PHYSICAL CONDITIONS IN MOLECULAR CLOUDS IN THE ARM AND INTERARM REGIONS OF M51
- Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800 (United States)
- Department of Astronomy, California Institute of Technology, Pasadena, CA 91125 (United States)
- National Astronomical Observatory of Japan, NAOJ Chile Observatory, Joaquin Montero 3000 Oficina 702, Vitacura, Santiago 763-0409 (Chile)
- Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
- Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (Japan)
- Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA (United Kingdom)
- Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano, 384-1305 (Japan)
- Institute for Cosmic Ray Research, University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa City, Chiba, 277-8582 (Japan)
- Department of Physics/Department of Cosmosciences, Hokkaido University, Kita-ku, Sapporo 060-0810 (Japan)
We report systematic variations in the emission line ratio of the CO J 2-1 and J = 1-0 transitions (R{sub 2-1/1-0}) in the grand-design spiral galaxy M51. The R{sub 2-1/1-0} ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions into the spiral arms and back into the downstream interarm regions. In the interarm regions, R{sub 2-1/1-0} is typically <0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far-infrared luminosities. However, the ratio rises to >0.7 (often 0.8-1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R{sub 2-1/1-0} is also high, {approx}0.8-1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low-density gas ({approx}< 10 K, {approx}< 300 cm{sup -3}) is required for the interarm GMCs, but this gas must become warmer and/or denser in the more active star-forming spiral arms. The ratio R{sub 2-1/1-0} is higher in areas of high 24 {mu}m dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (i.e., a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2-1) line relative to CO(1-0) in luminous star-forming regions suggests that some caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass, especially when galactic structures are resolved.
- OSTI ID:
- 22140080
- Journal Information:
- Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 761; ISSN ASJOAB; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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