Robert Sanders

MASS DISCREPANCIES IN GALAXIES - DARK MATTER AND ALTERNATIVES

Neutral hydrogen line observations of the extended rotation curves of spiral galaxies imply that there exist significant discrepancies between the luminous and dynamical mass beyond the bright optical discs. This means either that galaxies contain significant quantities of non-luminous matter (matter with a mass-to-light ratio very much higher than that of ordinary stars), or that the law of gravity on the scale of galaxies is not the usual Newtonian inverse square law. Attempts to account for the observed discrepancy in the context of these two explanations are reviewed here with emphasis given to the second and less conventional alternative. It is argued that the standard picture of spiral galaxy halo and disc formation in the context of cold dark matter cannot account for the observed systematics of the discrepancy — notably rotation curves which are seen to be flat and featureless from the bright inner regions where the visible matter dominates the dynamics (in some cases overwhelmingly) to the outer regions where the dark halo dominates. It is demonstrated that in those galaxies with well-observed rotation curves, the discrepancy apparently appears below a critical acceleration. Any dark matter explanation of the discrepancy must account for this fact. Moreover, this would also eliminate empirically motivated modifications of Newtons law in which the deviation from 1/r occurs beyond a fundamental length scale. The suggestion by Milgrom in which the force law becomes essentially 1/r below a critical acceleration (MOND) can account for most of the observed systematics of galaxy rotation curves and, significantly, leads to the observed luminosity-velocity relationship in spiral galaxies (the Tully-Fisher law). Generally covariant theories of gravity which predict this phenomenology in the weak-field limit are described. Although there is not yet a theory which obviously meets all of the requirements for a physically viable alternative to dark matter, a generalized scalar-tensor theory of the form suggested by Bekenstein (phase coupling gravitation) is the currently leading candidate and has the advantage of being testable locally.

GAS AS A TRACER OF BARRED SPIRAL DYNAMICS

The gravitational field of a barred spiral galaxy is described by a simple model consisting of two variable components: a central or nuclear disk and a homogeneous triaxial spheroid: and one fixed component: an extended disk or halo. The model is characterized by five free dimensionless parameters: the ratio of bar mass to central disk mass, two axial ratios of the spheroid, the ratio of the central disk length scale to the spheroid semimajor axis, and the corotation radius in units of the spheroid semimajor axis. Two-dimensional, time-dependent gas dynamical calculations are carried out in the gravitational field resulting from this mass model in order to generate a grid of hydrodynamical models on the five-dimensional parameter space. It is found that over a well-defined domain of this parameter space, the steady-state gas density and velocity distributions resemble the observed morphology and kinematics of the gas in actual SBb galaxies.

THE RECONFINEMENT OF JETS

The jets in several extragalactic radio sources appear to open with a more or less constant angle out to some critical distance from the nucleus, and then broaden much less rapidly or even become more narrow. This reconfinement shoulder seems to be a fairly typical property of jets observed with sufficiently high resolution to map the structure transverse to the jet axis. This may be due to the fact that the internal pressure in a free jet decreases very rapidly with distance from the nucleus, and therefore the jet would be expected to come into pressure equilibrium with an ambient medium. But the flow in free jets must be highly supersonic; thus, the process of reconfinement is certain to be accompanied by shocks. In the present paper, the structure of inviscid steady state jets in the presence of a realistic ambient medium is calculated by means of the method of characteristics. It is shown that reconfinement is accompanied by conical shocks which heat the jet causing it to reexpand as free jet. The typical jet shape can be reproduced by this process. If particle acceleration results from internal shocks, multiple attempts at reconfinement could produce symmetric knots in a two-sidedmorexa0» jet symmetrically placed with respect to the ambient medium.«xa0less

Explosions in galactic nuclei and the formation of double radio sources

Numerical hydrodynamical experiments have been carried out which attempt to define the conditions under which an initially isotropic explosion occurring at the center of a gaseous disk presumed to be in the nucleus of a giant elliptical galaxy will be directed along the rotational axis of the disk by the disk geometry itself. A very general condition for focusing an explosion is that the mass isotropically liberated by the explosion be less than the mass of the ambient disk medium within one vertical scale height of the explosion center. Moreover, it is shown that the degree of focusing necessary to account to double radio sources can occur only if the disk is initially in equilibrium with a highly centrally condensed gravitational field. (AIP)

