Mass and Light distributions in Clusters of Galaxies - Henry A ...

2.4 Distortion analysis of ACS/HST images
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Distortion g +
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Figure 2.6 – Distortion profile for the central r < 2 ′ area covered by ACS (filled circles)
together with the weaker distortions measured by Subaru at larger radius (open symbols).
The black points represent the combined blue and red sample of background galaxies, and
the green points include the cluster members. A remarkable saw-tooth pattern is visible
for the background galaxy distortions in the strong lensing region where the tangential and
radial critical radii are clearly visible corresponding to a maximum and a minimum in the
value of g T , respectively. In between the distortion passes through zero, where the degree
of tangential and radial distortion is equal, leaving images unchanged in shape at a radius
where κ = 1. The distortion of the green sample is consistent with zero in the inner region
where the cluster members dominate the sample but at larger radius the green and black
points merge for r > 3 ′ , indicating that there the green sample comprises predominately
background galaxies.
Outside the tangential critical curve we find that the tangential shear
of the background sample (black circles in Fig. 2.6) drops to g T,B ∼ 0.2 at
r = 2 ′ , in good agreement with the Subaru analysis at this radius, giving
us confidence in the consistency of our work. For the color range defined
above, which includes the cluster sequence and all the bluer members of the
cluster galaxy population, g T,G ∼ 0 over the full range of radius of the ACS
data (green circles in Fig. 2.6), indicating - as expected - that the galaxy
population in the this color range is dominated by cluster members with
negligible background contamination.
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