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Dwarf and low surface brightness galaxies are ideal objects to test modified Newtonian dynamics (MOND), because in most of these galaxies the accelerations fall below the threshold below where MOND supposedly applies. We have selected from the literature a sample of 27 dwarf and low surface brightness galaxies. MOND is successful in explaining the general shape of the observed rotation curves for roughly three quarters of the galaxies in the sample presented here. However, for the remaining quarter, MOND does not adequately explain the observed rotation curves. Considering the uncertainties in distances and inclinations for the galaxies in our sample, a small fraction of poor MOND predictions is expected and is not necessarily a problem for MOND. We have also made fits taking the MOND acceleration constant, a_0, as a free parameter in order to identify any systematic trends. We find that there appears to be a correlation between central surface brightness and the best-fit value of a_0, in the sense that lower surface brightness galaxies tend to have lower a_0. However, this correlation depends strongly on a small number of galaxies whose rotation curves might be uncertain due to either bars or warps. Without these galaxies, there is less evidence of a trend, but the average value we find for a_0 ~ 0.7*10^-8 cm s^-2 is somewhat lower than derived from previous studies. Such lower fitted values of a_0 could occur if external gravitational fields are important., 12 pages, accepted for publication in ApJ

We present MOND analysis for several of the lowest mass disc galaxies currently amenable to such analysis--with (baryonic) masses below 4x10^8 solar masses. The agreement is good, extending the validity of MOND and its predicted mass velocity relation, to such low masses.

The recent findings of Romanowsky et al., of an ``unexpectedly'' small mass discrepancy within 5 effective radii in several elliptical galaxies, are not surprising in the context of MOND. As we show here, they are, in fact, in full concordance with its predictions. One is dealing with high-surface-density galaxies with mean accelerations rather larger than the acceleration constant of MOND. These findings continue, and are now the extreme examples of, the trend predicted by MOND: the mass discrepancy sets in at larger and larger scaled radii in galaxies with larger and larger mean surface densities; or, equivalently, mean accelerations.

A sample of 197 X-ray emitting clusters of galaxies is considered in the context of Milgrom's modified Newtonian dynamics (MOND). It is shown that the gas mass, extrapolated via an assumed $\beta$ model to a fixed radius of 3 Mpc, is correlated with the gas temperature as predicted by MOND ($M_g \propto T^2$). The observed temperatures are generally consistent with the inferred mass of hot gas; no substantial quantity of additional unseen matter is required in the context of MOND. However, modified dynamics cannot resolve the strong lensing discrepancy in those clusters where this phenomenon occurs. The prediction is that additional baryonic matter may be detected in the central regions of rich clusters.

We study gravitational lensing by clusters of galaxies in the context of the generic class of unconventional gravity theories of the scalar--tensor type. For positive energy scalar fields with any dynamics, the bending of light by a weakly gravitating system (galaxy or cluster) is always smaller than the bending predicted by general relativity for the mass of visible and hitherto undetected matter (but excluding the scalar field energy). The same conclusion obtains within general relativity if a nonnegligible part of the mass in clusters is in coherent scalar fields, {\it i.e.\/} Higgs fields. Thus the observational claim that clusters of galaxies deflect light much more strongly than would be expected from the observable matter contained by them, cannot be interpreted in terms of some scalar-tensor unconventional gravity theory with no dark matter. If the observations eventually show that the matter distribution inferred via general relativity from the lensing is very much like that determined from the dynamics of test objects, then scalar--tensor unconventional gravity will be irrelevant for understanding the mass discrepancy in clusters. However, even a single system in which the dynamically determined mass significantly exceeds the lensing mass suggested by general relativity, would be very problematic for the dark matter picture, but would be entirely consistent with unconventional scalar--tensor gravity., Comment: plain TeX, 23 pages

Numerical hydrodynamical calculations have been carried out in order to investigate the response of a non-self-gravitating gaseous component to a rotating weak oval distortion in the axisymmetric gravitational field of a disk. The purpose of these time-dependent calculations was to determine the form of the steady-state gas density and flow distributions which would develop in the rotating frame of the oval distortion. This is possibly relevant to observed characteristics of gas distribution and kinematics in disk galaxies. The calculations reveal that the gas responds to the presence of the oval distortion by forming an open two-arm trailing spiral wave rotating with the angular speed of the oval distortion. The gas flow pattern is similar in form to predicted gas flow through small pitch-angle gravitational self-consistent spiral waves, although the deviations from uniform circular motion are large (approximately 75 km s/sup -1/ in regions of low gas density). These results may be understood from an investigation of periodic particle orbits in the rotating frame of the bar. Such an investigation implies that the presence of all three resonance regions (inner Lindblad, corotation, and outer Lindblad) within the gaseous disk is a necessary condition for such a spiral response. (AIP)

The response of rotating disks of gas to barlike perturbations in galactic gravitational fields is investigated. In particular, two-dimensional time-dependent numerical hydrodynamical calculations have been performed in order to determine the steady-state response of disks of gas to rotating barlike perturbations. Two types of barlike perturbations are considered: oval distortions in the axisymmetric gravitational field of the disk, and heterogeneous prolate spheroids. The calculations reveal that in the absence of gaseous self-gravity, a viscous differentially rotating disk of gas responds to a rotating barlike perturbation by forming a central gas bar with two trailing spiral waves. The local phase of the gas response is primarily a function of the number and spacing of the principal resonances in the disk. This result may be understood in terms of particle orbit theory. The gas response to barlike perturbations also depends on the relative strength and the effective axial ratio of the bar. In these calculations strong narrow bars produce offset shocks in the central gas bar. These shocks correspond in position to the long narrow dust lanes observed in many barred spiral galaxies.

Observations of the inner 2° of galactic longitude have been made in the 2.6 mm transition of carbon monoxide with 1'-2' beam spacings. The resulting maps show several large scale (> 100 pc) patterns which correlate well with features present in H I, including the nuclear disk and the 3 kpc arm. Several features are identified with absorption by very cold foreground clouds; the 3 kpc arm feature occurs in both emission and absorption.

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/

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-sidedmore » jet symmetrically placed with respect to the ambient medium.« less

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)

Galactic warps of the gaseous component beyond the boundary of a finite stellar disk or at radii greater than 5 radial scale heights in an exponential disk can persist for several rotation times. The persistence of systematic warping in such regions is due to the near spherical symmetry of the gravitational field. Assuming a finite disk model for the highly warped galaxy NGC 5907, it is found from a test particle calculation that the persistence of the observed warp for 5 x 10/sup 9/ years requires a spheroidal halo which is at least as massive as the stellar disk and which has an axial ratio not exceeding 1.5.

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.

La plus grande part de la matiere visible dans l'Univers 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 l'ordre de 10 15 M ○. Mpc −1 et la hauteur d'echelle 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