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Observations of ongoing HI accretion in nearby galaxies have only identifiedabout 10% of the needed fuel to sustain star formation in these galaxies. Mostof these observations have been conducted using interferometers and may havemissed lower column density, diffuse, HI gas that may trace the missing 90% ofgas. Such gas may represent the so-called "cold flows" predicted by currenttheories of galaxy formation to have never been heated above the virialtemperature of the dark matter halo. As a first attempt to identify such coldflows around nearby galaxies and complete the census of HI down to N(HI)~10^18cm^-2, I used the Robert C. Byrd Green Bank Telescope (GBT) to map thecircumgalactic (r < 100-200 kpc) HI environment around NGC 2997 and NGC 6946.The resulting GBT observations cover a four square degree area around eachgalaxy with a 5-sigma detection limit of N(HI)~10^18 cm^-2 over a 20 km/slinewidth. This project complements absorption line studies, which arewell-suited to the regime of lower N(HI). Around NGC 2997, the GBT HI datareveal an extended HI disk and all of its surrounding gas-rich satellitegalaxies, but no filamentary features. Furthermore, the HI mass as measuredwith the GBT is only 7% higher than past interferometric measurements. Aftercorrecting for resolution differences, the HI extent of the galaxy is 23%larger at the N(HI)~1.2x10^18 cm^-2 level as measured by the GBT. On the otherhand, the HI observations of NGC 6946 reveal a filamentary feature apparentlyconnecting NGC 6946 with its nearest companions. This HI filament hasN(HI)~10^18 cm^-2 and a FWHM of 55+-5 km/s and was invisible in pastinterferometer observations. The properties of this filament are broadlyconsistent with being a cold flow or debris from a past tidal interactionbetween NGC 6946 and its satellites.

Green Bank Telescope observations of low column density HI around NGC 2997 and NGC 6946