sagittarius debris in sdss stripe 82 zhu ling ( 朱玲 ) & martin. c. smith center for...
Post on 16-Dec-2015
221 Views
Preview:
TRANSCRIPT
Sagittarius debris in SDSS stripe 82
Zhu Ling ( 朱玲 ) & Martin. C. SmithCenter for Astrophysics, Tsinghua university
KIAA at Peking University
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 s
elected from SDSS stripe 82. • What constrains can this sample provide for the simulatio
n.
Majewski et al 2003 Niederste-Ostholt et al 2008
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
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 starsBS: Blue StragglersBMP: Blue metal poor starsMS: main sequence stars
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
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 system
atic velocity offset.
RA
radi
al v
eloc
ity
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=18.7d=40kpc
Surface gravity log(g)
g=18.3d~ 10 kpc
appa
rant
mag
nitu
de
g
RA
rad
ial
velo
cit
y
15<RA<50: 416
288 have log(g)
Velocity distributionBHB: 76BS: 91faint: 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
Rad
ial
velo
city
Rad
ial v
eloc
ity
g
BHBs VS BSs
BS: 91sigma: 15 km/s (18.7)
BHB: 76sigma: 9.5 km/s (11.4)
BHB: 76BS: 91
surface temperature metallicity
BHB
BS
BS: colder, higher metallicityBHB: hotter, lower metallicity
BHBS VS BSs
Compare with previous resultManaco et al 2007 RGB Majewski et al 2004 M giants
8.3+-0.9 km/s 10.4+-1.3 km/s
BS ~ 15 km/s
BHB ~ 9.5 km/s
BHB+BS+faint ~ 14.7 km/s
How can the data constrain the simulation
Law et al 2010
How the velocity dispersion varies in the simulation
Fellhauer et al 2006
Rad
ial
velo
city
Vel
oci
ty
disp
ersi
on
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
Thank youThank you
top related