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

2. (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

3. 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

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

5. 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

6. Measuring stellar populations

7. 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

8. 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)

9. 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

10. 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

11. 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

12. 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

13. Questions?