Interstellar Ultraviolet Ly <i>α</i> High Resolution Mapping from the New Horizons P-Alice Instrument

Abstract We present New Horizons Alice spectral observations of interstellar hydrogen Ly α emissions at a vantage point of 57 au from the Sun. The observations were conducted as a pair of orthogonal scans to produce high spatial angular resolution measurements of a dark patch of sky located at high Galactic latitudes. We find that the brightness of the UV background remains relatively constant over the selected sky region, with the exception of areas near a few bright stars and galaxies. Most stars do not affect the brightness of Ly α in this region; however, we detect a 25 Rayleigh Ly α enhancement near the location of hot subdwarf CD-38 222. This enhancement is consistent with nebular emission produced by ionizing photons from a bow shock as the object interacts with a high-latitude interstellar gas cloud. We do not find correlations of Ly α or background UV brightness with dust structures seen by the Planck space observatory. Our analysis at higher angular resolution and with the full Alice bandpass of 520–1870 Å confirms the results of the global Ly α maps produced by G. R. Gladstone et al. at lower spatial resolution. The near-constant Ly α is consistent with their interpretation that scattering of H within our Local Bubble produces a nearly isotropic Ly α distribution. Future Alice global scans in photometer mode can be used to assess changes in the Ly α sky as a function of solar distance. High-resolution FUV Ly α scans with the full spatial and spectral information should be repeated over the entire sky to resolve additional small-scale structures.