Galactic Lyman-α Background Detected by New Horizons

The Alice spectrograph on the New Horizons (NH) spacecraft is currently being used to periodically observe the Lyman-α (Lyα) interplanetary medium (IPM) background, which is primarily due to resonant scattering of solar Lyα emissions by interstellar hydrogen atoms as they pass through the solar system. Observations of IPM Lyα along six great circles spread over the entire sky at intervals of 30 degrees have been acquired on five occasions since the NH flyby of Pluto at 33 AU. The NH-Alice observations of the IPM background at Lyα are consistent with observations acquired three decades earlier by the UVS instruments on Voyager 1 and 2. Furthermore, the NH-Alice data indicate a more distant Lyα background of ~40 Rayleighs brightness (equivalent to ~52 nW/m²/sr in common astrophysical units). The repeated observations of the same regions of the sky show this distant background to be roughly uniform over the sky (or at least over the great circle swaths), and it is most likely the local galactic Lyα background, which has been speculated about for over 50 years. The observations show no appreciable correlation with cloud structure of the local interstellar medium, but the observed brightness can be used to constrain the local density of interstellar dust, which is the primary absorber of the galactic Lyα background. Details of these results, comparison to the last 50 years of Lyα background models, and plans for future observations will be presented here. This research was supported by NASA contract NASW02008 to SwRI.