Geometry of Spiral Galaxy Arms

The previous work (He, 2001) on image analysis of over one hundred nearby galaxies indicates that for any spiral galaxy image of small inclination there is evidence of two sets of iso-ratio curves in different scales which, like curvilineal coordinate lines, expand over different galaxy components. This result sets constraint on galaxy patterns and a general equation which carries this constraint is presented in the current paper. The equation has clear geometrical meaning. Its much simplified version which applies to galaxy arms, when solved, may be a guidance to understanding galaxy patterns. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of star formation on the evolution of Blue Compact Dwarf galaxies

We study the effect of different star formation regimes on the dynamical and chemical evolution of Blue Compact Dwarf galaxies. To do that, we adopt a 2-D hydrocode coupled with detailed chemical yields originating from SNeII, SNeIa and from intermediate-mass stars. We apply this kind of simulation to a model of galaxy resembling IZw18. This revised version was published online in August 2006 with corrections to the Cover Date.     

Infrared galaxies in compact groups

IR properties of Compact Group (CG) galaxies are presented and compared with IR properties of isolated galaxies. Despite CGs displaying more E-S0s and optically bright galaxies, no differences are retrieved concerning FIR emission. The observed lack of strong FIR enhancement in the CG sample is not surprising when interpreting FIR sources in CGs as accordant redshift projections, rather than as mergers in progress. This revised version was published online in August 2006 with corrections to the Cover Date.     

Phase-reference mapping of a weak-cored double-lobed source in the 1636+473 system using MERLIN and Global VLBI

We present maps and astrometric results from first epoch, simultaneous MERLIN and Global VLBI observations of the very weak core of a double-lobed radio source, using as reference the bright, compact, flat-spectrum quasar 1636+473 A, 20 arcsec away. The observations were made in May 1995 at 5 GHz. The phase-referenced VLBI map of the weak core, of resolution 1.5 mas, contains a high percentage of the core flux density seen in the MERLIN map. Further hybrid mapping iterations reveal a faint, one-sided core extension on the same side as the MERLIN jet. We explore the effect of both temporal and other coherence losses on the phase-reference map. We relate our results to the structural asymmetry in weak AGN cores within the context of unification models.     

Kinematical properties of early-type galaxies

Summary In the last decade, our understanding of early-type galaxies has greatly changed: from rather uninteresting oblate spheroids flattened by rotation to multicomponent stellar systems whose structure, formation mechanisms, and evolution, are far from being understood. This new scenario is mainly the consequence of the huge growth, in quantity and in quality, of kinematical data obtained from high signal to noise spectral data. Rotation curves and velocity dispersion profiles extending out to almost 2 effective radii are now available, together with line asymmetry measurements, for the stellar components of a fairly large sample of galaxies. For a few galaxies, outer halo tracers such as globular clusters and/or planetary nebulae allow to explore the kinematics out at 4 6r _e. In this article we focus on these data giving particular emphasis on the most recent results. Reference is given to other review articles complementing the approach presented here.     

10 μm Imaging of the Blue Compact Galaxy HE 2–10

We present the first results of a survey of Blue Compact Galaxies with a 10 m array. Blue Compact Galaxies (BCGs) are dwarf galaxies experiencing an intense burst of star formation. As dwarf systems, their main characteristics is a low heavy-elements abundance. Although dust is thought to be less abundant in these galaxies than in normal spirals, the presence of a compact starburst region favors a detection in the infrared, and 10 m imaging is perfectly suited to star formation studies in BCGs since it focuses on the hottest dust inside the star-forming regions.     

The redshift dependence of spectral index in powerful radio galaxies

We present and discuss in this paper the rest frame radio spectra (1–25 GHz) of a sample of fourteen radio galaxies atz >2 from the newly defined MRC/1Jy complete sample of 558 radio sources. These galaxies are among the most powerful radio sources known and range in luminosity from 1028-1028·8 watt Hz-1 at 1 GHz. We find that the median rest frame spectral index of this sample of galaxies atz >2 is significantly steeper than that of a matched luminosity sample of 3CRR galaxies which are at a much lower redshift (0.85 <z < 1.7). This indicates that spectral index correlates primarily with redshift, at least in the luminosity range considered here. The difference between the distributions of rest frame spectral curvatures for the two samples does not appear to be statistically significant. We suggest a new explanation for the steeper spectra of radio galaxies at high redshift involving steeper electron energy spectra at injection. Electron energy spectra are expected to steepen in a first-order Fermi acceleration process, at both non-relativistic and relativistic shock fronts, as the upstream fluid velocity decreases. This may well be the case at high redshifts: the hotter and denser circum-galactic medium at high redshifts could result in slower speeds for the hotspot and the jet material behind it. The smaller sizes of radio sources at higher redshifts provide support to this scenario. Since deceased.     

