Observations of ongoing H i accretion in nearby galaxies have only identified about 10 % of the needed fuel to sustain star formation in these galaxies .
Most of these observations have been conducted using interferometers and may have missed lower column density , diffuse , H i gas that may trace the missing 90 % of gas .
Such gas may represent the so-called “ cold flows ” predicted by current theories of galaxy formation to have never been heated above the virial temperature of the dark matter halo .
As a first attempt to identify such cold flows around nearby galaxies and complete the census of H i down to N _ { HI } \sim 10 ^ { 18 } cm ^ { -2 } , I used the Robert C. Byrd Green Bank Telescope ( GBT ) to map the circumgalactic ( r \lesssim 100-200 kpc ) H i environment around NGC 2997 and NGC 6946 .
The resulting GBT observations cover a four square degree area around each galaxy with a 5 \sigma detection limit of N _ { HI } \sim 10 ^ { 18 } cm ^ { -2 } over a 20 km s ^ { -1 } linewidth .
This project complements absorption line studies , which are well-suited to the regime of lower N _ { HI } .
Around NGC 2997 , the GBT H i data reveal an extended H i disk and all of its surrounding gas-rich satellite galaxies , but no filamentary features .
Furthermore , the H i mass as measured with the GBT is only 7 % higher than past interferometric measurements .
After correcting for resolution differences , the H i extent of the galaxy is 23 % larger at the N _ { HI } = 1.2 \times 10 ^ { 18 } cm ^ { -2 } level as measured by the GBT .
On the other hand , the H i observations of NGC 6946 reveal a filamentary feature apparently connecting NGC 6946 with its nearest companions .
This H i filament has N _ { HI } \sim 5 \times 10 ^ { 18 } cm ^ { -2 } and a FWHM of 55 \pm 5 km s ^ { -1 } and was invisible in past interferometer observations .
The properties of this filament are broadly consistent with being a cold flow or debris from a past tidal interaction between NGC 6946 and its satellites .