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ASTROARCHAEOLOGY Reconstructing Galaxy Formation Ken Freeman RSAA, ANU

ASTROARCHAEOLOGY Reconstructing Galaxy Formation Ken Freeman RSAA, ANU. August 2012. Galactic archaeology

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ASTROARCHAEOLOGY Reconstructing Galaxy Formation Ken Freeman RSAA, ANU

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  1. ASTROARCHAEOLOGY Reconstructing Galaxy Formation Ken Freeman RSAA, ANU August 2012

  2. Galactic archaeology Goal is to learn about the formation and evolution of the Galaxy, using surviving relics of star formation and accretion events. Data includes phase space variables and element abundances. Much of the dynamical information may have been lost because of heating and radial migration. Chemical coordinates are likely to be conserved. Almost all open and globular star clusters are chemically homogeneous for elements > Al dark halo • Start with some general points about the Galactic components. • Then look at some more specific GA issues stellar halo Thick disk Thin disk bulge

  3. The thick diskAlmost all spirals have thick disks: origin not understood yet. Thin and thick disk stars near the sun have different motions and [/Fe] - [Fe/H] distributions. The thick disk is old and its stars formed rapidly. thin thick thick disk thin disk Bensby 2012

  4. The Age-Metallicity relation for subgiants in the nearby thin disk (distance < 400 pc, age errors ~ 25%). Includes stars with [M/H] up to about +0.4. Current belief is that they formed in inner Galaxy and migrated radially. Radial migration is big issue in GA right now: how important is it ? The Galactic abundance gradient Measuring ages is difficult. We could use giants to study the disk at larger distances, but cannot measure isochrone ages. Asteroseismology ages for giants are not yet very accurate but will improve. Wylie de Boer, KCF 2013

  5. The bulge Bulge MDF Ness et al 2012 Boxy bulge forms from disk via bar-forming and bar buckling instabilities that occurred ~ 8 Gyr ago. The instabilities trap the stars of the inner Galaxy within the bulge. We see them now as snap-frozen relics in the bulge’s metallicity distribution function The same 3 components are seen all over the bulge but their weights change with position. C is the inner thick disk, B is the inner thin disk and A is the cold metal rich part of the thin disk which is dynamically very responsive.

  6. V U Hercules HR1614 Galactic archaeology of disk substructure. The galactic disk shows kinematical substructure, called stellarmoving groups. The stars of the moving groups are all around us. •Some are debris of star-forming aggregates in the disk (eg the HR1614 and Wolf 630 moving groups), partly dispersed into extended regions of the Galaxy: chemically homogenous, common age • Some are associated with dynamical resonances (bar) or spiral structure (eg Hercules moving group): typical sample of the disk - do not expect chemical or age homogeneity.No interest for GA •Others may be debris of infalling objects, as seen in CDM simulations. Part of our goal is to find these.

  7. • HR 1614 o field stars The HR 1614 stars are metal-rich disk stars, chemically homogeneous and have same age (2 Gyr). They are probably the dispersed relic of an old star forming event. De Silva et al 2007

  8. Although the disk does show some surviving kinematic substructure in the form of moving stellar groups, a lot of dynamical information was lost in the the subsequent heating and radial mixing by spiral arms and giant molecular clouds. HR 1614 is a rare example of a old dispersed cluster which is still identifiable both chemically and kinematically. Most dispersed aggregates would not now be recognizable dynamically However ... we are not restricted to dynamical techniques. Much fossil information is locked up in the detailed distribution of chemical elements in stars.

