2018ApJ...865...36P

We present initial results from a deep neutral hydrogen (H I) survey of the HALOGAS galaxy sample, which includes the spiral galaxies NGC 891, NGC 925, NGC 4414, and NGC 4565, performed with the Robert C. Byrd Green Bank Telescope (GBT). The resulting observations cover at least 4 deg² around these galaxies, with an average 5σ detection limit of 1.2 x 10¹⁸ 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 data match well at equal spatial resolutions. The H I mass fraction below H I column densities of 10¹⁹ cm^–2 is, on average, 2%. We discuss the possible origins of low column density H I of nearby spiral galaxies. The absence of a considerable amount of newly detected H I by the GBT indicates these galaxies do not have significant extended diffuse H I structures, and suggests future surveys planned with the SKA and its precursors must go at least as deep as 10¹⁷ cm^–2 in column density to significantly increase the probability of detecting H I associated with the cosmic web and/or cold mode accretion.