Studying the galactic outflow in NGC 1569


  • M. S. Westmoquette,

    Corresponding author
    1. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
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  • L. J. Smith,

    1. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
    2. Space Telescope Science Institute and European Space Agency, 3700 San Martin Drive, Baltimore, MD 21218, USA
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  • J. S. Gallagher III

    1. Department of Astronomy, University of Wisconsin-Madison, 5534 Sterling, 475 North Charter Street, Madison, WI 53706, USA
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We present deep WIYN Hα imaging of the dwarf irregular starburst galaxy NGC 1569, together with WIYN SparsePak spatially resolved optical spectroscopy of the galactic outflow. This leads on from our previous detailed analyses of the state of the interstellar medium (ISM) in the central regions of this galaxy. Our deep imaging reveals previously undetected ionized filaments in the outer halo. Through combining these results with our spectroscopy we have been able to redefine the spatial extent of the previously catalogued superbubbles, and derive estimates for their expansion velocities, which we find to be in the range 50–100 km s−1. The implied dynamical ages of ≲25 Myr are consistent with the recent star and cluster formation histories of the galaxy. Detailed decomposition of the multicomponent Hα line has shown that within a distinct region ∼700 × 500 pc in size, roughly centred on the bright super star cluster A, the profile is composed of a bright, narrow [full width at half-maximum (FWHM) ≲ 70 km s−1] feature with an underlying, broad component (FWHM ∼ 150 km s−1). Applying the conclusions found in our previous work regarding the mechanism through which the broad component is produced, we associate the faint, broad emission with the interaction of the hot, fast-flowing winds from the young star clusters with cool clumps of ISM material. This interaction generates turbulent mixing layers on the surface of the clouds and the evaporation and/or ablation of material into the outflow. Under this interpretation, the extent of the broad component region may indicate an important transition point in the outflow, where ordered expansion begins to dominate over turbulent motion. In this context, we present a multiwavelength discussion of the evolutionary state of the outflow.