CLUMPING AND THE INTERPRETATION OF kpc-SCALE MAPS OF THE INTERSTELLAR MEDIUM: SMOOTH H I AND CLUMPY, VARIABLE H{sub 2} SURFACE DENSITY
- National Radio Astronomy Observtory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
- Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904 (United States)
- California Institute for Technology, 1200 E California Blvd, Pasadena, CA 91125 (United States)
- Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
- Max Planck Institute fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
- Institut de Radioastronomie Millimetrique, 300 Rue de la Piscine, F-38406 Saint Martin d'Heres (France)
Many recent models consider the structure of individual interstellar medium (ISM) clouds as a way to explain observations of large parts of galaxies. To compare such models to observations, one must understand how to translate between surface densities observed averaging over large ({approx}kpc) scales and surface densities on the scale of individual clouds ({approx}pc scale), which are treated by models. We define a ''clumping factor'' that captures this translation as the ratio of the mass-weighted surface density, which is often the quantity of physical interest, to the area-weighted surface density, which is observed. We use high spatial resolution (sub-kpc) maps of CO and H I emission from nearby galaxies to measure the clumping factor of both atomic and molecular gas. The molecular and atomic ISM exhibit dramatically different degrees of clumping. As a result, the ratio H{sub 2}/H I measured at {approx}kpc resolution cannot be trivially interpreted as a cloud-scale ratio of surface densities. H I emission appears very smooth, with a clumping factor of only {approx}1.3. Based on the scarce and heterogeneous high-resolution data available, CO emission is far more clumped with a widely variable clumping factor, median {approx}7 for our heterogeneous data. Our measurements do not provide evidence for a universal mass-weighted surface density of molecular gas, but also cannot conclusively rule out such a scenario. We suggest that a more sophisticated treatment of molecular ISM structure, one informed by high spatial resolution CO maps, is needed to link cloud-scale models to kpc-scale observations of galaxies.
- OSTI ID:
- 22130641
- Journal Information:
- Astrophysical Journal Letters, Vol. 769, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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