EMPRESS. VI. Outflows Investigated in Low-mass Galaxies with M (*)=10(4)-10(7) M (circle dot): Weak Feedback in Low-mass Galaxies?

We study emission line profiles of 21 nearby low-mass (M (*) = 10(4)-10(7) M (circle dot)) galaxies in deep medium-high resolution spectra taken with Magellan/MagE. These low-mass galaxies are actively star-forming systems with high specific star formation rates of similar to 100-1000 Gyr(-1) that are well above the star formation main sequence and its extrapolation. We identify broad-line components of H alpha and [O iii]lambda 5007 emission in 14 out of the 21 galaxies that cannot be explained by the MagE instrumental profile or the natural broadening of line emission. We conduct double-Gaussian profile fitting to the emission of the 14 galaxies, and find that the broad-line components have line widths significantly larger than those of the narrow-line components, indicative of galactic outflows. The broad-line components have moderately large line widths of similar to 100 km s(-1). We estimate the maximum outflow velocities v (max) and obtain values of similar or equal to 60-200 km s(-1), which are found to be comparable to or slightly larger than the escape velocities. Positive correlations of v (max) with star formation rates, stellar masses, and circular velocities extend down into this low-mass regime. Broad- to narrow-line flux ratios (BNRs) are generally found to be smaller than those of massive galaxies. The small v (max) and BNRs suggest that the mass-loading factors eta can be as small as 0.1-1 or below, in contrast to the large eta of energy-driven outflows predicted by numerical simulations.