The dipole anisotropy of AllWISE galaxies
Monthly Notices of the Royal Astronomical Society Blackwell Publishing Inc. 477 (2018) 1772-1781
Authors:
M Rameez, R Mohayaee, S Sarkar, J Colin
Abstract:
We determine the dipole in the WISE galaxy catalogue. After reducing star
contamination to <0.1% by rejecting sources with high apparent motion and those
close to the Galactic plane, we eliminate low redshift sources to suppress the
non-kinematic, clustering dipole. We remove sources within {\pm}5{\deg} of the
super-galactic plane, as well as those within 1'' of 2MRS sources at redshift z
< 0.03. We enforce cuts on the source angular extent to preferentially select
distant ones. As we progress along these steps, the dipole converges in
direction to within 5{\deg} of the CMB dipole and its magnitude also
progressively reduces but stabilises at {\sim}0.012, corresponding to a
velocity >1000 km/s if it is solely of kinematic origin. However, previous
studies have shown that only {\sim}70% of the velocity of the Local Group as
inferred from the CMB dipole is due to sources at z < 0.03. We examine the Dark
Sky simulations to quantify the prevalence of such environments and find that
<2.1% of Milky Way-like observers in a {\Lambda}CDM universe should observe the
bulk flow (> 240 km/s extending to z > 0.03) that we do. We construct mock
catalogues in the neighbourhood of such peculiar observers in order to mimic
our final galaxy selection and quantify the residual clustering dipole. After
subtracting this the remaining dipole is 0.0048 {\pm} 0.0022, corresponding to
a velocity of 420 {\pm} 213 km/s which is consistent with the CMB. However the
sources (at z > 0.03) of such a large clustering dipole remain to be
identified.
