Science Results Enabled by SDSS AstrometricObservations

Zeljko Ivezic1, Mario Juric2, Nick Bond2,Jeff Munn3, Robert Lupton2, et al.

1 University of Washington2Princeton University

3 USNO Flagstaff

Astrometry in the Age of the Next Generation of Large TelescopesFlagstaff, Oct 17-20, 2004

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SDSS Astrometric Data Quality

1. Pipeline astrom developed by USNO (Pier et al. 2003)

2. Dynamic range: 14 < V < 22.5, exposure 54 sec, 5 bands

(ugriz) over 5 minutes

3. Absolute accuracy: < 50 mas

4. Relative band-to-band accuracy: ∼30 mas for sources not

limited by photon statistics, and ∼100 mas at the survey

limit

5. SDSS DR3: 141 million unique objects

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Science Results Based on SDSS AstrometricData

1. Solar System Objects: move during 5 minutes

2. Stellar Proper Motions:

• SDSS-POSS: 50 yrs baseline, g < 20, proper motion errors∼3 mas/yr

• SDSS-SDSS: 5 yrs baseline, g < 22, proper motion errors∼6 mas/yr

3. Stellar Parallaxes: ∼100 deg2, out to ∼10 pc, advantage offaint flux limit

4. Optical identifications for sources detected at other wave-lengths (FIRST, Chandra, 2MASS)

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SDSS Asteroid Observations

Moving objects in Solar System can be efficiently detected out to

∼ 20 AU even in a single scan: 5 minutes between the exposures

in the r and g bands

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Asteroids move during 5 minutes and thus appear to have pecu-

liar colors.

The images map the i-r-g filters to RGB. The data is taken in

the order riuzg, i.e. GR··B

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SDSS Asteroid Observations

• Moving objects must be efficiently found to prevent the con-

tamination of quasar candidates (and other objects with non-

stellar colors)

• Detected as moving objects with a baseline of only 5 minutes

• The sample completeness is 90%, with a contamination of 3%,

to a magnitudes fainter completeness limit than available before

• The velocity errors 2-10%, sufficient for recovery within a few

weeks

• Accurate (∼0.02 mag) 5-band photometry

• SDSS Moving Object Catalog is public at www.sdss.org

Detected 204,305 moving objects, 67,637 are identified with

known objects in Bowell’s catalog, 43,329 are unique

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Asteroid Counts

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Main SDSS Asteroid Results

• The size distribution for main-belt asteroids: measured

to a significantly smaller size limit (< 1 km) than possible

before, discovery of a change of slope at D ∼ 5 km, a smaller

number of asteroids compared to previous work by a factor

of ∼2 (N(D>1km) ∼0.75 million)

• Strong correlation between colors and position/dynamics:

Confirmation of color gradient: rocky S-type in the inner belt

vs. carbonaceous C type asteroids in the outer belt; dynam-

ical families have distinctive colors;

• Colors are correlated with the family age: space weath-

ering

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SMALL SIZEBUMP

D (km)

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COMPARISON OF ASTEROID SIZE DISTR� IBUTION: OBSERVATIONS AND MODELS

The asteroid size distribution (Davis 2002, in Asteroids III).

SDSS results:1) Extended the observed range to ∼300m2) Detected the second break at ∼5 km

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The semi-major axis v. (proper) inclination for known asteroids

from Bowell’s catalog that were observed by SDSS

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The semi-major axis v. (proper) inclination for known asteroids

color-coded using measured SDSS colors

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The osculating inclination vs. semi-major axis diagram.

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What is the meaning of different color shades?

• Chemistry, of course, for the gross differences (red vs. blue),

but what about different shades of red?

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Age (Years)

Col

our

Iannini

Karin

Brangane

Agnia

Massalia

Merxia

Gefion

Rafita

Eos

Koronis

Eunomia

Maria

Sol

ar S

yste

m

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

10 10 10 107 8 9 10

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What is the meaning of different color shades?

• Chemistry, of course, for the gross differences (red vs. blue)

• Within a given chemical class, colors also depend on age:

SDSS colors can be used to date asteroids

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Prospects for Proper Motion Studies

• SDSS-POSS proper motions limited by the POSS astromet-ric accuracy (0.15 arcsec) resulting in proper motion accu-racy of ∼3 mas/yr; usable to g ∼ 20 (recalibrated POSSastrometry by Munn et al.)

• SDSS-SDSS proper motions with 5 years baseline accurateto ∼6 mas/yr (using only 2 epochs); usable to g ∼ 22

• SDSS-LSST proper motions will be limited by the SDSSastrometric accuracy (∼30 mas): with 15 years baselineaccurate to ∼2 mas/yr This is >100 times more sensitivethan Luyten’s catalog (a standard resource for proper mo-tion studies)!

SDSS (and especially LSST) may revolutionize proper motionbased studies of the Galactic structure (2 mas/yr correspondsto 10 km/s at 1 kpc)!

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Photometrically and Astrometrically Variable Ob-

jects

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Tangential VelocityDistributions for M dwarfs

(D<1 kpc)• Top row: l=0, b∼45, vl and vb

for D=300 pc

• Middle row: l=90, b∼45, vl for

D=300 pc and D=800 pc

• Bottom row: l=180, D=300 pc,

vl for b∼45 and b∼-45

• Note strong non-Gaussianity:

asymmetric drift

• The main advantage of SDSS-

POSS sample: probes larger dis-

tances than possible before, ac-

curate distance estimates, large

number of sources: enormous

amount of detailed information!

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Thick Disk vs. Halo VelocityDistributions

• Top: vb, bottom: vl

• Turn-off stars selected in r vs.

g − r color diagram: black

• Further separated by u − g color

(metallicity proxy) into “halo”

(blue) and “thick disk” (red)

stars

• Note the strong lag

• Note strong non-Gaussianity:

asymmetric drift

• The main advantage of SDSS-

POSS sample: probes larger dis-

tances than possible before, ac-

curate distance estimates, large

number of sources: enormous

amount of detailed information!

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Conclusions

• It’s good to have accurate astrometry for a lot of faint

sources across a large chunk of the sky.

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Conclusions

• It’s good to have accurate astrometry for a lot of faint

sources across a large chunk of the sky.

• Especially when accurate multi-band photometry is also avail-

able.

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Conclusions

• It’s good to have accurate astrometry for a lot of faint

sources across a large chunk of the sky.

• Especially when accurate multi-band photometry is also avail-

able.

• And radial velocities, and variability information, and . . .

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