HII Region Ionization of the Warm Interstellar Medium: A Case Study of NGC 7538

POSTER

Abstract

The warm interstellar medium (WIM) is a low-density diffuse ionized component of the interstellar medium. Its ionization is likely maintained by photons from O-type stars, which are surrounded by spheres of fully ionized hydrogen, their HII regions. In order to ionize the WIM, these photons must escape through the photo-dissociation region (PDR), the boundary between the HII region within and the neutral medium surrounding it. Using data from the Green Bank Telescope, we analyze the ionizing radiation that is escaping through the PDR boundary of the HII region NGC~7538. We define the location of the PDR boundary, finding extended radio continuum and radio recombination line emission outside the PDR toward the north and east of the region. This suggests a non-uniform PDR morphology that is affecting the amount of radiation “leaking´´ through the PDR. We quantify the leaking photon fraction along the line of sight, and use a numerical model to estimate the leaking photon fraction in three dimensions of both radio continuum and H-alpha (656\,nm) emission. We detect carbon emission near the PDR and find a decrease in the helium-to-hydrogen ionic abundance ratio with increasing distance from the central position. This indicates a softening of the radiation field within the PDR.

Authors

  • Matteo Luisi

    West Virginia University

  • Loren Anderson

    West Virginia University

  • Scott Cushing

    National Radio Astronomy Observatory, Jodrell Bank Centre for Astrophysics, New York University Abu Dhabi, Naval Research Laboratory, CSIRO Astronomy and Space Sciences, George Mason University, ASTRON, NASA Goddard Space Flight Center, University of Wisconsin-Milwaukee,, Station de radioastronomie de Nancay, Columbia Astrophysics Laboratory, Max-Planck-Institut fur Physik, West Virginia Univ, West Virginia University, Lehigh University, Fraunhofer Institute for Physical Measurement Techniques, Department of Microsystems Engineering (IMTEK), Pennsylvania State University, Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA, Massachusetts Institute of Technology, University of Minnesota, Pennsylvania State Univ, University of Maryland, Towson University, Department of Physics, Villanova University, University of Virginia, Boston University, Univ of Maryland-College Park, Harvard University, The Pennsylvania State Unversity, University of Wisconsin-Madison, McMaster University, California State University - Long Beach, National Chiao Tung University, National Chiao Tung University, Taiwan, Ohio University, University of Rennes, UMR CNRS 6251, Rennes, France, University of Bordeaux, UMR 5804, Floirac, France, University of Bordeaux, UMR 5255, Talence, France, Sandia National laboratories, Livermore, CA, Department of Chemistry, West Virginia University, Morgantwon, WV, The Pennsylvania State University, University of Toronto, Departamento de F\'isica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile, Cinvestav-Unidad Queretaro, Queretaro-76230, Mexico, Department of Physics and Astronomy, West Virginia University, Morgantown, WV-26505-6315, USA, The University of Arizona, TU Dortmund, Dortmund, Naval Air Warfare Center - Aircraft Division

  • Scott Cushing

    National Radio Astronomy Observatory, Jodrell Bank Centre for Astrophysics, New York University Abu Dhabi, Naval Research Laboratory, CSIRO Astronomy and Space Sciences, George Mason University, ASTRON, NASA Goddard Space Flight Center, University of Wisconsin-Milwaukee,, Station de radioastronomie de Nancay, Columbia Astrophysics Laboratory, Max-Planck-Institut fur Physik, West Virginia Univ, West Virginia University, Lehigh University, Fraunhofer Institute for Physical Measurement Techniques, Department of Microsystems Engineering (IMTEK), Pennsylvania State University, Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA, Massachusetts Institute of Technology, University of Minnesota, Pennsylvania State Univ, University of Maryland, Towson University, Department of Physics, Villanova University, University of Virginia, Boston University, Univ of Maryland-College Park, Harvard University, The Pennsylvania State Unversity, University of Wisconsin-Madison, McMaster University, California State University - Long Beach, National Chiao Tung University, National Chiao Tung University, Taiwan, Ohio University, University of Rennes, UMR CNRS 6251, Rennes, France, University of Bordeaux, UMR 5804, Floirac, France, University of Bordeaux, UMR 5255, Talence, France, Sandia National laboratories, Livermore, CA, Department of Chemistry, West Virginia University, Morgantwon, WV, The Pennsylvania State University, University of Toronto, Departamento de F\'isica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile, Cinvestav-Unidad Queretaro, Queretaro-76230, Mexico, Department of Physics and Astronomy, West Virginia University, Morgantown, WV-26505-6315, USA, The University of Arizona, TU Dortmund, Dortmund, Naval Air Warfare Center - Aircraft Division

  • Scott Cushing

    National Radio Astronomy Observatory, Jodrell Bank Centre for Astrophysics, New York University Abu Dhabi, Naval Research Laboratory, CSIRO Astronomy and Space Sciences, George Mason University, ASTRON, NASA Goddard Space Flight Center, University of Wisconsin-Milwaukee,, Station de radioastronomie de Nancay, Columbia Astrophysics Laboratory, Max-Planck-Institut fur Physik, West Virginia Univ, West Virginia University, Lehigh University, Fraunhofer Institute for Physical Measurement Techniques, Department of Microsystems Engineering (IMTEK), Pennsylvania State University, Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA, Massachusetts Institute of Technology, University of Minnesota, Pennsylvania State Univ, University of Maryland, Towson University, Department of Physics, Villanova University, University of Virginia, Boston University, Univ of Maryland-College Park, Harvard University, The Pennsylvania State Unversity, University of Wisconsin-Madison, McMaster University, California State University - Long Beach, National Chiao Tung University, National Chiao Tung University, Taiwan, Ohio University, University of Rennes, UMR CNRS 6251, Rennes, France, University of Bordeaux, UMR 5804, Floirac, France, University of Bordeaux, UMR 5255, Talence, France, Sandia National laboratories, Livermore, CA, Department of Chemistry, West Virginia University, Morgantwon, WV, The Pennsylvania State University, University of Toronto, Departamento de F\'isica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile, Cinvestav-Unidad Queretaro, Queretaro-76230, Mexico, Department of Physics and Astronomy, West Virginia University, Morgantown, WV-26505-6315, USA, The University of Arizona, TU Dortmund, Dortmund, Naval Air Warfare Center - Aircraft Division