DISCOVERY OF A GIANT COOLING DISK IN NGC 1700

The images below show the young elliptical galaxy NGC 1700, in (left) a relatively short exposure from the Digitized Sky Survey, and (right) a longer exposure from Brown et al. (2000, MNRAS 317 406). The galaxy is ordinary in the inner parts but is parallelogram-shaped at low surface brightness levels. Our stellar kinematic maps show that the outer parts of the galaxy are rapidly rotating.

The results of a 43-kilosecond Chandra observation are shown below. Top left: X-ray contours (green) on the optical image. Over most of the galaxy, the X-ray emission is extremely flat-- far flatter than the starlight. Top right: soft X-ray band (0.3 to 0.8 keV); bottom left: hard X-ray band (0.8 to 2.7 keV); bottom right: hard-to-soft ratio image. Note that the softest emission is the flattest emission, i.e., the flattened emission is most prominent in the soft band, and the flattest regions have the lowest hardness ratios.

The flattened X-ray emission comes from hot interstellar gas in the galaxy. But the flattening is so extreme that, contrary to the usual assumption, the gas cannot be resting comfortably in hydrostatic equilibrium. A likely alternative is that the gas is rotating, and a simple theoretical model for rotating hot gas can reproduce the X-ray shape. The angular momentum of the gas matches that of the stars in the distorted outer part of the galaxy, and the cooling time matches the time since the last major merger. Evidently, the gas was acquired in that merger, and has since gradually settled into a rotationally flattened cooling disk.

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Last updated 31 January 02.