H II region ionization of the interstellar medium: a case study of NGC 7538

Journal Article · Mon Jun 20 04:00:00 EDT 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/824/2/125· OSTI ID:22868926

Luisi, Matteo; Anderson, L. D. ^[1]; Balser, Dana S. ^[2]; Bania, T. M. ^[3]; Wenger, Trey V. ^[4]

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1. Department of Physics and Astronomy, West Virginia University, Morgantown WV 26506 (United States)
2. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville VA 22903-2475 (United States)
3. Institute for Astrophysical Research, Department of Astronomy, Boston University, 725 Commonwealth Ave., Boston MA 02215 (United States)
4. Astronomy Department, University of Virginia, P.O. Box 3818, Charlottesville VA 22903-0818 (United States)

Using data from the Green Bank Telescope, we analyze the radio continuum (free–free) and radio recombination line (RRL) emission of the compact H ii region NGC 7538 (Sharpless 158). We detect extended radio continuum and hydrogen RRL emission beyond the photodissociation region (PDR) toward the north and east, but a sharp decrease in emission toward the south and west. This indicates that a non-uniform PDR morphology is affecting the amount of radiation “leaking” through the PDR. The strongest carbon RRL emission is found in the western PDR that appears to be dense. We compute a leaking fraction f {sub R} = 15 ± 5% of the radio continuum emission measured in the plane of the sky which represents a lower limit when accounting for the three-dimensional geometry of the region. We detect an average {sup 4}He{sup +}/H{sup +} abundance ratio by number of 0.088 ± 0.003 inside the H ii region and a decrease in this ratio with increasing distance from the region beyond the PDR. Using Herschel Space Observatory data, we show that small dust temperature enhancements to the north and east of NGC 7538 coincide with extended radio emission, but that the dust temperature enhancements are mostly contained within a second PDR to the east. Unlike the giant H ii region W43, the radiation leaking from NGC 7538 seems to only affect the local ambient medium. This suggests that giant H ii regions may have a large effect in maintaining the ionization of the interstellar medium.

• OSTI ID: 22868926
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 824; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
CARBON
COSMIC DUST
DISSOCIATION
EMISSION
HELIUM IONS
HYDROGEN
HYDROGEN IONS
INTERSTELLAR SPACE
IONIZATION
ISOTOPE RATIO
PHOTOLYSIS
RECOMBINATION
TELESCOPES
THREE-DIMENSIONAL CALCULATIONS