A Green Bank Telescope Search for Highly Extended HI Disks Around Spiral Galaxies

Recent UV absorption line studies suggest that a large fraction of missing baryons are in the warm ionized and neutral phases, with about half of Milky Way-mass galaxies containing absorption systems with HI column densities of 10¹⁸ cm^-2 or greater. This HI gas, which would have been difficult to detect with previous instruments, could be a significant contributor to the missing baryons. The Green Bank Telescope (GBT) presents a unique opportunity to detect this emission. We present results from GBT 21 cm observations of a sample of ten nearby optically luminous spirals, which reveal extended HI gas in half of our sample. The column densities of this extended HI are typically ~ 1 × 10¹⁹ cm^-2, as measured at distances of 100 kpc from the center of the galaxies.