Probing DM Halo Shapes Using Satellite Galaxy

Kinematics

Jeremy Bailin (Swinburne)

Chris Power, Brad Gibson (Swinburne), Peder Norberg (ETH), Dennis Zaritsky (Arizona)

Swinburne Galaxy Groups Workshop 24 May 2005

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

b/a

c/a

0.1 rvir

0.5 rvir

Hydro N-body

(Bailin et al. 2005;see alsoKazantzidiset al. 2004)

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)…)

Satellite Velocities Consistent With NFW Halos

(Prada et al. 2003)

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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

Constant term: interlopers

(Prada et al. 2003)

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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