The dust lanes in M81 - Tracers of the molecular gas?

Abstract

view Abstract Citations (18) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Dust Lanes in M81: Tracers of the Molecular Gas? Kaufman, Michele ; Elmegreen, Debra Meloy ; Bash, Frank N. Abstract The dust lanes in M81 are analyzed to provide high-resolution information about the distribution of molecular gas and to see how the dust lanes are related to other tracers of spiral structure. Digitized B and I band, H I, radio continuum, and Hα images are used in this study; this set of registered images allows us to measure the locations of the dust lanes relative to the H I gas and relative to the giant radio H II regions. The internal extinction of the dust features is determined from measurements on the B and I images. Many of the filaments appear to be clouds 100-200 pc above the midplane, rather than dust lanes in the midplane. By comparing the extinction of a dust filament with the H I surface density in the same direction, we deduce that the filaments are composed mainly of molecular gas. Although a few short dust filaments are found near the spiral velocity shock front in the H I gas, these filaments have neither greater length nor greater extinction than the filaments farther downstream. In fact, narrow dust filaments are spread over a zone that starts near the spiral shock front and extends 1.5 kpc downstream. Thus, the dust lane morphology in M81 differs from that in M51 and M83, where dominant, long dust lanes are situated along the inside edge of the spiral arms. In Elmegreen's theory of magnetic cloud collision fronts, the absence of a prominent, long, molecular dust lane at the spiral shock front in M81 follows from the fact that the average density-wave compression in M81 is about half that in M51. A comparison of the spiral arms of M81, M33, and M51 suggests that the dust lane morphology, the CO luminosity, and the ratio of free-free to nonthermal radio emission are characteristics that seem to be sensitive to the amplitude of the density-wave compression but do not necessarily correlate with the arm class. Half of the dust filaments measured in M81 show an alignment suggestive of a global instability produced by galactic shear. The dust filaments with high opacity often abut the upstream and downstream edges of giant H II region complexes. The dust features on the downstream side of the strings of giant H II regions have roughly the same internal extinction as those on the upstream side. These high-extinction filaments may be parts of dense shells produced by the expanding H II regions and driven out of the disk by radiation pressure. Publication: The Astrophysical Journal Pub Date: October 1989 DOI: 10.1086/167942 Bibcode: 1989ApJ...345..697K Keywords: Cosmic Dust; Galactic Structure; Interstellar Matter; Molecular Gases; Spiral Galaxies; Carbon Monoxide; Hydrogen Clouds; Interstellar Gas; Shock Fronts; Astrophysics; GALAXIES: INDIVIDUAL MESSIER NUMBER: M81; GALAXIES: INTERSTELLAR MATTER; GALAXIES: STRUCTURE; INTERSTELLAR: GRAINS full text sources ADS | data products SIMBAD (4) NED (4)