Guy B. Purcell

The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. II. Models

The first paper in this series presented optical (UBVI) and near-infrared (H) surface photometry and Hα Fabry-Perot interferometry of IC 4214, an excellent example of a multiringed galaxy with a weak bar. In the present paper, we analyze the nonaxisymmetric gravitational potential of IC 4214, as defined by the near-infrared data. The gas behavior is modeled with two-dimensional sticky particle simulations that employ a rigidly rotating nonaxisymmetric potential. Our goals are to interpret the morphology and velocity field of IC 4214 and to obtain a model from which the bar pattern speed, halo contribution, and relative bulge and disk masses can be deduced. We also study the orbital families in the derived IC 4214 model. A fairly good match, in both morphology and kinematics, is obtained for Ωbar ≈ 40 ± 5 km s-1 kpc-1, placing the bar corotation slightly beyond the end of the bar (rbar/rcr ≈ 0.72 ± 0.05). The deduced bar amplitude implies that the visible mass of the disk dominates over the unseen halo (Mdisk/Mhalo > 3–4). Moreover, our model accounts quantitatively for the large difference in the orientation of IC 4214 as derived in our first paper with photometric and kinematic methods.

An Optical and H I Study of NGC 5850: Victim of a High-Speed Encounter?

We present optical CCD surface photometry and VLA H I observations of NGC 5850, one of the largest and brightest barred spirals of the inner ring variety in the sky. The broadband images reveal numerous morphological peculiarities, particularly in the spiral arms. Structural asymmetries are more obvious in H I, the most pronounced being a large-scale displacement of gas to the west and northwest of the nucleus. Most of the (3.3 ± 0.1) × 109 M☉ of atomic hydrogen is concentrated in the prominent optical ring and faint spiral arms, with very low H I surface densities (ΣH I) in the bulge and interarm regions. We detect approximately 2 × 107 M☉ of H I in the southwestern half of the nuclear ring. The H I surface density drops rapidly outside the arms, and we find no evidence for either large-scale tidal features or an extended gas disk above 0.05 M☉ pc-2 (3 σ). Overall, the intensity-weighted H I velocity field appears fairly regular, yet still shows clear deviations from circular rotation that we attribute to a warped oval disk and streaming motions across the arms. Radio continuum emission at 20 cm is dominated by a faint bulge component that peaks at the optical nucleus. The spiral arms are not detected. The absence of extended X-ray emission throughout the NGC 5846 group and the close similarity between NGC 5850s optical and H I morphologies argue against ram pressure stripping through a dense intergalactic medium as the cause of the galaxys peculiar morphology. We attribute it instead to a high-speed encounter with the massive elliptical galaxy NGC 5846. We identify the peculiar spiral arms as a disrupted outer pseudoring. Star formation in NGC 5850 has not been enhanced relative to other intermediate field spirals, nor have significant gas masses been transported to the nucleus. This may be attributed to the recent (200 Myr) nature of the interaction.

An Optical, Near-Infrared, and Kinematic Study of Four Early-Type Resonance Ring Galaxies

?????We present optical and near-infrared surface photometry of four early-type galaxies considered to have resonance ring phenomena on the basis of morphology. We also present kinematics and rotation curves for three of the galaxies. The four galaxies are exceptional examples of resonance ring galaxies, and from these observations we derive the characteristic light distributions, the colors of the rings, and the near-infrared disk mass-to-light ratios. We also deproject the galaxies to examine intrinsic ring shapes and bar/ring alignments. From models of the rotation curves, we compute the Lindblad precession frequencies to examine resonance locations in the disks of the galaxies. Under the assumption that the outer rings and pseudorings seen in these galaxies are linked to the outer Lindblad resonance, we find that the inner rings of the sample galaxies lie near the inner 4:1 resonance and that the sharp ends of the bars lie on average at only 0.65 times the predicted radius of corotation.

The Structure and Dynamics of the Early-Type Resonance Ring Galaxy IC 4214. I. Observations

The southern galaxy IC 4214 is a bright, nearby example of a multiple ring early-type spiral galaxy with a weak bar. The galaxy shows three strong pseudoring features, including a nuclear ring, an inner ring, and a prominent R outer ring, each with distinctive characteristics. The galaxy is important because it is a classic resonance ring galaxy where the ring features can be linked to specific orbital resonances with the bar. We present in this paper a detailed UBVIH study of its photometric structure and a Fabry-Perot study of its kinematics to set the stage for a dynamical model in a separate paper. Despite the early Hubble type, ionized gas is well distributed in the inner disk regions, providing a well-sampled velocity field. Rapid rotation is found in the vicinity of the nuclear ring, where the rotation curve reaches a maximum, and the rotation velocities drop off somewhat with increasing radius in the outer regions. Noncircular motions are clearly seen in the observed velocity field. Analysis of both the velocity field and the shapes of isophotes give discrepant values for the inclination of the system, ranging from 47° to 50° from photometry to 55° to 58° from kinematics. A likely value of the inclination could be 52° if the inner and outer rings are intrinsically elongated and aligned perpendicular to each other.

Star formation in leading/trailing single arm galaxies: NGC 4378

de Vaucouleurs (1958) found that all members of a sample of nearly edge-on spirals had trailing arms relative to disk rotation. An exception was provided by the identification of a single arm in the galaxy NGC 4622 by Byrd et al. (1989). Buta, Crocker and Byrd (1992) observationally verified that an inner single arm occurs in the stellar disk of NGC 4622 and that it also shows the color/age position angle sequence of an m=1 leading density wave. Byrd, Freeman and Howard (1993) simulated NGC 4622’s pattern with a small mass companion plunging close to the center in a retrograde orbit in the disk plane. A single long-lived leading arm is created in the central regions. NGC 4378 is also a single armed galaxy (Rubin et al. 1978, Sandage 1961). Byrd, Freeman and Howard (1994) simulated the arm pattern of NGC 4378 by a direct planar grazing encounter with a small galaxy. In contrast to NGC 4622, the simulation arm for NGC 4378 trailed. We use new B and I images and models to determine that NGC 4378’s arm trails...