Applications of Helium-4 Doubly Forbidden Lines in Astronomical Spectroscopy

Overview

An important issue in modern observational astronomy is detecting low-energy gas and dust, primarily in galaxies; previous work includes extensive use of the 21cm hydrogen (HI) line in radio astronomy, as well as studies of molecular clouds and ionized HII regions. Helium-4, the most common isotope of helium, is difficult to detect in the interstellar medium through classical transitions; however, Helium-4 also can undergo forbidden transitions. One of these, between metastable states of Helium-4 in the 2s energy level, occurs experimentally at 1557nm in the near-infrared range; it is thus lower in energy and less subject to dust extinction, making it an excellent choice for the cold and warm neutral mediums. A fairly homogeneous sample of 50 nearby (z<1) spiral galaxies were chosen for the study. Near-infrared and radio spectra were processed, reduced, and analyzed through an independently developed procedure to correct errors, and to determine the spectral flux of the Doppler-broadened 1557nm line using a Gaussian line fit and a complex background approximation. Overall, in 31 galaxies, the line was detected at a statistically significant level with high confidence, and in 6, it was detected with moderate confidence. In addition, in the sample galaxies, as expected, the spectral line flux correlated with overall H-band magnitude as defined by the astronomical magnitude system, confirming the accuracy and potential of this spectral line. Preliminary work has been undertaken for the study of the 1557nm spectral line in main sequence and post-MS stars, including the development of a procedure to perform a Voigt approximation. Forbidden transition spectroscopy in Helium-4, in the future, may serve as a useful tool for mapping and tracing states and masses of helium in the universe, leading to a variety of insights. The 1557nm spectral line has the potential to be a detectable and significant source of data for future study of low-energy Helium-4 in the interstellar mediums of nearby galaxies. As a proof of concept, a set of equations have been derived to calculate the mass of Helium-4 in the galaxy from spectral flux, akin to hydrogen mass determination with the 21cm HI line.

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