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NOAO Newsletter - NOAO Highlights - September 2000 - Number 63


High Altitude Clouds

Based on a Contribution Solicited from Chris Howk

Chris Howk (Johns Hopkins) and Blair Savage (Wisconsin) are taking advantage of the excellent image quality provided by the WIYN 3.5-m telescope to study the interstellar medium (ISM) in the "disk-halo interface" of massive spiral galaxies in the local universe. Their optical broadband images of large edge-on spirals have revealed complex webs of dusty material stretching kiloparsecs away from the midplanes of these systems. Such clouds are seen in approximately 70% of the edge-on spirals in the local universe.

The presence of interstellar matter at large distances from the planes of spiral galaxies can be understood if the combined effects of supernovae and stellar winds provide enough kinetic energy to their surroundings to lift some of the material from the thin disks of these systems. In particular, the correlated explosions of multiple supernovae in OB associations are expected to shape the local interstellar material on large scales, perhaps opening an interstellar "chimney" through which hot, high-pressure gas can escape into the halo of a galaxy.

Caption: Two views of a WIYN V-band image of the edge-on spiral galaxy NGC 891, which exhibits prominent dust-bearing clouds in a thickened disk of interstellar material. The left panel shows the direct V-band image; the right panel shows the same image after the application of an unsharp masking technique. Individual clouds, observable through their extinction of background stellar light, have sizes on the order of a few hundred parsecs.

Such scenarios can provide for the presence of material far above the planes of spirals, but the dusty material viewed in the WIYN images is extraor-dinary. The clouds seen in these images are only visible because they are more dense than their immediate surroundings. Estimates of the masses of individual clouds seen in the WIYN images invariably suggest total gas masses in excess of 105 M. The clouds seen in these images are not tracing the hot, low-density gas expelled through interstellar chimneys. Although it is possible these images are showing the dense, swept-up walls of the chimneys through which this hot material flows, Howk and Savage suggest that the morphological structure of the material, its density, and lack of associated ionized material argue that the picture cannot be this simple. Instead, it is believed that the WIYN images are showing dense, probably molecular, material in the thickened interstellar disks of spirals. Such clouds could form via cooling instabilities at the intersections between the walls of several chimneys, or as expelled matter falls back toward the disks of the galaxies, sweeping up and shocking material in front of it.

Whatever the method of forming these high-z clouds, Howk and Savage estimate that 10% of the dense phase of the ISM in most spirals is found at large distances from the plane. In addition, there is preliminary evidence in the WIYN images for the presence of young OB associations at heights of 600-2000 parsecs from the planes of some of these spirals. If their identifications are confirmed spectroscopically in an upcoming program with the KPNO 4-m, the observed heights of these objects from the midplane are too large to be explained by formation in and subsequent ejection from the thin Population I disk. Instead, these objects may represent a newly identified mode of star formation in spiral galaxies: star formation in the thickened disks of interstellar material.

Caption: A close-up of the central regions of NGC 891. The insets (each 350 parsecs on a side) show a possible OB association and associated H II region centered approximately 1400 parsecs above the midplane of the galaxy, as seen in the V-band and H images.


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