EMPRESS. VI. Outflows Investigated in Low-mass Galaxies with M <sub>∗</sub> = 10<sup>4</sup>–10<sup>7</sup> M <sub>⊙</sub>: Weak Feedback in Low-mass Galaxies?
Yi Xu, Masami Ouchi, Michael Rauch, Kimihiko Nakajima, Yuichi Harikane, Yuma Sugahara, Yutaka Komiyama, Haruka Kusakabe, Seiji Fujimoto, Yuki Isobe, Ji Hoon Kim, Yoshiaki Ono, Fakhri S. Zahedy
Abstract
Abstract We study emission line profiles of 21 nearby low-mass ( M * = 10 4 –10 7 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 H α and [O iii ] λ 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 ∼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.