  9. sun Aquarius stream A recent example of chemical tagging in Galactic archaeology: Wylie de Boer et al (2012) used the chemical tagging techniques to identify the nature of the Aquarius stream (Williams et al 2011). This is a stream of halo stars identified from the RAVE survey. It is coming directly towards the sun from near l = 50o, b = -60o, and its stars extend along the line of sight from 10 kpc to 200 pc. The stream appears to be the debris of a disrupted globular cluster rather than a dwarf galaxy. Its stars • are homogeneous in heavy elements ([Fe/H] = 0.09 dex), • show the Na-O anticorrelation like GCs, and • lie with GC stars in the Ni-Na plane

  10. Field stars Globular cluster stars: M3, NGC 288, 362 dSph stars: For, Sgr, Scl, Leo I, Car [Ni/Fe] Aquarius stars [Na/Fe] Ni-Na relation for globular cluster stars and dwarf spheroidal galaxy stars. (Comes from slower star formation rate in dSph galaxies than globular clusters.) Aquarius stars are more consistent with globular cluster debris. Wylie de Boer et al (2012)

  11. Chemical Tagging (see poster #271: Wylie de Boer et al) Use the detailed chemical abundances of stars to tag or associate them to common ancient star-forming aggregates with similar abundance patterns ( Freeman & Bland-Hawthorn ARAA 2002) The detailed abundance pattern reflects the chemical evolution of the gas from which the aggregate formed. Chemical studies of the old disk stars in the Galaxy can help to identify disk stars that are the debris of common dispersed star-forming aggregates and also those which came in from outside in disrupting satellites Star clusters are chemically homogeneous in the heavier elements: De Silva et al (2009), Pancino et al (2009). Metallicity distributions within individual clusters have observed dispersions << 0.1 dex

  12. SNII +SNIa rise in s-process The detailed chemical properties of surviving satellites (the dwarf spheroidal galaxies) vary from satellite to satellite, and are different from the overall properties of the disk stars. Evolution of abundance ratios reflects different star formation histories Venn (2008) LMC Pompeia, Hill et al. 2008 Sgr Sbordone et al. 2007 FornaxLetarte PhD 2007 Sculptor Hill et al. 2008 + Geisler et al. 2005 CarinaKoch et al. 2008 + Shetrone et al. 2003 Milky-Way Venn et al. 2004

  13. We can think of a chemical space of abundances of elements Na, Mg, Al, Ca, Mn, Fe, Cu, Zr, Ba, Eu … for example (~ 25 measurable elements with HERMES). The dimensionality of this space is 8 to 9. Most disk stars inhabit a sub-region of this space. Stars from chemically homogeneous aggregates like clusters will lie in tight clumps In C-space. Stars which came in from satellites may be different enough to stand out from the rest of the disk stars in chemical space. With this chemical tagging approach, we may be able to • reconstruct old dispersed star-forming aggregates in the Galactic disk •put observational limits on the satellite accretion history of the Galaxy Chemical tagging needs a high resolution spectroscopic survey of about 106 stars, homogeneously observed and analysed….. this is a prime science driver for HERMES

  14. A major goal is to identify how important mergers and accretion events were in building up the Galactic disk and the bulge. Cold Dark Matter simulations predicts a high level of merger activity which conflicts with some observed properties of disk galaxies. Try to find the debris of groups of stars, now dispersed, that were associated at birth, either • because they were born together in a single Galactic star-forming event, or • because they came from a common accreted galaxy.

  15. HERMES isa new high-resolution fiber-fed multi-object spectrometer on the AAT spectral resolution 28,000 (also R = 48,000 mode: slit mask) 390 fibres over  square degrees 4 bands (BGRI) ~ 1000 Å First light early 2013 Main driver: the GALAH survey (chemical tagging, stellar astrophysics, Galactic structure and chemical evolution) Team of about 40, mostly from Australian institutions (Gaia-ESO survey and APOGEE survey in near-IR H-band have related goals)

  16. Galactic Archaeology with HERMES The GALAH survey (see poster #135: de Silva et al) We are planning a large stellar survey down to V = 14 (star density matches the fiber density) Cover about half the southern sky (|b| > 12o ) : 10,000 square degrees = 3000 pointings gives ~ 106 stars At V = 14, R = 28,000, expect SNR = 100 per resolution element in 60 minutes Do ~ 8 fields per night for ~ 400 clear nights (bright time program)

