Download Galaxy alignment within dark matter halos

Galaxy alignment within
cosmic structures
Weipeng Lin
Shanghai Astronomical Observatory, CAS, China
[email protected]
Theory of structure formation
 Origin：the tiny density deviation from primeval quantum
field fluctuation, amplified by gravitational instability
 Three dimensional collapse:
1D: pancakes
2D: filaments
3D: ellipsoidal structures
The galaxy alignment embed in the origin of cosmic
structures
Pangu simulation：1Gpc Box，30 billion particles
The Consortium of Chinese Computational Cosmology（C4）
What alignment/mis-alignment?
Satellite Galaxy
or DM substructures
Central
Galaxy
DM Halo
Major axis of central
galaxy
Major axis of halo
Why?
 Directly compared with observations (only for the
alignment between the orientation of central galaxy and
satellite galaxies)
 Provide angular distribution information for structure
(other than number count/Lumi.F/Mass F or 2PCF)：
galaxy distribution follows dark matter but with galaxy
bias and dependence on galaxy properties
 Set constrains on structure formation and galaxy
formation models
 Remove contamination on shear measurement of
gravitational lensing (remove intrinsic alignment, G-I,II)
Observational Statistics (2D)
 Large sample from all sky surveys, i.e., 2dF, SDSS
Sales & Lambas, 2004; Brainerd, 2005; Yang et al., 2006
 HOD/CLF model to construct galaxy groups/clusters and
link them with DM halos
 Measure the angles between BCG major axis and the
position vectors of satellite galaxies.
 Significant alignment found for all the galaxies.
 The strength of alignment also depend on optical
properties (color) of central galaxies and satellite galaxies.
Brainerd T., 2005, ApJL (SDSS)
Results of Yang X, et al.,2006(SDSS)
Theoretical studies, how?
 Semi-analytical model of galaxy formation
 N-Body only simulations
 Hydro-dynamical simulation with star formation: directly
simulated “Galaxies”
Key ingredients: tidal stripping, dynamical friction, orbital
evolution, feedback (stellar or AGN)
N-body simulations
 Related satellite galaxies with sub-structures
 Suffer from tidal stripping
 No ‘orphan’ galaxies within the inner halos
 Lack of information for BCG orientation and galaxy color
Semi-analytical Model (SAM)
 Using N-body simulation, but populating ‘galaxies’ using
semi-analytical model. There are color for galaxies.
 Satellite galaxies are related with sub-structures,
suffering from tidal stripping.
 There are ‘orphan’ galaxies but lack of position
information.
 Lack of information for BCG orientation, assumption need.
 The predicted alignment strength is stronger than
observational results.
Kang, X., et al., 2007 (SAM)
Hydro-dynamical Simulations
 Direct simulation of ‘galaxies’ (centrals and satellites)
 There are galaxy properties, i.e., stellar mass, color,
metallicity, orientation, stellar formation time, …
 Problems: over-cooling/inefficient feedback/wrong color,
morphology which is difficult (except, The Illustris
Project using Volker Springel’s AREPO code)
Size: 100 Mpc/h
N_DM: 512^3
N_GAS: 512^3
Cosmology:
WMAP3
Code: P-Gadget-2
Performed at
Shanghai
Supercomputing
Center @2005
Exercise I. N-body simulation
Improvement:
 Using the shape of inner halo for BCG orientation
 Relating satellite galaxies with sub-structures or
neighboring small dark matter halos
 Using formation-time of DM structures to infer ‘galaxy
color’, i.e., early-formed  red color, late-formed
blue color
 Study the dependence of alignment on MAH
(formation/accretion history)
Reference: Wang, Y., Lin, W.P., et al., 2014, ApJ, 786, 8
Alignment with different layer of DM halo
Tri-axial model of halo
iso-density ellipsoid (Jing & Suto, 2002)
Assuming mis-alignment of BCG with DM
host halo (Gaussian, mean~0°, width~
25°)
Dependence on accretion time
Dependence on satellite formation time
Results I.
 BCG is mis-aligned with the major axis of DM halo, the
usage of inner halo for BCG orientation can improve the
prediction of satellite alignment.
 The late-accreted sub-structures are better aligned with
the DM outer halo than the early-accreted counterparts.
 Formation time seems useless for ‘galaxy color’, why?
(Environment is more important, recycle, quenching)
 However, due to strong tidal stripping in N-body
simulation at halo center, there is no ‘orphan’ galaxies
which should be red and carry strong alignment signal.
Simulations with star formation is necessary.
Exercise II. N-body/SPH simulation
 Define galaxies by stellar FoF groups
 Direct measurement of BCG major axis (no assumption)
 Applying stellar synthesis model to calculate galaxy
luminosity and color
 Directly compared with observational results
 This is the first attempt to use gas simulations to directly
studying the satellite-central galaxy alignment.
Reference: Dong, X.C., Lin, W.P., et al., 2014, ApJ Letter,
791, L33
Galaxy Color from stellar synthesis model
van den Bosch +2008
Too Blue!
Because of
insufficient
feedback
Results II.
 For all satellite-central pairs, the predicted alignment
strength is well consistent with observational results.
 The dependence of alignment on satellite color is consistent
with observational results.
 However, due to the wrong color for central galaxies, the
predicted alignment for centrals is wrong.
 If the color of central galaxies is defined by host halo mass in
stead, the alignment prediction will fit with observational
results.
 Metallicity is a better tracer for satellite alignment.
 Gas simulations with AGN feedback is required for the correct
color of central galaxies and the alignment color dependence.
Galaxy alignment on large scale
Large scale
structure
defined by
tidal field
On-going project
Zoom-in simulation of galaxy clusters, with star formation,
in aim to studying the formation and evolution of dwarf
elliptical galaxies within the environment of clusters.
I am interested in the using of Multi-dark gas simulations
of galaxy clusters.
Main Points
 Simulations with galaxy formation are necessary for the
study of alignment dependence on galaxy color.
 AGN feedback are very important for the study of galaxy
alignment dependence on color.
 The prediction of alignment from simulations are in
good agreement with observations, indicating the
theory of cosmic structure formation is correct for the
spatial distribution of galaxies.
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[email protected]
Research Center for Galaxy Formation and Cosmology
Shanghai Astronomical Observatory, CAS, China