The remarkable starburst-driven outflow in NGC 2782

Abstract

We show that the starburst-driven outflow in the peculiar galaxy NGC 2782 forms a well-defined collimated bubble which has an extent of $\sim$ 1 kpc and a closed shell at its edge, as seen in H$α$, [O III], and 5 GHz radio continuum. The shell coincides with the maximum in intensity and linewidths of [O III] lines in a blueshifted emission nebula which was previously detected via optical spectroscopy by Boer et al. (1992). Such a remarkable outflow morphology has not been observed to date in any other starburst galaxy of comparable luminosity. The radio continuum map reveals a second bubble of similar size on the opposite side of the nucleus, forming a striking double-bubble outflow morphology. We argue from the morphology and short timescale ($\sim$ $4 \times 10^6$ years) of the outflow that it is dynamically younger than freely-expanding outflows seen in other galaxies which harbor circumnuclear starbursts of comparable luminosity, e.g., M82. We suggest that the outflow in NGC 2782 is in the early stage where thermal instabilities have not yet completely ruptured the outflow bubble.We present evidence that the outflow is driving warm and hot ionised gas, and possibly cold molecular gas, out of the central kpc of the galaxy. We estimate the contribution of the hot, warm, and cold phases of the ISM to the energetics of the outflow. This study is based on our optical BVR, H$α$, and [O III] observations from the WIYN telescope and OVRO CO interferometric data, along with available 5 GHz radio continuum and ROSAT X-ray maps.