Centaurus A – NGC 5128

Summary. At a distance of 3.4 Mpc, NGC 5128 (Centaurus A) is by far the nearest active radio galaxy. It is often considered to be the prototype Fanaroff-Riley Class I ‘low-luminosity’ radio galaxy, and as such it plays an important role in our understanding of a major class of active galaxies. Its proximity has spawned numerous detailed investigations of its properties, yielding unrivalled but still incomplete knowledge of its structure and dynamics. The massive elliptical host galaxy is moderately triaxial and contains a thin, strongly warped disk rich in dust, atomic and molecular gas and luminous young stars. Its globular cluster ensemble has a bimodal distribution of metallicities. Deep optical images reveal faint major axis extensions as well as a system of filaments and shells. These and other characteristics are generally regarded as strong evidence that NGC 5128 has experienced a major merging events at least once in its past. The galaxy has a very compact, subparsec nucleus exhibiting noticeable intensity variations at radio and X-ray wavelengths, probably powered by accretion events. The central object may be a black hole of moderate mass. Towards the nucleus, rich absorption spectra of atomic hydrogen and various molecular species suggest the presence of significant amounts of material falling into the nucleus, presumably ‘feeding the monster’. Emanating from the nucleus are linear radio/X-ray jets, becoming subrelativistic at a few parsec from the nucleus. At about 5 kpc from the nucleus, the jets expand into plumes. Huge radio lobes extend beyond the plumes out to to 250 kpc. A compact circumnuclear disk with a central cavity surrounds the nucleus. Its plane, although at an angle to the minor axis of the galaxy, is perpendicular to the inner jets. The jet-collimating mechanism, probably connected to the circumnuclear disk, appears to precess on timescales of order a few times 10 years. This review summarizes the present state of knowledge of NGC 5128 and its associated radio source Centaurus A. Underlying physical processes are outside its scope: they are briefly referred to, but not discussed. Received 30 December 1997     

Observations of the Sunyaev–Zel''dovich effect at high angular resolution towards the galaxy clusters A665, A2163 and CL0016+16

We report on the first observation of the Sunyaev–Zel'dovich (SZ) effect, a distortion of the Cosmic Microwave Background radiation (CMB) by hot electrons in clusters of galaxies, with the Diabolo experiment at the IRAM 30 m telescope. Diabolo is a dual-channel 0.1 K bolometer photometer dedicated to the observation of CMB anisotropies at 2.1 and 1.2 mm. A significant brightness decrement in the 2.1 mm channel is detected in the direction of three clusters (Abell 665, Abell 2163 and CL0016+16). With a 30 arcsec beam and 3 arcmin beamthrow, this is the highest angular resolution observation to date of the SZ effect. Interleaving integrations on targets and on nearby blank fields have been performed in order to check and correct for systematic effects. Gas masses can be directly inferred from these observations.     

Simulations of stellar disks in galaxies with non-Newtonian interactions

We describe gravitationalN-body simulations to investigate whether various non-Newtonian interactions between the stars of a system could explain the flat rotational curves which are characteristic of actual isolated spiral galaxies. It is shown that replacing the standard Newtonian interaction by the models of Sanders (1984), Kuhn and Kruglyak (1987) and Milgrom (1983), no massive halo (or dark matter) is required to produce the flat rotational curves of the systems under consideration. All models also generate the exponential surface mass density distribution which is in agreement with that observed in disk-shaped galaxies. In relation to the spiral structure of galaxies, we present the evidence that the non-Newtonian interactions can reproduce the multiple armed patterns in stellar disks without dark matter.