NGC 4138 - A Case Study in Counterrotating Disk Formation

The Sa(r) galaxy NGC 4138 has been recently found to contain an extensive counterrotating disk which appears to be still forming. Up to a third of the stars in the disk system may be on retrograde orbits. A counterrotating ring of H II regions, along with extended counterrotating H I gas, suggests that the retrograde material has been recently acquired in the gas phase and is still trickling in. Using numerical simulations, we have attempted to model the process by which the counterrotating mass has been accreted by this galaxy. We investigate two possibilities: continuous retrograde infall of gas, and a retrograde merger with a gas-rich dwarf galaxy. Both processes are successful in producing a counterrotating disk of the observed mass and dimensions without heating up the primary significantly. Contrary to our experience with a fiducial cold, thin primary disk, the gas-rich merger works well for the massive, compact primary disk of NGC 4138 even though the mass of the dwarf galaxy is a significant fraction of the mass of the primary disk. Although we have restricted ourselves mainly to coplanar infall and mergers, we report on one inclined infall simulation as well. We also explore the possibility that the H-alpha ring seen in the inner half of the disk is a consequence of counterrotating gas clouds colliding with corotating gas already present in the disk and forming stars in the process.Comment: To appear in ApJ, 21 pages, LaTeX (aaspp4) format, 17 figs (gzipped tar file) also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr2/ or at http://www-astronomy.mps.ohio-state.edu/~thakar

Kinematic Evidence of Minor Mergers in Normal Sa Galaxies: NGC3626, NGC3900, NGC4772 and NGC5854

BVRI and H-alpha imaging and long-slit optical spectroscopic data are presented for four morphologically normal and relatively isolated Sa galaxies, NGC3626, NGC3900, NGC4772 and NGC5854. VLA HI synthesis imaging is presented for the first 3 objects. In all 4 galaxies, evidence of kinematic decoupling of ionized gas components is found; the degree and circumstances of the distinct kinematics vary from complete counterrotation of all of the gas from all of the stars (NGC3626) to nuclear gas disks decoupled from the stars (NGC5854) to anomalous velocity central gas components (NGC3900 and NGC4772). In the 3 objects mapped in HI, the neutral gas extends far beyond the optical radius, R_HI/R_25 > 2. In general, the HI surface density is very low and the outer HI is patchy and asymmetric or found in a distinct ring, exterior to the optical edge. While the overall HI velocity fields are dominated by circular motions, strong warps are suggested in the outer regions. Optical imaging is also presented for NGC 4138 previously reported by Jore et al. (1996) to show counterrotating stellar components. The multiwavelength evidence is interpreted in terms of the kinematic "memory" of past minor mergers in objects that otherwise exhibit no morphological signs of interaction.Comment: 26 pages, 15 figures, accepted for publication in Astron. J., postscript figures available at ftp://culebra.tn.cornell.edu/pub/haynes/figures.tar.g