EMPRESS. VI. Outflows Investigated in Low-Mass Galaxies with
  $M_*=10^4-10^7~M_\odot$: Weak Feedback in Low-Mass Galaxies?
  We study emission line profiles of 21 nearby low-mass
($M_*=10^4-10^7~M_\odot$) 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
$\mathrm{sSFR}\sim100-1000~\mathrm{Gyr}^{-1}$ that are well above the
star-formation main sequence and its extrapolation. We identify broad-line
components of H$\alpha$ and [OIII]$\lambda 5007$ 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 board-line components have moderately
large line widths of $\sim 100$ km s$^{-1}$. We estimate the maximum outflow
velocities $v_\mathrm{max}$ and obtain values of $\simeq 60-200$ km s$^{-1}$,
which are found to be comparable to or slightly larger than the escape
velocities. Positive correlations of $v_\mathrm{max}$ 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 $v_\mathrm{max}$ 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.
