SDSS RR Lyrae

Željko Ivezić

Robert Lupton

David Schlegel

Vernesa Smolčić

David Johnston

Jim Gunn

Jill Knapp

Michael Strauss

and hundreds more· · ·

University of Chicago,Fermilab,

Institute for Advanced Study,Japanese Participation Group,

Johns Hopkins University,Los Alamos National Laboratory,Max-Planck Institute (MPIA),Max-Planck Institute (MPA),New Mexico State University

University of PittsburghPrinceton University,

United States Naval Observatory,University of Washington,

University of Zagreb

Satellites and Tidal Tails, May 26-30, 2003, La Palma

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Summary

• There is mounting evidence for the existence of a tidal streamin the halo that is associated with the Sgr Dwarf galaxy

• The spatial and velocity distributions of ∼3,000 candidate RRLyrae stars discovered by SDSS provide strong confirmation for

the reality of this feature

• SDSS data provide significant new constraints for the modelsof Sgr Dwarf tidal stream, as well as for the overall halo structure

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The Comparison of Models for

Sgr Dwarf Tidal Stream; top:

Helmi & White (2000), mid-

dle: Ibata et al. (2001), bot-

tom: Delgado-Martinez et al.

(2003)

The green dot: the Sun

Models differ and can be con-

trained using observations of

RR Lyrae stars (which are lu-

minous standard candles ob-

servable to D∼100 kpc)

How to find RR Lyrae stars us-

ing SDSS data?

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data completeness

360 330 300 270 240 210 180 150 120 90 60 30 0

SDSS RR Lyrae candidates from Ivezic et al. (2000)

• First ∼150 SDSS RRLyrae from commissioning

data: Sgr Dwarf “northern

clump”

• Detection method: twoepochs and accurate colors

• confirmed by QUEST

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data completeness

360 330 300 270 240 210 180 150 120 90 60 30 0

SDSS RR Lyrae candidates from Ivezic et al. (2000)

360 330 300 270 240 210 180 150 120 90 60 30 0

1092 SDSS RR Lyrae candidates with |Dec|

0 1 2 321

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0 1 2 321

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QUEST RR Lyrae in SDSS

0 1 2 3 4-0.5

0

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0 1 2 3 4-0.5

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1.5

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0 1 2-0.5

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1.5

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0 1 2-0.5

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RR Lyrae stars have very specific colors!

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• All stars:1 in ∼ 104 is RR Lyraecompleteness is 100%

• Blue box:1 in ∼20 is RR Lyraecompleteness is 100%

• Red corner:1 in ∼2 is RR Lyraecompleteness is 25%

• Magenta box:Ivezic et al. (2000)

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Variability Studies with SDSS

• Imaging vs. Imaging Data

– Southern Survey: ∼ 200 deg2, up to ∼ 12 epochs

– Converging Strips: ∼ 30% of the area

– Strip Overlaps: ∼ 10% of the area

– Commissioning Data: ∼ 100 deg2, 2 epochs, 2 days apart

• Imaging vs. Spectrophotometric Data

• Imaging vs. Other Data (e.g. POSS)

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How good is SDSS photometry? Not only that true errors aresmall (0.02 mag), but the error estimates are very accurate!

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Variability-Based Selection

Criteria for SDSS RR Lyrae:

• Colors have to be consis-tent with “RR Lyrae” box

• Repeated ObservationsMust Indicate Variability:

1) Processing flags

!SATUR, !BLENDED,

!MOVING

2) r < 20.7 and σr < 0.3

3) Variation in g and r:

(∆g)2+(∆r)2 > 0.152,

at least 5σ significant

4) Correlated variation in

g and r:

∆g ∼ 1.4∆r

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• Blue dots: 1092 variability-selected RR Lyrae candidates

with |Dec| < 1.25

• Contours: Ibata et al. (2001)model

• Black dots: all stars withRR Lyrae colors and

multiple observations

• Green dots:variability-selected

RR Lyrae candidates

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CDM predictions by Bullock,

Kravtsov & Weinberg (2001)Observed Distribution

of SDSS RR Lyrae

Models predicts more clustering than seen in the data (outside

the Sgr Dwarf tidal stream): e.g. observe the 50–75 kpc ring.

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Sgr Dwarf clump is coherent

in velocity space!

Do the models correctly predict

the velocity distribution in the

tidal stream?

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300 240 180 120 60 0 -60

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Contours: Ibata et al. (2001)

Green dashed line: radial velocity

300 240 180 120 60 0 -60

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-200

-150

-100

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150

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Helmi & White (2000)

for non-rotating halo

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data completeness

-250 -200 -150 -100 -50 0 50 100 150 200 25021

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range of Sun’s reflex motion

Contours: Ibata et al. (2001)

Such m vs. vrad diagrams in small

excellent method for comparing

data completeness

-250 -200 -150 -100 -50 0 50 100 150 200 25021

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range of Sun’s reflex motion

sky patches provide an

data and model predictions

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data completeness

-250 -200 -150 -100 -50 0 50 100 150 200 25021

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range of Sun’s reflex motion

Contours: Ibata et al. (2001)

data completeness

-250 -200 -150 -100 -50 0 50 100 150 200 25021

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range of Sun’s reflex motion

Helmi & White (2000)

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Conclusions• SDSS is producing a large

and robust sample of distant

RR Lyrae stars

• Even with the increase inthe sample size by a factor of

20, the Sgr Dwarf tidal stream

remains the most pronounced

feature

• Areas not associated withthe Sgr Dwarf tidal stream

seem to show less structure

than predicted by the CDM

models

• The data is in qualitativeagreement with models for Sgr

Dwarf tidal stream, but there is

room for quantitative improve-

ment

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