Galaxy alignment within cosmic structures Weipeng Lin Shanghai Astronomical Observatory, CAS, China

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? Major axis of halo Satellite Galaxy or DM substructures DM Halo Central Galaxy Major axis of central galaxy

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

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 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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