The Nature of the Halo of the Galaxy as Revealed by SDSS/SEGUE Timothy C. Beers Dept. of Physics & Astronomy and JINA: Joint Institute for Nuclear Astrophysics Michigan State University

2 The Sloan Digital Sky Survey The most ambitious astronomy project ever undertaken –Obtain accurately calibrated imaging of 10,000 square degrees of (northern) sky, in five filters (ugriz) –Obtain medium-resolution spectroscopy for 1,000,000 galaxies 100,000 quasars Has been fully operational since ~ Jan 1999 Completed its primary imaging mission in July 2005

3 SEGUE: The Sloan Extension for Galactic Understanding and Exploration Use existing SDSS hardware and software to obtain: –3500 square degrees of additional ugriz imaging at lower Galactic latitudes Stripes chosen to complement existing areal coverage; includes several vertical stripes through Galactic plane Medium-resolution spectroscopy of 250,000 “optimally selected” stars in the thick disk and halo of the Galaxy 200 “spectroscopic plate” pairs of 45 / 135 min exposures Objects selected to populate distances from 1 to 100 kpc along each line of site Proper motions available (from SDSS) for stars within ~ 5 kpc

4 8 kpc KV G MSTO/F BHB/BS K III SEGUE uses stellar probes of increasing absolute brightness to probe increasing distances in the disk, thick disk and Milky Way halo. d < 1 kpc d < 6 kpc d < 15 kpc d < 50 kpc d < 100 kpc Other spectroscopic surveys will not probe as deep, for instance, Blue Horizontal Branch Stars (BHBs) from a survey with V< 12 are from a volume within 1.5 kpc of the sun. r = 1.5kpc Streams and outer halo stars Inner and outer halo stars thin, thick disk stars

Completed SEGUE Survey 5

6 Overview of our Galaxy…. So far… Dark Halo Halo Thick Disk and Metal-Weak Thick Disk Thin Disk Bulge

7 Nature of the Galactic Halo(s) Conclusions First The structural components of the stellar populations in the Galaxy have been known for (at least) several decades: –Bulge / Thin Disk / Thick Disk (MWTD) / Halo New results from SDSS have now revised this list (Carollo et al. 2007, Nature, 450, 1200) : –Halo  Halos –Inner Halo: Dominant at R < kpc Highly eccentric (slightly prograde) orbits Highly eccentric (slightly prograde) orbits Metallicity peak at [Fe/H] = -1.6 Metallicity peak at [Fe/H] = -1.6 Likely associated with major/major collision of massive components early in galactic history Likely associated with major/major collision of massive components early in galactic history –Outer Halo: Dominant at R > kpc Uniform distribution of eccentricity (including highly retrograde) orbits Uniform distribution of eccentricity (including highly retrograde) orbits Metallicity peak around [Fe/H] = -2.2 Metallicity peak around [Fe/H] = -2.2 Likely associated with accretion from dwarf-like galaxies over an extended period, up to present Likely associated with accretion from dwarf-like galaxies over an extended period, up to present

Nature Article, Vol. 450, , 2007

9 A Sample of SDSS “Calibration Stars” In total, over 30,000 calibration stars, comprising two different sets: – – Spectrophotometric calibration stars: Mainly F and G turnoff stars Apparent magnitude range: 15.5 < g < 17.0 Color range: 0.6 < (u-g) < 1.2 ; 0.0 < (g-r) < 0.6 – – Telluric calibration stars: They are fainter: 17.0 < g < 18.5 Cover the same color range Spectroscopy: S/N > 30 for the first set and 20 < S/N < 30 for the second set

10 Spatial Distribution of Sample Distribution of the full sample of over 30,000 SDSS stars in the Z-R plane. The red points indicate the 20,000 stars that satisfy our criteria of a ‘local sample’, with meaningful measurements of proper motions.

