1. Sagittarius debris in SDSS stripe 82 Zhu Ling ( 朱玲 ) & Martin. C. Smith Center for Astrophysics, Tsinghua university KIAA at Peking University

2. Motivation Sagittarius dwarf, being accreted by the Milky Way, forms lengthy tidal streams wrap entirely around the Milky way. It can provide important constrain to the shape, orientation, and mass of the Milky way dark matter halo. Velocity dispersion of the streams is an Important constraint. Relatively large sample of sag debris in a small region selected from SDSS stripe 82. What constrains can this sample provide for the simulation. Majewski et al 2003 Niederste-Ostholt et al 2008

3. Star overdensity traces the sag streams 76 82 86 79 Star overdensity traces the sag streams. The stream passes through S82, S86, but not S76, maybe S79 Over 25,000 stars in stripe 82 have spectra, also about 10,000 in 79 and 86 in SDSS dr7. Overdensity of BHBs, BSs, K/M giants and MSTOs may trace sag streams (yanny et al 2000, yanny et al 2009). Belokurov et al 2006

4. Select sample from S82 Aim: A BHB+BS sample as clean as possible. Selection Steps: 1) Color selection (A-color stars=BHB + BS + A type MS + BMP) 2) Cut in RA 3) Cut in g0 and surface gravity. (distance) BHB: Blue Horizontal Branch stars BS: Blue Stragglers BMP: Blue metal poor stars MS: main sequence stars

5. 1) color selection - 0.3 < g-r < 0 & 0.8 < u-g < 1.5 (yanny et al 2001) g-r A-color stars = BHBs + BSs + *** u-g

6. 2) cut in RA Lots of smooth halo stars exist. their velocity distribution centered at 0. Sag stream only passes a small RA region. Sag debris have a systematic velocity offset. RA radial velocity

7. 3) Cut in distance BHB: log(g) < 3.8, 17.2 < g < 18.7 typical absolute mag g0=0.7 (with dispersion ~ 0.2 mag) BS: log(g)> 3.8, 18.3 < g < 19.2 typical absolute mag g0 2.7 (with dispersion ~ 1 mag) faint: with no log(g) measurement, g > 18.7. g=17.2 d=20kpc g=18.7 d=40kpc Surface gravity log(g) g=18.3 d~ 10 kpc apparant magnitude g RA radial velocity 15<RA<50: 416 288 have log(g)

8. Velocity distribution BHB: 76 BS: 91 faint: 123 We can not kick out the smooth halo stars which exist at the same RA & distance. Another structure exists: a more compact, distant stream (Yanny et al. 2009, Newberg et al. 2009) To the sag stream, a velocity gradient exist along RA, not along distance. mean= -137 km/s. sigma=14.7 (19.3) km/s large error bar avg :13 km/s BSs and faint stars have been shifted 2 mag left, so magnitude represents distance. BHB+BS+fiant Radial velocity g

9. BHBs VS BSs BS: 91 sigma: 15 km/s (18.7) BHB: 76 sigma: 9.5 km/s (11.4) BHB: 76 BS: 91

10. surface temperaturemetallicity BHB BS BS: colder, higher metallicity BHB: hotter, lower metallicity BHBS VS BSs

11. Compare with previous result Manaco et al 2007 RGBMajewski et al 2004 M giants 8.3+-0.9 km/s10.4+-1.3 km/s BS ~ 15 km/s BHB ~ 9.5 km/s BHB+BS+faint ~ 14.7 km/s

12. How can the data constrain the simulation Law et al 2010

13. How the velocity dispersion varies in the simulation Fellhauer et al 2006 Radial velocity Velocity dispersion

14. Summary We select BHBs + BSs sample of sag debris in SDSS dr7. BHBs and BSs show different velocity dispersion in this region. BHBs: 76 stars. Velocity dispersion: 9.5 km/s BSs : 91 stars. Velocity dispersion: 15 km/s BHB+BS+faint: 76+91+123 stars. Velocity dispersion: 14.7 km/s. BSs are metal richer than BHBs, and BSs are with little lower temperature. The velocity dispersion of this sample are larger than previous results. Analysis of simulation data shows that the velocity dispersion varies along the stream. There are also a large number of red giants in the data, which we are looking into We are improving the analysis by using Markov Chain Monte Carlo techniques

15. Thank you