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 theNa D absorption doublet (5890A, 5896A) for six dwarf starburst galaxies and twomore luminous starbursts: M82 and NGC 1614. The absorption features wereseparated into multiple components and separated into stellar and interstellarparts based on kinematics. We find that three of the dwarfs show outflows, withan average blueshift of 27 km/s. This is small compared to the highest velocitycomponents in NGC 1614 and M82 (blueshifted by 150 km/s and 91 km/s,respectively); these two brighter galaxies also show more complex absorptionprofiles than the dwarfs. None of the outflow speeds clearly exceed the escapevelocity of the host galaxy. Sightlines in NGC 2363 and NGC4214 apparentlyintersect expanding shells. We compare the shocked gas velocity (v_NaD) to theionized gas velocity (v_Halpha) and interpret the velocity difference as eithera trapped ionization front (NGC 4214) or a leaky HII region (NGC 2363). Thedwarfs show N(NaD) = 10^(11.8-13.7) cm^-2, while the Na D columns in M82 andNGC 1614 are 10^13.7 cm^-2 and 10^14.0 cm^-2, respectively. The mass ofexpelled gas is highly sensitive to outflow geometry, dust depletion, andionization fraction, but with a simple shell model we estimate neutral outflowgas masses from ~10^6 M_solar to ~10^10 M_solar.Comment: 36 pages, including 9 figures. Accepted for publication in Astrophys.