Transcript Mining the Milky Way Galaxy with SDSS

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
– 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
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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
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SEGUE uses stellar probes of increasing
absolute brightness to probe
increasing distances in the disk, thick
BHB/BS
disk and Milky Way halo.
K III
d < 100 kpc
d < 50 kpc
MSTO/F
Streams and outer halo stars
d < 15 kpc
thin, thick
disk stars
G
d < 6 kpc
KV
Inner and outer halo stars
d < 1 kpc
r = 1.5kpc
8 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 4sun.
Completed SEGUE Survey
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Overview of our Galaxy…. So far…
Dark Halo
Halo
Thin Disk
Bulge
Thick Disk and
Metal-Weak Thick Disk
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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
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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
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for the second set
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
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sample’, with meaningful measurements of proper motions.
Another View of the Local Volume
Sun
d < 4 kpc
8 kpc
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Quantities Required for Analysis
Astrometry
Positions, proper motions
Radial velocities
Magnitudes and Colors
Stellar physical parameters
Chemical composition
Distances
Effective temperature
Surface gravity
Metallicity ([Fe/H])
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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)
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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
W
U
V
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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
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[Fe/H] vs. V Component
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MDF for Retrograde Stars
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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
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[Fe/H] vs. Eccentricity / The History
ELS 1962
[Fe/H] ~ -1.5
~0
Chiba & [Fe/H]
Beers (2000)
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Halos
Thick disk + MWTD
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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
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The Retrograde Outer Halo
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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.
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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.
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Fractions of Stars
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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
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Adding Back the Thick Disk
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Implications
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 metalpoor 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)
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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] < 0.0
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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.
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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
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