Stephan's Quintet is a compact group of five galaxies that are strongly gravitationally interacting. This group is shown below, in an optical image from the Digitized Sky Survey.

NGC 7320C, NGC 7319, NGC 7318A, NGC 7318B, and NGC 7317 all have similar velocities, and so are believed to be physically associated. NGC 7320 has a much lower velocity, and so is generally thought to be a foreground galaxy, not part of the group.

To the north of NGC 7318B is a concentration of atomic hydrogen gas with on-going star formation (identified as Star Formation Region A in this figure).

Using the 12 meter telescope on Kitt Peak in Arizona, with Curt Struck (Iowa State), I have searched for the 2.6mm line of carbon monoxide at this position.

Note that two CO lines are detected at this position, at 6000 km/s and 6700 km/s. Using the standard Galactic CO to H2 conversion factor, these lines correspond to molecular gas masses of 109 solar masses (from Smith and Struck 2001, Astronomical Journal, Volume 121, p. 710).

To see another example of extra-disk molecular gas, see my web page on NGC 2782.

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To see Jane Charlton's Hubble Space Telescope image of Stephan's Quintet, click here.