A Census of the Extended Neutral Hydrogen Around 18 MHONGOOSE Galaxies

We present the analysis of the diffuse, low column density H i environment of 18 MHONGOOSE galaxies. We obtained deep observations with the Robert C. Byrd Green Bank Telescope and reached down to a 3σ column density detection limit of N HI = 6.3 × 1017 cm−2 over a 20 km s−1 line width. We analyze the environment around these galaxies, with a focus on H i gas that reaches column densities below N HI = 1019 cm−2. We calculate the total amount of H i gas in and around the galaxies, revealing that nearly all of these galaxies contained excess H i outside of their disks. We quantify the amount of diffuse gas in the maps of each galaxy, defined by H i gas with column densities below 1019 cm−2, and find a large spread in percentages of diffuse gas. However, by binning the percentage of diffuse H i into quarters, we find that the bin with the largest number of galaxies is the lowest quartile (0%–25% diffuse H i). We identified several galaxies that may be undergoing gas accretion onto the galaxy disk using multiple methods of analysis, including azimuthally averaging column densities beyond the disk, and identifying structure within our integrated intensity (moment 0) maps. We measured H i mass outside the disks of most of our galaxies, with rising cumulative flux even at large radii. We also find a strong correlation between the fraction of diffuse gas in a galaxy and its baryonic mass, and we test this correlation using both Spearman and Pearson correlation coefficients. We see evidence of a dark matter halo mass threshold of M halo ∼ 1011.1 M ☉ in which galaxies with high fractions of diffuse H i all reside below. It is in this regime that cold-mode accretion should dominate. Finally, we suggest a rotation velocity of v rot ∼ 80 km s−1 as an upper threshold to find diffuse-gas-dominated galaxies.