GALACTIC ARCHAEOLOGY Reconstructing Galaxy Formation. Ken Freeman Australian National University

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1 GALACTIC ARCHAEOLOGY Reconstructing Galaxy Formation Ken Freeman Australian National University Källén Seminar, Lund, November 27, 2013

2 The abundance of chemical elements in stars Stars form from the interstellar gas. The chemical composition at the surface of a star reflects the composition of the gas from which it formed. Once a star has formed, although its interior composition evolves, its surface composition does not change significantly as the star ages (except for some of the elements lighter than Mg).

3 The light of a star contain much information about its chemical composition. From its spectrum, we can measure the abundance of 30 or more of the chemical elements at the surface of the star. The next slide shows the spectrum of the sun at resolving power λ/ λ = 80,000 or about 4 km s -1. At 4000 Å (blue), this detail has a scale of 0.05 Å

4 High resolution spectrum (R ~ 80,000) 17Å blue light window on the Sun revealing detailed chemistry of Fe, Cr, Ti, V, Co, Mg, Mn, Nd, Cu, Ce, Sc, Gd, Zr, Dy

5 Stars are mostly born in clusters of 103 to 106 stars. In almost all star clusters, the chemical abundances of its stars are observed to be identical over almost all of the elements. The gas from which clusters form is very well homogenised. The abundances are different from cluster to cluster, depending on the gas from which they formed.

6 Most open star clusters do not live very long. After about 20 Myr, they lose mass as their stars age, and the clusters disrupt and are spread around the Galaxy. But the stars remember their chemical compositions as they disperse around the Galaxy. The stars of surviving open clusters have identical abundances at the level to which they can be measured (e.g. de Silva 2009). If we can find stars in the Galaxy with identical compositions over 25 or 30 elements, then they are probably part of the debris of one common disrupted star cluster. These debris stars are the fossil remnant of the disrupted cluster.

7 [Fe/H] Te Te Fe, Ca, Ba abundances for stars in the open cluster Collinder 261 (de Silva et al 2007) rms scatter: [Fe/H] < 0.02, [Ca/H] < 0.05 [Ba/H] < 0.03 dex Te

8 Formation of the star clusters is a major part of the assembly of the Galactic disk. This is all still poorly understood - most of our knowledge about galaxy formation comes from computer simulations. We would like to find an observational way to determine the history of the assembly of the Galaxy. Continuing infall of gas and accretion of small galaxies also contributes to building up the Galaxy. Using chemical methods, we can find the fossil remains of the disrupted clusters and small galaxies which built up the Milky Way. In this way, we can determine observationally the history of its assembly. The technique is called chemical tagging.

9 Summary so far Star clusters are the main way in which stars form and build up the Galactic disk Star clusters are chemically homogeneous Most star clusters disrupt after about 107 years and their stars are spread around the Galactic disk We can identify the debris of ancient disrupted star clusters, using chemical techniques, and then measure their ages. In this way, we can determine the history of the assembly of the Galactic disk.

10 Galactic Archaeology Finding fossils of Galaxy formation using chemical techniques Joss Bland-Hawthorn and I started thinking about this in We wrote an Annual Reviews article on Galactic Archaeology in 2002 on this subject.

11 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 dynamical phase space variables (position and velocity) and element abundances of stars. Much of the dynamical information may have been lost because of heating and radial migration. The chemical properties of the stars are conserved. dark halo stellar halo Thick disk Thin disk bulge Start with some general points about the Galactic components. Then look at some more specific GA issues

12 The Galactic Thin Disk The thin disk is the major stellar component of the Milky Way. Its mass is about M The mean element abundance decreases as the radius increases, and there is a wide range of abundance at a given radius and age.

13 The Age-Metallicity relation for subgiants in the nearby thin disk (distance < 400 pc, age errors ~ 25%). Includes stars with [Fe/H] up to about Luck et al 2006 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 Wylie de Boer, KCF 2013

14 Galactic archaeology of disk substructure. The galactic disk shows kinematical substructure, called stellar moving groups. The stars of the moving groups are all around us. Some are debris of star-forming Some are associated with dynamical V aggregates in the disk (eg the HR1614 and Wolf 630 moving groups), partly dispersed into extended regions of the Galaxy: chemically homogenous, common age. resonances (bar) or spiral structure (eg Hercules moving group): typical sample of the disk - do not expect or see chemical or age homogeneity (Bensby et al 2007). U Others may be debris of infalling objects, as seen in ΛCDM simulations. Part of our goal is to find these. Hercules HR1614

15 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

16 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.

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

18 The Galactic thick disk is interesting for chemical tagging. It is old, with sub-solar [Fe/H] and α-enhanced (formed quickly). Its formation is not yet understood. Many possible ways to form thick disks. They may have formed through the dissolution of giant clumps in the early Galaxy, as seen now in forming disks at high-z...

