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

TL;DR

This study analyzes H-alpha emission line profiles in star-forming galaxies, revealing that boxy profiles are common and depend on galaxy mass, orientation, and star formation activity, consistent with a rotating disk with a ring-like emission region.

Contribution

It provides the first large-scale statistical analysis linking H-alpha line profile shapes to galaxy physical and geometric parameters, supporting a rotating disk model.

Findings

Boxy H-alpha profiles are prevalent in massive, edge-on star-forming galaxies.

The fraction of boxy profiles increases with stellar mass and inclination.

Velocity differences between components follow the Tully-Fisher relation.

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 ($\Sigma_{\rm SFR}$), and stellar mass ($M_*$), with the trend being much stronger 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 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.