Panoramic GALEX Far- and Near-Ultraviolet Imaging of M31 and M33

摘要: We present Galaxy Evolution Explorer (GALEX) far-UV (FUV) and near-UV (NUV) mosaic observations covering the entirety of M31 M33. For both targets, we measure decline surface brightness (in FUV NUV) changes in - NUV color as a function galactocentric radius. These UV radial profiles are compared to distribution ionized gas traced by Hα emission. find that extent emission, is greater than observed H II regions diffuse gas. determine ultraviolet fraction M33 using our compare it obtained from wide-field narrowband imaging. The appears be remarkably constant near 0.65 over large range radius, with departures higher values circumnuclear and, most notably, at limit disk. suggest increase radii could indicate substantial portion emission beyond this point not generated situ but rather scattered dust, after originating vicinity disk's outermost regions. variation was also measured. found generally 0.4 rising toward galaxy center, up 0.6. made no attempt correct measurements for position-dependent extinction, so quoted best interpreted upper limits given plausibly extinction stellar clusters relative their surroundings.