NEUTRAL HYDROGEN IN ELLIPTICAL GALAXIES - A BIMODAL DISTRIBUTION

A sample of 46 elliptical galaxies with published neutral hydrogen upper limits of detections is analyzed by a maximum likelihood technique. The sample is most likely drawn from a parent distribution which is bimodal in the ratio of hydrogen mass to blue luminosity (M/sub H/ /sub i//L/sub B/). The character of this bimodal distribution is such that about 70% of all ellipticals are undetectable at the sensitivities of present surveys (M/sub Hi//L/sub B/ <0.003) and the remainder contain neutral hydrogen at the level of M/sub Hi//L/sub B/roughly-equal0.03. The significance of this bimodal distribution is high even if galaxies in the core of the Virgo cluster are excluded from the sample.

HOT GUSTS FROM ELLIPTICAL GALAXIES

The pulsed wind is an intrinsic mechanism for removing gas from elliptical galaxies and is based upon the suggestion that Type I supernovae originate in a young stellar population. In this model, a period of gas accumulation is followed by cooling and star formation which is followed by a burst of supernova activity and a short-lived but intense wind. The process repeats on a time scale of (5--10) x 10/sup 8/ years. The model leads naturally to a bimodal distribution of elliptical galaxies in neutral hydrogen content, with ellipticals being either gas rich at the level of 10/sup 8/--10/sup 9/ M/sub sun/ of H I or gas poor with essentially no neutral hydrogen.

NONCIRCULAR GAS MOTIONS IN THE INNER GALAXY

It is shown that the observed motion of neutral hydrogen in the inner 1000 pc of the Galaxy is, for the most part, consistent with flow on circular streamlines in the potential of the Galactic bulge as derived from the observed distribution of near infrared emission. The implied mass distribution is also consistent with recent kinematic determinations of the stellar mass in the inner few parsecs of the bulge. The non-circular gas motion seen between two and four kpc is most likely due to flow on elliptical streamlines in the presence of a weak bar distortion of the Galactic disk. Circular gas motion in the region of the bulge and elliptical streaming further out is an observed characteristic of flow in barred galaxies and is consistent with our present theoretical understanding of such systems. The implication is that non-circular motions of the molecular clouds in the inner 200 pc have a non-gravitational origin. A possible mechanism for exciting such motions is an accretion event resulting from an encounter of a molecular cloud with a massive black hole. A starburst leading to a high supernovae rate 107 years ago in the inner 50 pc is an alternative explanation. Observations of molecular cloud regions in the nuclei of external normal galaxies could distinguish between alternative mechanisms.

GRAVITATIONAL IMAGING BY SUPERCLUSTERS

La plus grande part de la matiere visible dans lUnivers semble concentree dans des superamas-filaments fins et longs (10-100 Mpc) separes par des vides de meme etendue. Si la densite lineaire totale dans ces structures est de lordre de 10 15 M ○. Mpc −1 et la hauteur dechelle perpendiculaire inferieure a 1 Mpc, les superamas doivent se comporter comme des lentilles gravitationnelles (amplification, images multiples) pour les sources du fond avec z>0,5. Cela implique que les superamas se composent de matiere sombre sans lien avec les galaxies individuelles ou les amas de galaxies. On cite plusieurs exemples possibles

Periodic Orbits and Gas Flow in Barred Spiral Galaxies

One purpose for studying the gas flow in barred spiral galaxies is to use the observed distribution and kinematics of the gas as a tracer of the underlying gravitational field. By comparing model hydro dynamical calculations with observations of actual systems, one would like to define three basic properties of barred galaxies: n n1) n nThe bar strength. How significant is the deviation from axial symmetry in the region of the bar, measured by some parameter such as qt, maximum aximuthal force in terms of the mean radial force (Sanders and Tubbs, 1980). n n n n n2) n nThe mean radial distribution of matter. Clearly in a system with large deviations from circular motion, the “rotation curve” gives no direct information on the radial mass distribution. n n n n n3) n nThe angular velocity of the bar. Where is the co-rotation radius (or Lagrange points) with respect to the bar axes? Are other principal resonances present?