Detection of the effect of cosmological large-scale structure on the orientation of galaxies

Ignacio Trujillo, Conrado Carretero & Santiago G. Patiri2006, ApJ, 640, L111

A theoretical motivationThe Tidal Torque Theory (TTT):

Spin of spiral galaxies is generated by tidal torques operating on the primordial material destined to form a galaxy (Peebles 1969; White 1984)

Millennium gas project

Pearce et al. (2006)

A theoretical motivation: stability of the large-scale structure with cosmic time

The TTT prediction:

There must be local correlations between the galaxy rotation axes and the surrounding matter field

s

n

A theoretical motivation

The TTT prediction:

Lee (2004) provides an analytical description of the effect

sn

sn

deg

P (

cos

)

0

1

0 8020 6040

C=0

C=0.3

C=0.5

C=0.7

C=1

Lee (2004)

Detection problems (1)

s

Estimating spin vector : galaxy orientation is degenerated

ς: Inclination is estimated from the semi-minor to semi-major axis b/a

ς=±arcsin(b/a)

Solution: select Edge-on (i.e. ς≤12°) or Face-on (i.e. ς≥78°)

Detection problems (1)

s

Estimating spin vector : galaxy orientation is degenerated

ς: Inclination is estimated from the semi-minor to semi-major axis b/a

ς=±arcsin(b/a)

Solution: select Edge-on (i.e. ς≤12°) or Face-on (i.e. ς≥78°)

Detection problems (2)

Cosmic planes are not measured in real space but in redshift space:

cz=H0d+v0 => Redshift distortion => Finger of God (FOG)

Uncertainty in position of individual galaxies is:

±4.2 h-1 Mpc

Characterization of planes is degenerated

Void based method: a new approach

Search for orientation in the shells of big (>10 h-1 Mpc) voids

Advantages:

-They are not spurious structures

- Uncertainty in centre position: ±2.5 h-1 Mpc

- r: the vector joining the centre of the void to the centre of the galaxy is a

good approximation of the vector n that describes the

matter distribution

Disadvantages:

- Large voids are scarce

n

© T

he

Vir

go P

roye

ct

Void based method: estimating Void based method: estimating θθ1) Searching voids: (x,y,z,r)void

HB Void Finder (Patiri et al. 2006)

2) Selecting galaxies in the shell

3) Selecting edge-on/face-on (spin)

4)

||||

·arccos

rs

rs

SDSS DR3

3732 square degrees 5.5x105 galaxiesbJ = 18.8 mag (90%)

Surveys used: 2dFGRS and SDSS DR3

1500 square degrees 2.2x105 galaxies bJ = 19 mag (90% )

2dFGRS

Maximizing the volume and number of galaxies imply estimate voids using all galaxies brighter than:

MbJ=-19.3+5log(h)

z<0.14 (2dFGRS)

z<0.13 (SDSS)

Our sample

Voids (>10 h-1 Mpc) + shells (4 h-1 Mpc) within the survey:

149 (2dFGRS)

321 (SDSS DR3)

Voids which contain at least one edge-on/face-on spiral in their shells:

49 (2dFGRS)

129 (SDSS)

Final number of galaxies:

60 (edge-on; 2dFGRS)

118 (edge-on; SDSS)

23 (face-on; SDSS)

Total: 201

Results

Statistical Tests for the combined sample

(rejection of null hypothesis):

Kolmogorov-Smirnov: 99.6%

Χ2-test: 99.8%

Deviation of sin(θ): 99.7%

Best strength correlation parameter c:

True value (without redshift distortion) must be higher!!!

1.02.07.0

Summary

1. First statistical significant observation of alignments between disk galaxy orientation and large scale structure

2. Confirmation of one of the main prediction of the Tidal Torque Theory

3. Estimation of the strength correlation parameter c

4. Potential explanation of the Holmberg Effect