The Nature of the Halo of the The Nature of the Halo of the Galaxy as Revealed by Galaxy as Revealed by

SDSS/SEGUE SDSS/SEGUE

Timothy C. BeersTimothy C. Beers

Dept. of Physics & Astronomy andDept. of Physics & Astronomy andJINA: Joint Institute for Nuclear JINA: Joint Institute for Nuclear

AstrophysicsAstrophysicsMichigan State UniversityMichigan State University

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The Sloan Digital Sky SurveyThe Sloan Digital Sky SurveyThe most ambitious astronomy project ever undertakenThe most ambitious astronomy project ever undertaken

– Obtain accurately calibrated imaging of Obtain accurately calibrated imaging of 10,000 10,000 square degrees of (northern) sky, square degrees of (northern) sky, in five filters (in five filters (ugrizugriz))

– Obtain medium-resolution spectroscopy forObtain medium-resolution spectroscopy for

1,000,000 galaxies1,000,000 galaxies

100,000 quasars100,000 quasars

Has been fully operational since ~ Jan 1999Has been fully operational since ~ Jan 1999Completed its primary imaging mission in July 2005Completed its primary imaging mission in July 2005

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SEGUESEGUE: : TheThe SSloanloan EExtensionxtension forfor GGalacticalactic UUnderstandingnderstanding andand EExplorationxploration

Use existing SDSS hardware and software to obtain:Use existing SDSS hardware and software to obtain:

– 3500 square degrees3500 square degrees of additional of additional ugrizugriz imaging at lower imaging at lower Galactic latitudesGalactic latitudes

Stripes chosen to complement existing areal coverage; Stripes chosen to complement existing areal coverage; includes several vertical stripes through Galactic planeincludes several vertical stripes through Galactic plane

Medium-resolution spectroscopy of Medium-resolution spectroscopy of 250,000 “optimally 250,000 “optimally selected”selected” stars in the thick disk and halo of the Galaxy stars in the thick disk and halo of the Galaxy

200 “spectroscopic plate” pairs of 45 / 135 min exposures200 “spectroscopic plate” pairs of 45 / 135 min exposures

Objects selected to populate distances from 1 to 100 kpc Objects selected to populate distances from 1 to 100 kpc along each line of sitealong each line of site

Proper motions available (from SDSS) for stars within ~ 5 kpcProper motions available (from SDSS) for stars within ~ 5 kpc

448 kpc

KV

G

MSTO/F

BHB/BS

K IIISEGUE uses stellar probes of increasingabsolute brightness to probeincreasing 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, thickdisk stars

Completed SEGUE SurveyCompleted SEGUE Survey

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Overview of our Galaxy…. So far…Overview of our Galaxy…. So far…Dark Halo

Halo

Thick Disk and Metal-Weak Thick Disk

Thin Disk

Bulge

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Nature of the Galactic Halo(s) Nature of the Galactic Halo(s) Conclusions FirstConclusions First

The structural components of the stellar populations in the Galaxy The structural components of the stellar populations in the Galaxy have been known for (at least) several decades:have been known for (at least) several decades:

– Bulge / Thin Disk / Thick Disk (MWTD) / HaloBulge / Thin Disk / Thick Disk (MWTD) / Halo

New results from SDSS have now revised this list New results from SDSS have now revised this list (Carollo et al. 2007, Nature, 450, 1200) :(Carollo et al. 2007, Nature, 450, 1200) :

– Halo Halo Halos Halos

– Inner Halo:Inner Halo: Dominant at R < 10-15 kpcDominant at R < 10-15 kpc Highly eccentric (slightly prograde) orbitsHighly eccentric (slightly prograde) orbits Metallicity peak at [Fe/H] = -1.6Metallicity peak at [Fe/H] = -1.6 Likely associated with major/major collision Likely associated with major/major collision

of massive components early in galactic history of massive components early in galactic history

– Outer Halo:Outer Halo: Dominant at R > 15-20 kpcDominant at R > 15-20 kpc Uniform distribution of eccentricity Uniform distribution of eccentricity

(including highly retrograde) orbits (including highly retrograde) orbits Metallicity peak around [Fe/H] = -2.2Metallicity peak around [Fe/H] = -2.2 Likely associated with accretion from dwarf-like Likely associated with accretion from dwarf-like

galaxies over an extended period, up to galaxies over an extended period, up to presentpresent

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A Sample of SDSSA Sample of SDSS “Calibration Stars”“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

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Spatial Distribution of SampleSpatial 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.

