Abstract
In many astrophysical systems, smoothly-varying large-scale variations
coexist with small-scale fluctuations. For example, a large-scale velocity or
density gradient can exist in molecular clouds that exhibit small-scale
turbulence. In redshifted 21cm observations, we also have two types of signals
- the Galactic foreground emissions that change smoothly and the redshifted
21cm signals that change fast in frequency space. Sometimes the large-scale
variations make it difficult to extract information on small-scale
fluctuations. We propose a simple technique to remove smoothly varying
large-scale variations. Our technique relies on multi-point structure functions
and can obtain the magnitudes of small-scale fluctuations. It can also help us
to filter out large-scale variations and retrieve small-scale maps. We discuss
applications of our technique to astrophysical observations.
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