  17. Fractional contribution from Galactic components Dwarf Giant Thin disk 0.58 0.19 Thick disk 0.11 0.07 Halo 0.02 0.03

  18. Old disk dwarfs are seen out to distances of about 1 kpc Disk clump giants ……………………………. 5 Halo giants …………………………………… 15 About 9% of the thick disk stars and about 14% of the thin disk stars pass through our 1 kpc dwarf horizon Assume that all of their star formation aggregates are now azimuthally mixed right around the Galaxy, so all of their formation sites are represented within our horizon

  19. Simulations Bland-Hawthorn & KCF 2004, 2010) show that a random sample of 106 stars with V < 14 would allow detection of about • 20 thick disk dwarfs from each of about 3,000 star formation sites • 10 thin disk dwarfs from each of about 30,000 star formation sites * A smaller survey means less stars from a similar number of sites

  20. • Can we detect ~ 30,000 different disk sites using chemical tagging techniques ? Yes: we would need ~ 7 independent chemical element groups, each with 5 measurable abundance levels to get enough independent cells (57) in chemical space. (48 is also OK) • Are there 7 independent elements or element groups ? Yes: we can estimate the dimensionality of chemical space …

  21. The dimensionality of the HERMES chemical space The 25 HERMES elements: Li C O Na Al K Mg Si Ca Ti Sc V Cr Mn Fe Co N Cu Zn Y Zr Ba La Nd Eu The HERMES bands (BGRI) were chosen to ensure measurable lines of these elements from the major nucleosynthesis processes. Also H and H. May get a few more elements (~ 29) in some stars. The variation of these elements from star to star is highly correlated. What is the dimensionality of the C-space ?

  22. Ting, KCF, CK et al (2011) made principal component analysis (PCA) of element abundances [X/Fe] from several catalogs: • metal-poor stars (Barklem, Cayrel: 281 stars: [Fe/H] < -2) • metal-rich stars (Reddy: 357 stars: [Fe/H] > -1) • open clusters (Carrera & Pancino: 78 clusters) • Fornax dwarf galaxy (Letarte et al: 80 stars) PCA includes detailed simulation of effects of observational errors on the apparent dimensionality of the C-space, element by element. Outcome: the HERMES C-space has dimensionality = 8 to 9 for all of these samples: The principal components are vectors in C-space of element abundances [X/Fe], identifiable with nucleosynthetic processes.

  23. The principal components are vectors in C-space of element abundances [X/Fe]. The components are eigenvectors of the correlation matrix, and are all orthogonal in the 25-dimensional C-space: therefore the higher components are projections on hyperplanes normal to the more prominent components. The structure of the first principal components is clear, but it is not so obvious for the others because of the projection. The number of significant components is similar for metal-rich and metal-poor stars, but the actual components are different. The structure of the main PCA components reflects the dominant nucleosynthetic processes for each sample (see Ting et al 2011 for extensive discussion)

  24. e.g. the first two principal components for the low metallicity stars -3.5 < [Fe/H] < -2 (CEMP stars excluded) The first component includes all of the n-capture elements and the alpha elements. Probably related to core-collapse SN producing alpha elements plus the r-process contribution to n-capture elements. The second component shows anticorrelation of alpha elements with Fe-peak and n-capture elements - maybe related to “normal” core-collapse SN which don’t contribute to r-process.

  25. • The Ting (2011) principal components are based on samples of a few hundred stars. The GALAH sample of about a million stars should help to delineate the nature of the principal components more clearly. • The C-space for open clusters, which have Galactocentric radii RG = 6 to 20 kpc, has about one more dimension than the metal-rich solar neighborhood stars. De Silva et al 2009

  26. Chemical tagging in the inner Galactic disk (expect ~ 200,000 survey giants in inner region of Galaxy) The old (> 1 Gyr) surviving open clusters are mostly in the outer Galaxy, beyond a radius of 8 kpc. Expect many broken open and globular clusters in the inner disk : good for chemical tagging recovery using giants, and good for testing radial mixing theory. Open clusters are on near-circular orbits - in the absence of radial mixing, their dispersed debris would be confined to a fairly narrow annulus around the Galaxy. The radial extent of chemically tagged cluster debris can give a direct measure of how significant radial mixing is. TheNa/O anticorrelation is unique to globular clusters, and will help to identify the debris of disrupted globular clusters.

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