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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. The Sloan Digital Sky Survey. The most ambitious astronomy project ever undertaken

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the nature of the halo of the galaxy as revealed by sdss segue

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

the sloan digital sky survey
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
segue the s loan e xtension for g alactic u nderstanding and e xploration
SEGUE: TheSloanExtensionforGalacticUnderstandingandExploration
  • 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

SEGUE uses stellar probes of increasing

absolute brightness to probe

increasing distances in the disk, thick

disk and Milky Way halo.


d < 100 kpc


d < 50 kpc

Streams and outer halo stars


d < 15 kpc


thin, thick

disk stars

d < 6 kpc

Inner and outer halo stars


d < 1 kpc

r = 1.5kpc

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.

8 kpc


Overview of our Galaxy…. So far…

Dark Halo


Thin Disk


Thick Disk and

Metal-Weak Thick Disk

nature of the galactic halo s conclusions first
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 < 10-15 kpc

Highly eccentric (slightly prograde) orbits

Metallicity peak at [Fe/H] = -1.6

Likely associated with major/major collision of massive components early in galactic history

    • Outer Halo:Dominant at R > 15-20 kpc

Uniform distribution of eccentricity (including highly retrograde) orbits

Metallicity peak around [Fe/H] = -2.2

Likely associated with accretion from dwarf-like galaxies over an extended period, up to present

a sample of sdss calibration stars
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

spatial distribution of sample
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.

quantities required for analysis
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])
the segue stellar parameter pipeline sspp
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)
galactic velocity components uvw




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
derivation of orbital parameters
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 (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
flattened inside spherical outside inversion from kinematics to density prediction
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
decoupling the inner outer halo
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

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 Zmax.


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.

decoupling the metal weak thick disk
Decoupling the Metal-Weak Thick Disk

By fixing the velocity ellipsoid

of the inner halo, and

restricting range on Zmax,

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

  • 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
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-rphotometric 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] < 0.0
kinematics at the ngp
Kinematics at the NGP

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
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