New Horizons Detection of the Local Galactic Lyman-α Background

Abstract Since 2007 the Alice spectrograph on the New Horizons (NH) spacecraft has been used to periodically observe the Lyman- α (Ly α ) emissions of the interplanetary medium (IPM), which mostly result from resonant scattering of solar Ly α emissions by interstellar hydrogen atoms passing through the solar system. Three observations of IPM Ly α along a single great circle were made during the NH cruise to Pluto, and these have been supplemented by observations along six great circles (spread over the sky at 30° intervals), acquired one month before and one day after the NH flyby of Pluto, and on a further five occasions since then, out to just over 47 au from the Sun. These data indicate a distant Ly α background of 43 ± 3 Rayleigh brightness (equivalent to 56 ± 4 nW m −2 sr −1 ), which is present in all directions (i.e., not only in the upstream direction, as previously reported). This result is found independently by: (1) the falloff with distance from the Sun of the IPM Ly α brightness observed by NH–Alice in several directions on the sky, and (2) the residual between the observed brightness and a model brightness accounting for the resonantly scattered solar Ly α component alone. The repeated observations show that this distant Ly α background is constant and uniform over the sky, and represents the local Galactic Ly α background. The observations show no strong correlation with the cloud structure of the local IPM. The observed brightness constrains the absorption coefficient of interstellar dust at Ly α to 0.2 ± 0.01 kpc −1 .