Gaussian Decomposition of λ21 cm H i profiles, the Critical Ionization Velocity, and the Interstellar Helium Abundance

Abstract

Following an established protocol of science—that results must be reproducible—we examine the Gaussian fits to Galactic λ21 cm (H i) emission profiles obtained by two seemingly complementary methods using data from the Leiden–Argentine–Bonn all-sky survey. One is based on the method used by Verschuur, the other by Nidever et al. (2008). The comparisons led to the identification of four problems that might arise when an algorithm is applied to huge databases without close monitoring: (1) different methods of calculating measuring the goodness of fit; (2) an ultra-broad component found to imperfectly bridge the gap between low- and intermediate-velocity gas; (3) the lack of an imposed spatial coherence allowing different components to appear and disappear in profiles separated by a fraction of a beamwidth; and (4) multiple, fundamentally different solutions for profiles at both the north and south Galactic poles. Confirming evidence emerges from this study of an underlying component with a line width of an order 34 km s−1. If this feature is the result of the critical ionization velocity effect acting on interstellar helium, it can be used to calculate its interstellar abundance. Analysis of H i profiles in an area in the southern Galactic hemisphere using multitelescope data gives a helium abundance of 0.094 ± 0.035, in excellent agreement with the accepted cosmic abundance of 0.085.