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Another View of the Local VolumeAnother View of the Local Volume

d < 4 kpc

8 kpc

Sun

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Quantities Required for AnalysisQuantities 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])

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The SEGUE Stellar Parameter The SEGUE Stellar Parameter PipelinePipeline (SSPP)(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)

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Galactic Velocity Components Galactic Velocity Components (UVW)(UVW)

Proper motions obtained from the re-calibrated USNO-B Proper motions obtained from the re-calibrated USNO-B Catalog, typical accuracy Catalog, typical accuracy 3-4 mas/yr3-4 mas/yr (Munn et al. 2004) (Munn et al. 2004)

Used in combination with the measured radial velocities and Used in combination with the measured radial velocities and estimated distances from the estimated distances from the SSPP SSPP to derive the full space to derive the full space motion components (U, V, W) relative to the local standard of motion components (U, V, W) relative to the local standard of restrest

V

W

U

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Derivation of Orbital ParametersDerivation 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 (rperi) closest approach of an orbit to the Galactic center

The apo-galactic distance (rapo) the farthest extent of an orbit from the Galactic center

Zmax the maximum distance of stellar orbits above or below the Galactic plane

Orbital eccentricity

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[Fe/H] vs. V Component[Fe/H] vs. V Component

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MDF for Retrograde StarsMDF for Retrograde Stars

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Flattened Inside / Spherical Outside Flattened Inside / Spherical Outside Inversion from Kinematics to Density PredictionInversion from Kinematics to Density Prediction

By making simplifying assumptions By making simplifying assumptions about nature of galactic potential, about nature of galactic potential, e.g., that the Jeans theorem appliese.g., that the Jeans theorem applies

One can invert motions to recover One can invert motions to recover the underlying density field – the underlying density field – “armchair cartography”“armchair cartography”

May & Binney (1986) May & Binney (1986) Sommer-Larsen & Zhen (1990)Sommer-Larsen & Zhen (1990)Chiba & Beers (2000)Chiba & Beers (2000)

Note progression from flattened to Note progression from flattened to spherical with decreasing metallicityspherical with decreasing metallicity

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[Fe/H] vs. Eccentricity / The History[Fe/H] vs. Eccentricity / The History

ELS 1962

[Fe/H] ~ 0

[Fe/H] ~ -1.5

Chiba & Beers (2000)

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Halos

Thick disk + MWTD

Decoupling the Inner/Outer HaloDecoupling the Inner/Outer Halo

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The Retrograde Outer HaloThe Retrograde Outer Halo

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Stars with at all [Fe/H]

Results of a three componentfit 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 Zmax.

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Stars at [Fe/H] < -1.0

Results of a three componentfit 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 StarsFractions of Stars

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Decoupling the Metal-Weak Thick DiskDecoupling the Metal-Weak Thick Disk

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By fixing the velocity ellipsoidof the inner halo, and restricting range on Zmax,it is clear that inner haloalone cannot account forthe 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 DiskAdding Back the Thick Disk

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ImplicationsImplicationsOne can nowOne can now target target outer-haloouter-halo stars in order to stars in order to elucidate their chemical histories elucidate their chemical histories ([([αα/Fe], [C/Fe]),/Fe], [C/Fe]), and and possibly their accretion historiespossibly their accretion histories

One can nowOne can now preferentially preferentially SELECT outer-haloSELECT outer-halo stars based on proper motion cuts in the local volume stars based on proper motion cuts in the local volume (SDSS-III/SEGUE-2)(SDSS-III/SEGUE-2)

One can nowOne can now take advantage of the lower [Fe/H], in take advantage of the lower [Fe/H], in general, of outer-halo stars to find the most metal-general, of outer-halo stars to find the most metal-poor stars poor stars (all three stars with [Fe/H] < -4.5 have (all three stars with [Fe/H] < -4.5 have properties consistent with outer halo properties consistent with outer halo membership)membership)

One can soonOne can soon constrain models for formation / constrain models for formation / evolution of the Galaxy that take evolution of the Galaxy that take all of the chemical all of the chemical and kinematic informationand kinematic information into account (e.g., into account (e.g., Tumlinson 2006) Tumlinson 2006)

A Metallicity Map of the Milky Way A Metallicity Map of the Milky Way

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Kinematics at the NGPKinematics at the NGP

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By choosing directionsclose to the NGP, the proper motions (obtainedfrom a re-calibration of the USNO-B catalog)sample only the U and Vvelocity components.

This enables determinationof the rotational propertiesfor Galactic componentsas a function of distance and metallicity

This map shows results forsome 60,000 stars.

The Future of Metallicity MappingThe Future of Metallicity Mapping

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