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The Dipole Anisotropy of the 2MASS
Redshift Survey
Pirin Erdoğdu
(University of Nottingham)
and the 2MRS Team
MNRAS, in press
OUTLINE
 What is the 2MASS Redshift Survey?
 What is the acceleration on our Local Group
of galaxies?
 Why do I think I am right?
Main Assumptions
 CMB dipole is due to the motion of the Sun.
 Linear theory of density perturbations is
acceptable.
 The 2MASS has constant mass-to-light ratio
and linear bias.
The 2MASS Redshift Survey
z ≤ 0.01 0.01 < z ≤ 0.025 0.025 < z < 0.05 0.05 ≤ z
Number Weighting Technique
If we have a perfect 3-dimensional
galaxy catalogue with same mass, with no selection
effects, no z-distortions and with linear biasing then

Linear theory:
Linear theory:

r
G
g 0  G   d r m r 3 
r
b
0
3
rˆi
i r 2
i
H0
v0  
g 0 
4G 

  m
b
0.6
From CMB: vLG = 627
km/sec towards l=273o and
b=29o
From galaxy cat.
The 2MRS Redshift Distribution
The Rocket Effect
(Kaiser, 1987)
cz=H0r + (vpec- v(0)).(r/r)
LG
Gal 2 with d = 100 km/sec v pec= -20 km/sec
Thus calculated distance=80 km/sec
Thus calculated dipole = 64 % more than the
Real dipole
Gal 1 with d=100 km/sec v pec = 20 km/sec
Thus calculated distance=120 km/sec
Thus calculated dipole= 44% less than
the real dipole
Net effect: A spurious contribution to the LG dipole from
All the galaxies.
Flux Weighting Technique
If we have a perfect 2-dimensional
galaxy catalogue with no selection effects, constant mass-tolight ratio and with linear biasing then
ˆ
ri
M

g (0)  G  M i 2  G
ri
L
i
M
 4G
L
since
ˆ
 Si ri
i
L
S
2
4r
ˆ
ri
Li 2

ri
i
Luminosity Weighting Function
Notice the Zone of Avodiance
Star: CMB
Triangle: 2MRS – Lum
Weighted
Triangle: 2MRS-Num
Weighted
Circle: 2MASS by Maller
etal 2003
Upside down Triangle:
IRAS PSCz Dipole
Upside down Triangle:
2MASS dipole of PSCz
galaxies
Triangle: 2MRS – Lum
Weighted without Maffei 1,
2, Dwingeloo 1, IC342, M87
Blue square: Dipole from XRay Clusters (Kocevski &
Ebeling 2005)
Shapley
Centaurus
Hydra
To summarise…
 The flux weighted galaxy dipoles are better.
 Most of the LG dipole is caused by the tug-of-war
between the GA and Per-Pis. The dipole
converges by ~ 60 Mpc/h.
 If dipole converged β2mrs = 0.4±0.1
 Small discrepancy between LG and CMB dipoles
are due to ZoA, 2nd and 3rd assumptions
mentioned in the earlier slide and structure
beyond. But no big deal!