Download Probing DM Halo Shapes Using Satellite Galaxy Kinematics

Swinburne Galaxy Groups Workshop 24 May 2005
Probing DM Halo Shapes
Using Satellite Galaxy
Kinematics
Jeremy Bailin (Swinburne)
Chris Power, Brad Gibson (Swinburne), Peder Norberg (ETH),
Dennis Zaritsky (Arizona)
Outline
1. Predicted Shapes of Dark Matter
Halos
2. Observational Tests of Halo Shapes
3. Testing Halo Shapes With Satellite
Galaxy Kinematics
a) Observational Methodology
b) Theoretical Methodology
Introduction
• Goal: Test dark matter on group and
galaxy scales
• Cosmological N-body simulations
predict:
– Galaxies/groups/clusters form in dark
matter halos
– Halos are triaxial
Dark Matter Halos
Halo Shapes
(Bailin &
Steinmetz
2005)
Baryons Make Halos Rounder
0.1 rvir
0.5 rvir
c/a
Hydro
N-body
(Bailin et al.
2005;
see also
Kazantzidis
et al. 2004)
b/a
Observational Tests of Halo
Shapes
• Tidal streams around galaxies (Ibata et al.
2001; Helmi 2004; Johnston et al. 2005; Martínez-Delgado et al.
2004…)
– Results are not consistent
• Weak gravitational lensing by galaxies
(Hoekstra et al. 2004)
– Mean projected b/a≈0.7
• X-ray/SZ contours of groups and
clusters (Buote et al. 2002; Lee & Suto 2003)
– Consistent with simulations
Observational Tests of Halo
Shapes
• Positions of satellite galaxies
– Simulations: Subhalos tend to lie near the
major axis of the halo (Knebe et al. 2004; Zentner
et al. 2005; Agustsson & Brainerd 2005; Lee & Kang 2005)
– Observations of Clusters: Cluster galaxies
tend to lie near the (X-ray isophote, BCG)
major axis of the cluster (West 1994)
Observational Tests of Halo
Shapes
• Positions of satellite galaxies
– Observations of Galaxies: contraversial!
Holmberg Effect: Satellites tend to lie along the
poles of parent galaxies (Holmberg 1969; Zaritsky et
al. 1997; Sales & Lambas 2004 (2dFGRS))
Anti-Holmberg Effect: Satellites tend to lie near
the plane of parent galaxies (Brainerd 2004
(SDSS))
Satellite Kinematics
• Regardless of position, kinematics of
satellites are determined by the halo
potential
• Stacking satellite galaxies in large
surveys have been used to measure
mass, outer radial profile of galaxy
halos (Zaritsky et al. 1997; Prada et al. 2003 (SDSS); van
den Bosch et al. 2004 (2dFGRS)…)
|V|
Satellite Velocities Consistent
With NFW Halos
(Prada
et al.
2003)
R
Testing Halo Shapes Using
Satellite Kinematics
• Idea: Compare kinematics of satellite
galaxies at different positions around
parent galaxy (BGG)
– Expectation: Smaller velocities along halo
minor axis than along halo major axis
• Requires LARGE galaxy redshift
survey
– SDSS, 2dFGRS
Observational Issues
• Removal of interlopers
– Traditional approach: fit Gaussian +
constant to velocity distribution
Gaussian term: velocities of
real satellites
N
Constant term: interlopers
(Prada et al. 2003)
|V|
Observational Issues
• Removal of interlopers
– Constant is wrong: interlopers are
clustered in velocity space too
– Iterative selection approach can mostly
eliminate them (van den Bosch et al. 2004)
• Ensure primaries are sufficiently
isolated that kinematics is unaffected by
tidal field
• Getting correct results from the SDSS
SQL server is “non-trivial”
Theoretical Interpretation
• What are the predicted velocities of
satellite galaxies along the major vs.
minor axis of the parent halo?
– What are the predicted velocities of
subhalos?
• Requires many cosmological halos simulated at
sufficiently high resolution
– Which subhalos correspond to observed
satellites?
• Many more subhalos than observed satellites!
Subhalos and Satellites
• Possible mappings:
– Subhalo velocities = satellite velocities
– DM particle velocities = satellite velocities
– Only use subhalos most likely to host
satellites
• Most massive at infall?
• Highest collapse z?
• Conditional Luminosity Function?
– Semi-analytic prescription?
Summary
• Dark matter halos in CDM are triaxial
• The kinematics of satellite galaxies
trace the halo potential
• We are analyzing satellite galaxies in
SDSS and 2dFGRS to determine if the
kinematics are different along the major
vs. minor axis of the parent galaxy
• We are analyzing high resolution
cosmological halos to interpret any
difference in terms of the halo shape