Extragalactic Archaeology Unlocking galaxy formation histories with SAMI Nic Scott ASA Sydney, 5th July 2016

(Extra)Galactic archaeology Goal is to understand the formation of the Galaxy by studying the collective properties of its stars But why just our galaxy? Can apply the same principle to all* other galaxies Modified approach – work with integrated spectra instead of individual stars * “All” being those bright enough to produce moderate S/N continuum spectra SDSS3

Early stellar population work Trager et al. (2000) Thomas et al. (2005) Trends of i) colour and ii) absorption line strength with galaxy properties identified early on – (roughly in the 80s when CCD spectra became common) Early population models applied to relatively small samples of early-type galaxies to derive ages and metallicities

Example data Example data from the SAMI Early Data Release (Allen et al. 2015) Upcoming release of ~800 galaxies (remember Andy’s talk!)

Why SAMI? The sample: Hexabundles: The spectra: Large sample of galaxies Broad range in morphology, mass and environment Hexabundles: Spatially resolved spectra of many galaxies Or a very large aperture to obtain high S/N spectra of low-mass galaxies The spectra: Large spectral range covering many prominent absorption features Intermediate resolution suitable for most galaxies – allows us to separate emission and absorption

Measuring stellar populations

Mass dependence All global SSP parameters measured within a 1 Re aperture Luminosity-weighted age shows complicated mass dependence Metallicity increases with mass, with a break at M* ~ 1010

Focus on metallicity Morphology Environment Metallicity doesn’t care about morphology, except for late-type spirals Metallicity is enhanced in high-density environments (environment measurements from GAMA survey via Sarah Brough)

Focus on metallicity Metallicity (at fixed mass) also depends on size More compact galaxies have higher metallicity NB This plot has been adaptively smoothed to highlight underlying trend

Focus on age Local density Host halo mass Age shows strong dependence on morphology (unsurprisingly) Age depends on environment but correlations with local density and host halo mass are different – at low galaxy mass, halo mass is more important

Focus on age Age also depends on size – more compact galaxies are older at fixed mass Flat age-mass relation at low masses is because the mass-size relation changes at low masses

Summary SAMI can measure stellar populations of 1000s of galaxies, providing the first large sample of galaxies with broad mass, morphology and environment coverage Morphology strongly influences age (at fixed mass) but metallicity and abundance pattern largely unchanged Environment has a similar effect on age – but beware the morphology- density relation! Hints that halo environment is more important than local environmental density… Single structural parameters are not great predictors of stellar populations – use the size-mass plane. Age trend becomes flatter at low mass

Questions?