11 Another View of the Local Volume d < 4 kpc 8 kpc Sun

12 Quantities Required for Analysis Astrometry Positions, proper motions Radial velocities Magnitudes and Colors Distances Stellar physical parameters Effective temperature Surface gravity Chemical composition Metallicity ([Fe/H])

13 The SEGUE Stellar Parameter Pipeline (SSPP) Estimates with different number of approaches: – – Effective Temperature (Teff) – – Surface gravity (log g) – – [Fe/H] (see Lee et al. 2007a,b) Typical internal errors are: – – σ (Teff) ~ 100 K to 125 K – – σ (logg) ~ 0.25 dex – – σ ([Fe/H]) ~ 0.20 dex External errors are of similar magnitude (Allende Prieto et al. 2007)

14 Galactic Velocity Components (UVW) Proper motions obtained from the re-calibrated USNO-B Catalog, typical accuracy 3-4 mas/yr (Munn et al. 2004) Used in combination with the measured radial velocities and estimated distances from the SSPP to derive the full space motion components (U, V, W) relative to the local standard of rest V W U

15 Derivation of Orbital Parameters We adopt an analytic Stäckel-type gravitational potential -- flattened, oblate disk and a spherical massive halo We derive: The peri-galactic distance (r peri ) closest approach of an orbit to the Galactic center The apo-galactic distance (r apo ) the farthest extent of an orbit from the Galactic center Z max the maximum distance of stellar orbits above or below the Galactic plane Orbital eccentricity

16 [Fe/H] vs. V Component

17 MDF for Retrograde Stars

18 Flattened Inside / Spherical Outside Inversion from Kinematics to Density Prediction By making simplifying assumptions about nature of galactic potential, e.g., that the Jeans theorem applies One can invert motions to recover the underlying density field – “armchair cartography” May & Binney (1986) Sommer-Larsen & Zhen (1990) Chiba & Beers (2000) Note progression from flattened to spherical with decreasing metallicity

19 [Fe/H] vs. Eccentricity / The History ELS 1962 [Fe/H] ~ 0 [Fe/H] ~ -1.5 Chiba & Beers (2000)

20 Halos Thick disk + MWTD

Decoupling the Inner/Outer Halo Carollo et al. (2008, in prep) –New (more detailed) analysis of SDSS calibration stars (through DR-7) –Around 20K unique in local sample (instead of 10K) –Obtain fractions of TD, MWTD, Inner Halo, Outer Halo as a function of |Z| and [Fe/H] –Determine velocity ellipsoids of all (recognized) components 21

The Retrograde Outer Halo 22

23 Stars with at all [Fe/H] Results of a three component fit of a thick disk, an inner and outer halo to the velocity distribution with respect to the Galactic center Note the very different behavior that results as one moves to larger and larger cuts on Z max.

24 Stars at [Fe/H] < -1.0 Results of a three component fit of a thick disk + inner and outer halo to the velocity distribution with respect to the Galactic center Note the two cuts on: Zmax < 10 kpc Zmax > 10 kpc And the very different behavior that results.

Fractions of Stars 25

Decoupling the Metal-Weak Thick Disk 26 By fixing the velocity ellipsoid of the inner halo, and restricting range on Z max, it is clear that inner halo alone cannot account for the shape of the velocity distribution, even for [Fe/H] < -1.0 We need an additional component – the Metal-Weak Thick Disk

Adding Back the Thick Disk 27

28Implications One can now target outer-halo stars in order to elucidate their chemical histories ([α/Fe], [C/Fe]), and possibly their accretion histories One can now preferentially SELECT outer-halo stars based on proper motion cuts in the local volume (SDSS-III/SEGUE-2) One can now take advantage of the lower [Fe/H], in general, of outer-halo stars to find the most metal- poor stars (all three stars with [Fe/H] < -4.5 have properties consistent with outer halo membership) One can soon constrain models for formation / evolution of the Galaxy that take all of the chemical and kinematic information into account (e.g., Tumlinson 2006)

A Metallicity Map of the Milky Way Based on the spectroscopic determinations of atmospheric parameters from the SSPP, one can calibrate a u-g vs. g-r photometric estimator of [Fe/H] For main-sequence F and G stars Accuracy on the order of 0.25 dex –Set by photometric errors of a few percent –Covers region -2.0 < [Fe/H] <

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Kinematics at the NGP 33 By choosing directions close to the NGP, the proper motions (obtained from a re-calibration of the USNO-B catalog) sample only the U and V velocity components. This enables determination of the rotational properties for Galactic components as a function of distance and metallicity This map shows results for some 60,000 stars.

The Future of Metallicity Mapping Sky-Mapper (Australia) will use a modified set of ugriz filters to obtain similar depth maps of the entire southern hemisphere. LSST will use ugriz photometry, go substantially deeper than SDSS, and obtain more accurate photometry, enabling metallicity mapping to extend out to 100 kpc, with metallicity down to at least [Fe/H] = -3, and perhaps lower; proper motions as well One can expect results for several hundred million stars 34