| Abstract | We combine broad-band and narrow-band CCD images, K-band photometry, long-slit optical spectroscopy, and 21-cm radio observations to investigate the nature of the hitherto unclassified field galaxy UGC 11000. The spectroscopy and 21-cm data yield a heliocentric velocity of 1400 km s^-1^ for UGC 11000, which is about a factor of 4 larger than previously assumed. We study the morphology of the galaxy by a difference-frame technique that enhances deviations from the overall light distribution. Whereas the outer isophotes have a regular, elliptical form, the inner ~70% of the galaxy are dominated by a barred-spiral pattern. Extending from a small, centrally located nucleus is an 5-shaped bar that is twisted by about 20^deg^ to 30^deg^ with respect to the outer isophotes. Two spiral-arm fragments are visible oriented perpendicularly to the bar. The galaxy is clearly of a late type, but since some spiral arms remain discernible, we put forward a classification as SB (s)dm IV for UGC 11000. The total absolute corrected blue magnitude is - 17.8. For a Hubble constant of 50 km s^-1^ Mpc^-1^, the distance to UGC 11000 becomes 28 Mpc^-1^ its diameter is 7 kpc. The morphology, colors, luminosity profiles, and spectra are used to study the stellar populations of UGC 11000. We distinguish three dominant stellar populations: a bulge with colors corresponding to early-F main- sequence stars, a young disk population with median colors of late-A type associated with the spiral-arm pattern and numerous, giant H I, regions, and an older population of G to K stars corresponding to the outermost regions of the galaxy displaying regular, elliptical contours. The H II, regions have sizes of typically 2.2", or 0.3 kpc, normal excitations and abundances, and the reddened emission lines indicate the presence of hotter-than-A stars in the spiral arms. The giant H II regions form a ring pattern which might trace the Outer Lindblad Resonance predicted for barred potentials. The star-forming activity of the galaxy is high in this H II-region ring, whereas the nucleus and the bar appear to be inactive. UGC 11000 was also detected by IRAS. Although the field might be confused by galactic cirrus. it is likely that the IRAS emission comes from dust heated by young stars embedded in the H II regions. The galaxy has a sizeable amount of neutral gas associated with it, providing plenty of fuel for ongoing massive-star formation. UGC 11000 appears to be a field galaxy at its new velocity, meaning that it is not an obvious member of a group of galaxies. It has probably evolved without any environmental influence. The location of the H II regions is consistent with model expectations for barred spirals. Hence. a density-wave scenario seems appropriate even on this small a scale, i. e., dwarf-galaxy size, and there is no need to call for external triggering mechanisms. |
