A Keck/HIRES Study of Kinematics of the Cold Interstellar Medium in Dwarf Starburst Galaxies

We have obtained high-resolution Echelle spectra (R = 30,000-50,000) of the Na D absorption doublet (λλ5890, 5896) for six dwarf starburst galaxies and two more luminous starbursts: M82 and NGC 1614. The absorption features were separated into multiple components and separated into stellar and interstellar parts on the basis of kinematics. We find that three of the dwarfs show outflows, with an average blueshift of 27 km s-1. This is small compared with the highest velocity components in NGC 1614 and M82 (blueshifted by 150 and 91 km s-1, respectively); these two brighter galaxies also show more complex absorption profiles than the dwarfs. None of the outflow speeds clearly exceed the escape velocity of the host galaxy. Sight lines in NGC 2363 and NGC 4214 apparently intersect expanding shells. We compare the shocked gas velocity (vNa D) with the ionized gas velocity (vHα) and interpret the velocity difference as either a trapped ionization front (NGC 4214) or a leaky H II region (NGC 2363). The dwarfs show NNa D = 1011.8-1013.7cm-2, while the Na D columns in M82 and NGC 1614 are 1013.7 and 1014.0 cm-2, respectively. The mass of expelled gas is highly sensitive to outflow geometry, dust depletion, and ionization fraction, but with a simple shell model we estimate neutral outflow gas masses from ~106 to ~1010 M☉.

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