EMPRESS. VI. Outflows Investigated in Low-mass Galaxies with M ∗ = 104-107 M ⊙: Weak Feedback in Low-mass Galaxies?

We study emission line profiles of 21 nearby low-mass (M _* = 10⁴-10⁷ M _⊙) 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 ~100-1000 Gyr^-1 that are well above the star formation main sequence and its extrapolation. We identify broad-line components of Hα and [O III]λ5007 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 ~100 km s^-1. We estimate the maximum outflow velocities v _max and obtain values of ≃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 η can be as small as 0.1-1 or below, in contrast to the large η of energy-driven outflows predicted by numerical simulations.