A Counterrotating Bulge in the Sb Galaxy NGC 7331

We have found that the bulge of the large, nearby Sb galaxy NGC 7331 rotatesretrograde to its disk. Analysis of spectra in the region of the near-IR Ca IItriplet along the major axis shows that, in the radial range between 5'' and~20'', the line of sight velocity distribution of the absorption lines has twodistinct peaks, and can be decomposed into a fast-rotating component withv/sigma > 3, and a slower rotating, retrograde component with v/sigma between 1-- 1.5. The radial surface brightness profile of the counter-rotating componentfollows that of the bulge, obtained from a 2-dimensional bulge-diskdecomposition of a near-infrared K-band image, while the fast rotatingcomponent follows the disk. At the radius where the disk starts to dominate theisophotes change from being considerably boxy to very disky. Although a number of spiral galaxies have been found that contain cold,couter-rotating disks, this is the first galaxy known to have a boxy, probablytriaxial, fairly warm, counter-rotating component, which is dominating in thecentral regions. If it is a bar seen end-on, this bar has to be thicker thanthe disk. We find that NGC 7331, even though it is a fairly early-type spiral,does not have a conventional, co-rotating bulge. The fact that the innercomponent is retrograde makes us believe that it was formed from infallingmaterial, in either stellar or gaseous form (e.g. Balcells & Quinn 1990).Another possibility however is that the structure has been there since theformation of the galaxy. In this case it will be a challenge to explain thelarge change in orientation of the angular momentum when going outwardradially.Comment: 13 pages latex, including 4 figures and 1 B/W plate. Accepted for Astrophysical Journal Letters. Revised version incorporating some small last-minute change