A GBT Survey of the HALOGAS Galaxies and Their Environments I: Revealing
  the full extent of HI around NGC891, NGC925, NGC4414 & NGC4565
  We present initial results from a deep neutral hydrogen (HI) survey of the
HALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925,
NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope
(GBT). The resulting observations cover at least four deg$^2$ around these
galaxies with an average 5$\sigma$ detection limit of 1.2$\times$10$^{18}$
cm$^{-2}$ over a velocity range of 20 km s$^{-1}$ and angular scale of 9.1$'$.
In addition to detecting the same total flux as the GBT data, the spatial
distribution of the GBT and original Westerbork Synthesis Radio Telescope
(WSRT) data match well at equal spatial resolutions. The HI mass fraction below
HI column densities of 10$^{19}$ cm$^{-2}$ is, on average, 2\%. We discuss the
possible origins of low column density HI of nearby spiral galaxies. The
absence of a considerable amount of newly detected HI by the GBT indicates
these galaxies do not have significant extended diffuse HI structures, and
suggests future surveys planned with the SKA and its precursors must go
\textit{at least} as deep as 10$^{17}$ cm$^{-2}$ in column density to
significantly increase the probability of detecting HI associated with the
cosmic web and/or cold mode accretion.
