The Nature of the Halo of the Galaxy as Revealed by SDSS/SEGUE

1
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
SEGUE uses stellar probes of increasing absolute
brightness to probe increasing distances in the
disk, thick disk and Milky Way halo.
K III
d lt 100 kpc
BHB/BS
d lt 50 kpc
Streams and outer halo stars
MSTO/F
d lt 15 kpc
G
thin, thick disk stars
d lt 6 kpc
Inner and outer halo stars
KV
d lt 1 kpc
r 1.5kpc
Other spectroscopic surveys will not probe as
deep, for instance, Blue Horizontal Branch Stars
(BHBs) from a survey with Vlt 12 are from a
volume within 1.5 kpc of the sun.
8 kpc
5
Completed SEGUE Survey
6
Overview of our Galaxy. So far
Dark Halo
Halo
Thin Disk
Bulge
Thick Disk and Metal-Weak Thick Disk
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 lt 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 gt 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

8
Nature Article, Vol. 450, 1020-1025, 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 lt g lt 17.0
• Color range 0.6 lt (u-g) lt 1.2 0.0 lt (g-r) lt
  0.6
• Telluric calibration stars
• They are fainter 17.0 lt g lt 18.5
• Cover the same color range
• Spectroscopy S/N gt 30 for the first set and 20
  lt S/N lt 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
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
• s (Teff) 100 K to 125 K
• s (logg) 0.25 dex
• s (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

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 (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

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 -1.5
Fe/H 0
20
(No Transcript)
21
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

22
The Retrograde Outer Halo
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 Zmax.
24
Stars at Fe/H lt -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
lt 10 kpc Zmax gt 10 kpc And the very different
behavior that results.
25
Fractions of Stars
26
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 lt -1.0 We need an additional component
the Metal-Weak Thick Disk
27
Adding Back the Thick Disk
28
Implications

• One can now target outer-halo stars in order to
  elucidate their chemical histories (a/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 lt
  -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)

29
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 lt Fe/H lt 0.0

30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
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.
34
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