The Structure and Metallicity Gradient in the Extreme Outer Disk of NGC 7793

Studies of outer regions of spirals disks are fundamental to our understanding of both the process of galaxy assembly and the subsequent secular evolution of galaxies. In an earlier series of papers we explored the extent and abundance gradient in the outer disk of NGC 300 and found an extended purely exponential disk with a metallicity gradient which flattens off in the outermost regions. We now continue the study of outskirts of pure disk spirals with another Sculptor Group spiral, NGC 7793. Using Gemini Multi Object Spectrograph camera at Gemini South, we trace the disk of NGC 7793 with star counts out to ~9 scale lengths, corresponding to 11.5 kpc at our calibrated distance of 3.61+/-0.53 Mpc. The outer disk of NGC 7793 shows no evidence of a break in its light profile down to an effective surface brightness of ~30 mag/arcsec^2 (~3 mag/arcsec^2 deeper than what has been achieved with surface photometry) and exhibits a non-negative abundance gradient within the radial extent of our data.Comment: 11 pages, 9 figures, ApJ accepte

Isolated Star Formation: A Compact HII Region in the Virgo Cluster

We report on the discovery of an isolated, compact HII region in the Virgo cluster. The object is located in the diffuse outer halo of NGC 4388, or could possibly be in intracluster space. Star formation can thus take place far outside the main star forming regions of galaxies. This object is powered by a small starburst with an estimated mass of \sim 400\msun and age of \sim 3\myr. From a total sample of 17 HII region candidates, the present rate of isolated star formation estimated in our Virgo field is small, \sim 10^{-6} Msun arcmin}^{-2} yr^{-1}. However, this mode of star formation might have been more important at higher redshifts and be responsible for a fraction of the observed intracluster stars and total cluster metal production. This object is relevant also for distance determinations with the planetary nebula luminosity function from emission line surveys, for high-velocity clouds and the in situ origin of B stars in the Galactic halo, and for local enrichment of the intracluster gas by Type II supernovae.Comment: 5 pages, LaTeX, 1 figure. ApJ Letters, in press (scheduled Dec 1, 2002

The Properties and Gaseous Environments of Powerful Classical Double Radio Galaxies

The properties of a sample of 31 very powerful classical double radio galaxies with redshifts between zero and 1.8 are studied. The source velocities, beam powers, ambient gas densities, total lifetimes, and total outflow energies are presented and discussed. The rate of growth of each side of each source were obtained using a spectral aging analysis. The beam power and ambient gas density were obtained by applying the strong shock jump conditions to the ends of each side of the source. The total outflow lifetime was obtained by applying the power-law relationship between the beam power and the total source lifetime derived elsewhere for sources of this type, and the total outflow energy was obtained by combining the beam power and the total source lifetime. Composite profiles were constructed by combining results obtained from each side of each source. The composite profiles indicate that the ambient gas density falls with distance from the central engine. The source velocities, beam powers, total lifetimes, and total energies seem to be independent of radio source size. This is consistent with the standard model in which each source grows at a roughly constant rate during which time the central engine puts out a roughly constant beam power. The fact that the total source lifetimes and energies are independent of radio source size indicates that the sources are being sampled at random times during their lifetimes.Comment: 7 pages, 5 figures, to appear in "Extragalactic Jets: Theory and Observation from Radio to Gamma Ray", eds. T. A. Rector and D. S. De Young, ASP conference series, Replaced version has minor textual correction

Extended HI spiral structure and the figure rotation of triaxial dark halos

The HI disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to 22 optical scalelengths and shows spiral arms reaching far beyond the optical component. None of the previous theories for spiral structure provide likely explanations for these very extended spiral arms. Our numerical simulations first demonstrate that such large spiral arms can form in an extended gas disk embedded in a massive triaxial dark matter halo with slow figure rotation, through the strong gravitational torque of the rotating halo. We then show that the detailed morphological properties of the developed spirals and rings depend strongly on the pattern speed of the figure rotation, the shape of the triaxial halo, and the inclination of the disk with respect to the plane including the triaxial halo's long and middle axes. These results strongly suggest that the dark matter halo of NGC 2915 is triaxial and has figure rotation. Based on these results, we also suggest that dynamical effects of triaxial halos with figure rotation are important in various aspect of galaxy formation and evolution, such as formation of polar ring galaxies, excitation of non-axisymmetric structures in low surface-brightness galaxies, and gas fueling to the central starburst regions of BCDs.Comment: 13 pages 2 figures (fig.2 = jpg format), accepted by ApJ

The dark matter halo shape of edge-on disk galaxies: IV. UGC 7321

This is the fourth paper in a series in which we attempt to put constraints on the flattening of dark halos in disk galaxies. We observed for this purpose the HI in edge-on galaxies, where it is in principle possible to measure the force field in the halo vertically and radially from gas layer flaring and rotation curve decomposition respectively. As reported in earlier papers in this series we have for this purpose analysed the HI channel maps to accurately measure all four functions that describe as a function of galactocentric radius the planar HI kinematics and 3D HI distribution of a galaxy: the radial HI surface density, the HI vertical thickness, the rotation curve and the HI velocity dispersion. In this paper we analyse these data for the edge-on galaxy UGC 7321. We measured the stellar mass distribution (M=3 × 108 M⊙ with M/LR\la≈ 0.2), finding that the vertical force of the gas disk dominates the stellar disk at all radii. Measurements of both the rotation curve and the vertical force field showed that the vertical force puts a much stronger constraint on the stellar mass-to-light ratio than rotation curve decomposition. Fitting of the vertical force field derived from the flaring of the HI layer and HI velocity dispersion revealed that UGC 7321 has a spherical halo density distribution with a flattening of q = c/a = 1.0 ± 0.1. However, the shape of the vertical force field showed that a non-singular isothermal halo was required, assuming a vertically isothermal HI velocity dispersion. A pseudo-isothermal halo and a gaseous disk with a declining HI velocity dispersion at high latitudes may also fit the vertical force field of UGC 7321, but to date there is no observational evidence that the HI velocity dispersion declines away from the galactic plane. We compare the halo flattening of UGC 7321 with other studies in the literature and discuss its implications. Our result is consistent with new n-body simulations which show that inclusion of hydrodynamical modelling produces more spherical halos

The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128

The halos of elliptical galaxies are faint and difficult to explore, but they contain vital clues to both structure and formation. We present the results of an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby elliptical NGC 5128. We extend the work of Hui et al.(1995) well into the halo of the galaxy--out to distances of 100 and 50 kpc along the major and minor axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these 780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe exist at distances up to 80 kpc (~15 r_e), showing that the stellar halo extends to the limit of our data. This study represents by far the largest kinematic study of an elliptical galaxy to date, both in the number of velocity tracers and in radial extent. We confirm the large rotation of the PNe along the major axis, and show that it extends in a disk-like feature into the halo. The rotation curve of the stars flattens at ~100 km/s with V/sigma between 1 and 1.5, and with the velocity dispersion of the PNe falling gradually at larger radii. The two-dimensional velocity field exhibits a zero-velocity contour with a pronounced twist, showing that the galaxy potential is likely triaxial in shape, tending toward prolate. The total dynamical mass of the galaxy within 80 kpc is ~5 x 10^{11} M_sun, with M/L_B ~ 13. This mass-to-light ratio is much lower than what is typically expected for elliptical galaxies.Comment: 21 pages, 13 figures (figures 3-8 best viewed in color), accepted for publication in the Astrophysical Journa