Johnston et al (2022). Nonetheless, the literature is not unanimous in finding more metals in the central regions of galaxies, with different results reported by Goddard et al (2017), Ferreras et al (2019), Domínguez Sánchez et al (2020) and Pak et al (2021).…”

“…Studies finding negative age gradients in galaxies include Sánchez-Blázquez et al (2014), González Delgado et al (2015), Goddard et al (2017), Zheng et al (2017) and Domínguez Sánchez et al (2020). Studies reporting specifically that bulges are older than discs include Fraser-McKelvie et al (2018), Pak et al (2021) and Johnston et al (2022). However, a substantial number of studies find that the bulge and disc have similar stellar ages (Tabor et al 2019;Ferreras et al 2019;Sánchez-Blázquez et al 2011, 2014Goddard et al 2017;Zibetti et al 2020;Barsanti et al 2021;Johnston et al 2022), while others report the inner regions of galaxies are younger than the outer regions (Johnston et al 2012(Johnston et al , 2014Goddard et al 2017;Fraser-McKelvie et al 2018;Santucci et al 2020).…”