19 The clump cluster galaxies Many high-z galaxies show massive starbursting clumps: masses up to 109 M and star formation rates of ~ 20 M yr -1. These clumps are short-lived (< 108 yr) and may disperse to form the thick disk (Bournaud et al 2009). If this is correct, the thick disk would have formed from a relatively small number of clumps. If these massive clumps were chemically homogenous, then it will be fairly easy to identify the debris of a small number of clumps from their distribution in chemical space. Genzel et al 2010

20 rapidly rotating disk & thick disk The stellar halo slowly rotating halo Zmax < 2 kpc Rotational velocity of nearby stars relative to the sun vs [m/h] (V = -232 km/s corresponds to zero angular momentum) (Carney et al 1990)

21 Widely believed now that the stellar halo ([Fe/H] < -1) comes mainly from accreted debris of small satellites: Searle & Zinn Halo-building accretions are still happening now: eg Sgr dwarf Is there a halo component that formed dissipationally early in the Galactic formation process? eg Eggen, Lynden-Bell & Sandage 1962, Samland & Gerhard 2003 ELS 1986 MDFs for dwarf spheroidal satellites are not like the metallicity distribution in the halo (Venn 08), but were maybe more similar long ago when most of the halo was accreted. Faintest satellites are more metal-poor and consistent with the MW halo in their [alpha/fe] behaviour

22 Is there a halo component that formed dissipationally early in the Galactic formation process? Hartwick (1987) : metal-poor RR Lyrae stars show a two-component halo: a flattened inner component and a spherical outer component. Carollo et al (2010 ) identified a two-component halo plus thick disk in sample of 17,000 SDSS stars, mostly with [Fe/H] < -0.5 <V> is the mean rotation, σ the velocity dispersion <V> σ [Fe/H] Thick disk Inner halo Outer halo (retrograde) From comparison with simulations, Zolotov et al (2009) argue that the inner halo has a partly dissipational origin while the outer halo is made up from debris of faint metal-poor accreted satellites.

23 E (104 km2 s -2 ) Energy vs angular momentum for a sample of stars with [Fe/H] < Stars in red have apogalactic radii > 15 kpc and have increasingly retrograde <Lz> with increasing energy Lz (kpc km s -1 ) Carollo et al 2013

24 The Bulge The boxy appearance of the bulge is typical of galactic bars seen edge-on. Where do these bar/bulges come from? They are very common. About 2/3 of spiral galaxies show some kind of central bar structure in the near infrared. M83 B K

25 The bars can come naturally from instabilities of the disk. A rotating disk is often unstable to forming a flat bar structure at its center. This flat bar in turn is often unstable to vertical buckling which generates the boxy appearance. Formation of this kind of boxy-peanut bar/bulge does not need mergers Shen, 2010: see also models by Athanassoula, Martines-Valpuesta QuickTime and a FLIC ت decompressor are needed to see this picture.

26 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. Bulge MDF Ness et al 2012 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.

27 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 SNII +SNIa Venn (2008) LMC Sgr Fornax Sculptor Pompeia, Hill et al Sbordone et al Letarte PhD 2007 Hill et al Geisler et al Carina Koch et al Shetrone et al Milky-Way Venn et al rise in s-process

28 We can think of a chemical space of abundances of elements Na, Mg, Al, Ca, Mn, Fe, Cu, Zr, Ba, Eu for example (~ 25 to 30 measurable elements: 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

29 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.

30 HERMES is a new high-resolution fiber-fed multi-object spectrometer on the 4-m AAT in Australia spectral resolution 30,000 (also R = 50,000 mode: slit mask) 390 fibres over π square degrees 4 bands (BGRI) ~ 1000 Å First light October 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)

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32 Hermes has two resolution modes: 30,000 and 50,000 The fibers are reimaged at spectrograph end R = 30,000 no mask R = 50,000 with mask psf fwhm = 4 pixels psf fwhm = 2 pixels Same spectral coverage: light loss from mask is 0.7 mag

33 Hermes first light: October 19, 2013

34 See some high-sn spectra of τ Ceti (dwarf, T = 5300, [Fe/H] = -0.5) taken during HERMES commissioning (R ~ 30,000).

35 Hβ Blue A Green A λ

36 Hα Red A atmospheric A-band (O2) O-triplet IR A

37 Element abundances The chemical pipeline matches the stellar spectra to synthetic spectra via an automated version of MOOG (Wylie, Sneden). This works over the effective temperature range K. We will observe stars outside this temperature range, but will not measure their abundances at this stage. Unusual stars (e.g. double lined binaries, chromospherically active stars) will be filtered out of the sample using spectral morphological techniques before entering the chemical pipeline (Matijevic et al 2012)

38 Galactic Archaeology with HERMES The GALAH survey 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 = 30,000, a 60 min exposure gives SNR = 100 per resolution element Do ~ 8 fields per night for ~ 400 clear nights (bright time program)

39 Galaxia survey tool (Sharma & Bland-Hawthorn) : choose fields with high enough stellar density (> 130 stars deg -2) and low reddening.

40 Distribution of stars over Te for the GALAH survey : see the turnoff stars with Te ~ 6000 K and the red clump giants with Te ~ 4800 K Sharma 2012

41 Fractional contribution from Galactic components Thin disk Thick disk Halo Dwarf Giant

42 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 the debris of their birth clusters is now azimuthally mixed right around the Galaxy, so all of their formation sites are represented within our horizon

43 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

44 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

45 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 Hα and Hβ. May get a few more elements (~ 30) in some stars. The variation of these elements from star to star is highly correlated. What is the dimensionality of the C-space? From principal component analysis (Ting et al 2012), it is 8 to 9, but the principal components (vectors in C-space) change with metallicity.

46 HERMES and GAIA GAIA is a major element of a HERMES survey GAIA (~ 2015) will provide precision astrometry for about 109 stars For V = 14, σπ = 10 µas, σµ = 10 µas yr -1 : this is GAIA at its best (1% distance errors at 1 kpc, 0.7 km s -1 velocity errors at 15 kpc) accurate transverse velocities for all stars in the HERMES sample, and accurate distances for all of the survey stars therefore accurate color-(absolute magnitude) diagram for all of the survey stars: may give independent check that chemically tagged groups have common age.

47 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. The Na/O anticorrelation is unique to globular clusters, and will help to identify the debris of disrupted globular clusters.

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