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We assemble a sample of disk star-forming galaxies from the Sloan Digital SkySurvey Data Release 7, studying the structure of H$\alpha$ emission lines,finding a large fraction of this sample contains boxy H$\alpha$ line profiles.This fraction depends on galaxy physical and geometric parameters in thefollowing way: (1) it increases monotonically with star formation rate per unitarea ($\Sigma_{\rm SFR}$), and stellar mass ($M_*$), with the trend being muchstronger with $M_*$, from $\sim$0% at $M_*=10^{10}M_{\odot}$ to about 50% at$M_*=10^{11}M_\odot$; (2) the fraction is much smaller in face-on systems thanin edge-on systems. It increases with galaxy inclination ($i$) while $i <60\,^{\circ}$ and is roughly a constant of 25% beyond this range; (3) for thesources which can be modeled well with two velocity components, blueshifted andredshifted from the systemic velocity, these is a positive correlation betweenthe velocity difference of these two components and the stellar mass, with aslope similar to the Tully-Fisher relation; (4) the two components are verysymmetric in the mean, both in velocity and in amplitude. The four findingslisted above can be understood as a natural result of a rotating galaxy diskwith a kpc-scale ring-like H$\alpha$ emission region.

Boxy H$\alpha$ Emission Profiles in Star-Forming Galaxies