1996/02/29 by F. Prada, Francisco Prada, C. Gutierrez +3 · 2 citations
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Brightness #Bulge #Galaxies: Formation, Evolution, Phenomena #Galaxy #Physics #RADIUS #Radial velocity #Spectral line #Stars #Stellar, planetary, and galactic studies #Surface brightness #astro-ph
paper · pdf · doi:10.1086/310044
13 pages latex, including 4 figures and 1 B/W plate. Accepted for Astrophysical Journal Letters. Revised version incorporating some small last-minute changes
arxiv created 1996/03/14 · openalex publication_date 1996/05/20 · arxiv updated 2017/01/04 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
We have 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'', the line-of-sight velocity distribution of the absorption lines has two distinct peaks and can be decomposed into a fast-rotating component with v /σ > 3, and a slower rotating, retrograde component with v /σ ~ 1-1.5. 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. We find that 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 us believe that it was formed from infalling material in either stellar or gaseous form (see, e.g. Balcells & Quinn). Another possibility, however, 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.