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

Abstract

We assemble a sample of disk star-forming galaxies from the Sloan Digital Sky Survey 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 the following way: (1) it increases monotonically with star formation rate per unit area (${\Sigma }_{\mathrm{SFR}}$), and stellar mass ($M_{\ast }$), with the trend being much stronger with $M_{\ast }$, from $\sim$0% at $M_{\ast }=10^{10}{M}_{\odot }$ to about 50% at $M_{\ast }=10^{11}{M}_{\odot }$; (2) the fraction is much smaller in face-on systems than in 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 the sources which can be modeled well with two velocity components, blueshifted and redshifted from the systemic velocity, these is a positive correlation between the velocity difference of these two components and the stellar mass, with a slope similar to the Tully-Fisher relation; (4) the two components are very symmetric in the mean, both in velocity and in amplitude. The four findings listed above can be understood as a natural result of a rotating galaxy disk with a kpc-scale ring-like H$\alpha$ emission region.

8 pages, 6 figures, accepted by MNRAS