Download Unlocking galaxy formation histories with SAMI

Extragalactic Archaeology
Unlocking galaxy formation histories with SAMI
ASA Sydney,
5th
Nic Scott
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
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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
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Example data
› Example data from the SAMI Early Data Release (Allen et al. 2015)
› Upcoming release of ~800 galaxies (remember Andy’s talk!)
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Why SAMI?
› The sample:
- 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
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Measuring stellar populations
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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
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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)
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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
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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
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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
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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 morphologydensity 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
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Questions?