Linda S. Sparke

Line variations in quasars and Seyfert galaxies

Quantitative estimates of the sizes of broad-line regions in three well-studied Seyfert I galaxies are presented, and differences between these and previously reported size estimates for NGC 4151 are discussed. Both Lick Observatory and IUE satellite data on NGC 4151 are analyzed, and it is shown that both are consistent with the high-ionization filaments (those emitting C IV, He II, and the high-order Balmer lines) mostly lying about 5 lt-day from the central source. The low-ionization filaments are 3 or 4 times further out. The Ohio State data on Akn 120 show the H-beta emitting region to be about 6 lt-day from the central source, and IUE data suggest that the Ly-alpha region in Mrk 509 is about 10 lt-day out. The present results imply that the densities in broad-line regions are a couple of orders of magnitude higher than in current standard models. For the objects studied here, the light crossing times are of the same order of magnitude as the continuum variability time scales, so kinematic mapping of the broad-line regions will be extremely difficult. 56 references.

The Quadruple Gravitational Lens PG 1115+080: Time Delays and Models

Optical photometry is presented for the quadruple gravitational lens PG 1115+080. A preliminary reduction of data taken from 1995 November to 1996 June gives component C leading component B by 23.7 ? 3.4 days and components A1 and A2 by 9.4 days. A range of models has been fitted to the image positions, none of which gives an adequate fit. The best-fitting and most physically plausible of these, taking the lensing galaxy and the associated group of galaxies to be singular isothermal spheres, gives a Hubble constant of 42 km s-1 Mpc-1 for ? = 1, with an observational uncertainty of 14%, as computed from the B - C time delay measurement. Taking the lensing galaxy to have an approximately E5 isothermal mass distribution yields H0 = 64 km s-1 Mpc-1, while taking the galaxy to be a point mass gives H0 = 84 km s-1 Mpc-1. The former gives a particularly bad fit to the position of the lensing galaxy, while the latter is inconsistent with measurements of nearby galaxy rotation curves. Constraints on these and other possible models are expected to improve with planned Hubble Space Telescope observations.

Gas inflow in barred galaxies: effects of secondary bars

We report results of high-resolution hydrodynamical simulations of gas flows in barred galaxies, with a focus on gas dynamics in the central kiloparsec. In a single bar with an inner Lindblad resonance, we find either near-circular motion of gas in the nuclear ring, or a spiral shock extending towards the galaxy centre, depending on the sound speed in the gas. From a simple model of a dynamically possible doubly barred galaxy with resonant coupling, we infer that the secondary bar is likely to end well inside its corotation. Such a bar cannot create shocks in the gas flow, and therefore will not reveal itself in colour maps through straight dust lanes: the gas flows induced by it are different from those caused by the rapidly rotating main bars. In particular, we find that secondary stellar bars are unlikely to increase the mass inflow rate into the galactic nucleus.

An Imaging Survey of Early-Type Barred Galaxies

This paper presents the results of a high-resolution imaging survey, using both ground-based and Hubble Space Telescope images, of a complete sample of nearby barred S0-Sa galaxies in the field, with a particular emphasis on identifying and measuring central structures within the bars: secondary bars, inner disks, nuclear rings and spirals, and off-plane dust. A discussion of the frequency and statistical properties of the various types of inner structures has already been published. Here we present the data for the individual galaxies and measurements of their bars and inner structures. We set out the methods we use to find and measure these structures, and how we discriminate between them. In particular, we discuss some of the deficiencies of ellipse fitting of the isophotes, which by itself cannot always distinguish between bars, rings, spirals, and dust, and which can produce erroneous measurements of bar sizes and orientations.

