project: intrinsic alignment in weak lensing Figure 1:
Top: A dark matter halo is 'spun up' by the surrounding tidal field. Bottom: Baryons
associated with the halo will tend to 'fall in' and form a disc in the plane
perpendicular to the Figure.
Figure 2:
The IA correlation function signal obtained from the COMBO-17 data (points) along with
a prediction of the IA signal based on numerical simulations.
Galaxies are thought to become intrinsically aligned with one another during
the galaxy formation process. When a large cloud of dark matter, gas and dust is
collapsing in on itself, the cloud attains angular momentum due to the
gravitational pull of the surrounding mass distribution. This spinning motion
of the cloud causes the baryonic matter to 'fall in' and form a disc, as shown in
Figure 1, and the resulting galaxy becomes aligned in the direction of the tidal
field. Consequently, when several galaxies form in the same part of the Universe,
they each become aligned with the local tidal field and hence, with each other. As
well as being astrophysically interesting in their own right, such intrinsic
alignments (IA) can mimic the much sought-after weak lensing signal of the
large-scale structure of the Universe. The identification and removal of the IA
signal is likely to become an extremely important issue for the next generation of
wide-field weak lensing surveys.
By making use of the accurate redshift information in the COMBO-17 survey, we have been able to identify physically close pairs of galaxies. By removing these close pairs from a correlation function analysis, we have suppressed the IA signal to a negligible level. One can also use such a correlation function-based analysis to put limits on the amplitude of the IA signal --- a measurement of which would have important implications for galaxy formation theories. A first attempt at measuring the IA signal is shown in Figure 2 which shows the constraints as obtained from the COMBO-17 data along with a prediction for the IA signal motivated by results from numerical simulations.
Contact person: Michael Brown, ROE
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Last update Mar 1, 2003, CW