DISCOVERY OF A NEW TYPE OF GALAXY: ONE IN WHICH THE BULGE ROTATES RETROGRADE TO THE DISK 

A team of astronomers found that the bulge of the large, nearby Sb galaxy NGC 7331 rotates retrograde to its disk. Analysis of spectra in the region of the near-IR Ca II triplet along the major axis shows that, in the radial range between 5 and 20 arcseconds, the line-of-sight velocity distribution of the absorption lines has two distinct peaks and can be decomposed into a fast-rotating component and a slower rotating, retrograde component. The radial surface brightness profile of the counterrotating component follows that of the bulge, obtained from a two-dimensional bulge-disk decomposition of a near-infrared K-band image, while the fast-rotating component follows the disk. At the radius at which the disk starts to dominate, the isophotes change from being considerably boxy to being very disky.

Although a number of spiral galaxies have been found that contain cold, counterrotating disks, this is the first galaxy known to have a boxy, probably triaxial, fairly warm, counterrotating component, which is dominating in the central regions. If it is a bar seen end-on, this bar has to be thicker than the disk. NGC 7331, even though it is a fairly early-type spiral, does not have a conventional, corotating bulge. The fact that the inner component is retrograde makes the astronomers believe that it was formed from infalling material in either stellar or gaseous form. Another possibility discused by the discoverers is that the structure has been there since the formation of the galaxy. In this case, it will be a challenge to explain the large change in orientation of the angular momentum when going outward radially.