Inverted EEMD: a robust method to identify narrow absorption features form spectral data and cubes

ABSTRACT Extracting information from complex data is a challenge shared by multiple frontiers of modern astrophysical research. Among those, analysing spectra cubes, where the emission is mapped in the position–position–velocity space is a difficult task given the vast amount of information contained within. The cubes often contain a superposition of emissions and absorptions, where extracting absorption signatures is often necessary. One example is the extraction of narrow absorption structures in H i 21 cm emission spectra. These H i self-absorption (HISA) clouds trace the cold H i gas in interstellar space. We introduce an automatic and robust method called the inverted ensemble empirical mode decomposition (EEMD) method to extract narrow features from spectral cubes. Our method is based on the EEMD method, an established method to decompose 1d signals. The method is robust and parameter-free, making it useful in analysing spectral cubes containing localized absorption signals of different types. The inverted-EEMD method is suitable for the analysis of spectral cubes where it can produce a cube containing the absorption signal and one containing the unabsorbed signal, where cold clouds can be identified as coherent regions in the absorption map.