Orbits supporting bars within bars

High-resolution observations of the inner regions of barred disc galaxies have revealed many asymmetrical, small-scale central features, some of which are best described as secondary bars. Because orbital time-scales in the galaxy centre are short, secondary bars are likely to be dynamically decoupled from the main kiloparsec-scale bars. Here we show that regular orbits exist in such doubly barred potentials, and that they can support the bars in their motion. We find orbits in which particles remain on loops: closed curves which return to their original positions after two bars have come back to the same relative orientation. Stars trapped around stable loops could form the building blocks for a long-lived, doubly barred galaxy. Using the loop representation, we can find which orbits support the bars in their motion, and the constraints on the sizes and shapes of self-consistent double bars. In particular, it appears that a long-lived secondary bar may exist only when an inner Lindblad resonance is present in the primary bar, and that it would not extend beyond this resonance.

The dark halo of the polar-ring galaxy NGC 4650A

By analyzing the rotation of its central disk and outer polar ring, limits are placed on the flattening of the unseen halo of the galaxy NGC 4650A. A model consisting of a bulge, disk, axisymmetric dark halo, and massive polar ring is fitted to both rotation curves, and it is shown that the observations are consistent with a range of halo flattening, from spherical to as flat as an E8 elliptical galaxy. The best fits are obtained for dark halo flattenings in the range E3-E7. The substantial mass of the ring is probably responsible for a characteristic wiggle in the polar rotation curve; fits to this feature suggest that there may be considerably more mass in the ring than has been reported from neutral hydrogen measurements. 30 refs.

Triple Bars and Complex Central Structures in Disk Galaxies

We present an analysis of ground-based and Hubble Space Telescope images of three early-type barred galaxies. The first, NGC 2681, may be the clearest example yet of a galaxy with three concentric bars. The two other galaxies were previously suggested to be triple barred. Our analysis shows that while NGC 3945 is probably double barred, NGC 4371 has only one bar; but both have intriguing central structures. NGC 3945 has a large, extremely bright disk inside its primary bar, with patchy dust lanes, a faint nuclear ring or pseudoring within the disk, and an apparent secondary bar crossing the ring. NGC 4371 has a bright nuclear ring that is only marginally bluer than the surrounding bulge and bar. There is no evidence for significant dust or star formation in either of these nuclear rings. The presence of stellar nuclear rings suggests that the centers of these galaxies are dynamically cool and disklike.

Polar Ring Galaxies and the Tully-Fisher Relation: Implications for the Dark Halo Shape

We have investigated the Tully-Fisher relation for Polar Ring Galaxies (PRGs), based on near infrared, optical and HI data available for a sample of these peculiar objects. Many PRGs show larger HI line-widths than expected for the observed K band luminosity, and this result is confirmed by a larger sample of objects, based on B-band data. This observational evidence may be related to the dark halo shape and orientation in these systems: the larger rotation velocities observed in PRGs can be explained by a flattened polar halo, aligned with the polar ring.

A Model for Lopsided Galactic Disks

Many disk galaxies are lopsided; their brightest inner parts are displaced from the center of the outer isophotes or the outer contours of the H I disk. This asymmetry is particularly common in small, low-luminosity galaxies. We argue here that long-lived lopsidedness is a consequence of the disk lying off-center in the potential of the galaxys extended dark halo and spinning in a sense retrograde to its orbit about the halo center. The stellar velocity field predicted by our gravitational N-body simulations is clearly asymmetric.

A Dynamical Model for the Twisted Gas Disk in Centaurus A

The prominent minor-axis dust lane across the radio galaxy Cen A has been extensively mapped in the emission lines of Hα, H I, and CO, revealing an ordered velocity field which has been interpreted as that of a warped and twisted disk of gas seen nearly edge-on. Here it is shown that the geometry of this disk can be explained as that of a near-polar structure precessing about the symmetry axis of an approximately oblate galaxy potential which, like the stellar body, is nearly round at the center, becoming flattened farther out. Both freely precessing models and those taking account of the self-gravity of the gas are presented. The broken elliptical ring of H I gas recently found to be associated with the faint outer shells of Cen A can be explained naturally as a continuation of the same precessing disk. This precessing model for the gas disk assumes that the gravitational potential of Cen A is close to axisymmetric; precession should be qualitatively similar if the figure is triaxial and tumbling about its short axis in the same sense as the rotation of the stars.