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tokens: between and M * in the top panel of Figure 1 for isolated galaxies with ALFALFA data and the residuals () between the observed and the predicted in the bottom panel. Equation ( 2 ) is plotted as the solid curve. The dashed lines in the bottom panel are ±1 dex from the relation, which roughly corresponds to the 3 σ scatter. We use the −1 dex residual as a reference point throughout the paper as an arbitrary line of demarcation. Figure 1. Top panel: the isolated to M * relation of gas-rich galaxies. Only isolated galaxies with detections from the 70% ALFALFA survey are plotted here as black contours and dots. The fit from Equation ( 2 ) is plotted as the solid curve and noted in the legend. Bottom panel: H i mass residual ( −1 dex) as a function of stellar mass with the same symbols at the top panel. The ±1 dex (3 σ ) residuals are shown as dashed lines. 2.4. Emission Line Measurements Optical emission line flux ratio diagnostics are often used to differentiate between various physical phenomena that ionize the ISM of galaxies Baldwin et al. ( 1981 ), Veilleux Osterbrock ( 1987 ). Differentiating between excitation sources (actively accreting black holes, star formation, shocks from AGB stars, etc.) depends on the ionization mechanism strength. Reines Volonteri ( 2015 ) measure emission line strengths of ∼67000 galaxies selected from the NSA catalog. The authors cull the spectral catalog with an S/N cutoff of three and an equivalent width cutoff of one for the H α , [N ii ], [O iii ] emission lines and an S/N cutoff of two for the H β line. They iteratively subtract continuum and absorption lines using the models of Tremonti et al. ( 2004 ) from the observed spectrum, carefully masking out the emission lines. Next, a one- or two-component Gaussian is fit to the [S ii ] doublet and used as a template for the remaining emission lines of interest, excluding [O iii ]. We use the data set from this study without any modifications. 2.5. BPT Emission Line Diagnostics We examine the emission line diagnostics of all galaxies from Reines Volonteri ( 2015 ), regardless of environment or H i data using the [O iii ]/H β and [N ii ]/H α narrow-line BPT diagram (Baldwin et al. 1981 ; Veilleux Osterbrock 1987 ; Kewley et al. 2006 ). We adopt the division into the star-forming, composite, and AGN categorization that has been performed by Reines Volonteri ( 2015 ) using the Kewley et al. ( 2001 ) and Kauffmann et al. ( 2003 ) categorization. The Kewley et al. ( 2001 ) demarcation line represents the division between a theoretical maximal starburst model and emission that can only be explained by AGN activity. The Kauffmann et al. ( 2003 ) demarcation line is an empirical separation between the star-forming sequence and the AGN sequence. Galaxies below the Kauffmann et al. ( 2003 ) line are considered to be purely star-forming and galaxies above the Kewley et al. ( 2001 ) line are considered Seyferts and LINERs. The region between these two demarcation lines