Investigating Galaxy Quenching with the SDSS: Stellar Metallicities as a Tracer of Quenching Mechanisms J. Trussler1,2,*, R. Maiolino1,2, D. Goddard3, C. Maraston3, D. Thomas3, Y. Peng4 1Cavendish Laboratory, University of Cambridge, 2Kavli Institute for Cosmology, University of Cambridge 3University of Portsmouth, 4Peking University *jaat2@cam.ac.uk Local galaxies can be split up into two main classes: gas-rich star-forming galaxies and gas-poor passive galaxies. However, it is still unclear which mechanisms are primarily responsible for quenching star formation in galaxies. Peng et al. [1] analysed stellar metallicities of 26,000 SDSS galaxies and found that galaxies with stellar masses below 1011 M are primarily quenched due to the halting of the supply of cold gas (strangulation). We take this analysis further by making use of the much larger spectroscopic sample in SDSS DR7, analysing the stellar metallicities of 70,000 local galaxies. We study the systematic difference in stellar metallicity between star-forming and passive galaxies (see Fig. 1) to put constraints on the possible quenching mechanisms and timescales. We analyse the light-weighted stellar metallicities of 70,000 SDSS galaxies, obtained using the spectral fitting code FIREFLY [2]. The bimodality in the star formation rate – stellar mass plane is used to divide galaxies into star-forming, green valley and passive. We compare the observed metallicity differences between star-forming and passive galaxies with the predictions of simple closed-box models, to determine whether local galaxies quench primarily through strangulation. Galaxy group catalogues [3] are used to further split the galaxy population into centrals and satellites to investigate the dependence of galaxy quenching on environment. The observed average stellar metallicity difference between star-forming and quenched galaxies is well matched by the predictions of a closed-box model for galaxy evolution (Fig. 2), indicating that local galaxies quench primarily through strangulation. We find that the typical quenching timescale is 2 Gyr and is roughly independent of stellar mass. The sample is further divided into central and satellite galaxies and again we determine the average stellar metallicity difference between star-forming and passive galaxies. We find that the metallicity differences are similar for both central and satellite galaxies, except at intermediate stellar masses, where the stellar metallicity enhancement of centrals is slightly larger (~0.1 dex) than that of satellites. Figure 1. The stellar mass-stellar metallicity relation for star-forming galaxies (blue curve), green valley galaxies (green curve) and passive galaxies (red curve) with 0.02 < z < 0.085. Galaxies are binned in 0.1 dex stellar mass bins and the median stellar metallicity in each bin is plotted. Error bars correspond to the 1σ error on the mean value. Figure 2. The observed average stellar metallicity difference between star-forming and passive galaxies is shown by the black curve. Error bars represent the 1σ error on the metallicity difference. The coloured lines show the stellar metallicity difference predicted by a simple closed-box model at different times after strangulation. We find that the typical quenching timescale is 2 Gyr. Figure 3. The blue curve shows the average stellar metallicity difference between star-forming central and passive central galaxies. The red curve shows the average stellar metallicity difference between star- forming satellite and passive satellite galaxies. We also investigate the dependence of the strangulation signal on the local overdensity (overdensity data taken from [4]). Fig. 4 shows that the metallicity difference for central galaxies depends very weakly on the local overdensity, even over an overdensity range of 2 dex. Satellite galaxies also show a weak dependence on overdensity, except in the smallest overdensity bin, where an excess metallicity difference is observed at