Classifying Objects by Medium-Band Observations
- a spectrophotometric 17-filter survey -

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