Quantitative Method for the Optimal Subtraction of Continuum Emission from Narrow-band Images : Skewness Transition Analysis

Abstract

We present an objective method to remove the stellar continuum emission from narrow-band images to derive emission-line images. The method is based on the skewness of the pixel histogram of the residual images. Specifically, we exploit a transition in the skewness of the signal in the continuum-subtracted image, which appears when the image changes from being under-subtracted to over-subtracted. Tests on one-dimensional artificial images demonstrate that the transition identifies the optimal scaling factor μ to be used on the broad-band image IB in order to produce the optimal line-emission image IE, i.e., IE =IN - μ IB, with IN the original (un-subtracted) narrow-band image. The advantage of this method is that it uses all information-bearing pixels in the final image, and not just a sub-set of those pixels (the latter being common in many traditional approaches to stellar continuum removal from narrow-band images). We apply our method to actual images, both from ground-based and space facilities, in particular to WFPC2 and ACS images from the Hubble Space Telescope, and we show that it is successful irrespective of the nature of the sources (point-like or extended). We also discuss the impact on the accuracy of the method of non-optimal images, such as those containing saturated sources or non-uniform background, and present `workarounds' for those problems.