C32, A Young Star Cluster in IC 1613

The Local Group irregular galaxy IC 1613 has remained an enigma for many years because of its apparent lack of star clusters. We report the successful search for clusters among several of the candidate objects identified many years ago on photographic plates. We have used a single HST WFPC2 pointing and a series of images obtained with the WIYN telescope under exceptional seeing conditions, examining a total of 23 of the previously published candidates. All but six of these objects were found to be either asterisms or background galaxies. Five of the six remaining candidates possibly are small, sparse clusters and the sixth, C32, is an obvious cluster. It is a compact, young object, with an age of less than 10 million years and a total absolute magnitude of M_V = -5.78+/-0.16 within a radius of 13 pc.Comment: 5 pages, 5 figures, to be published in the May 2000 issue of the PAS

Lumpy Structures in Self-Gravitating Disks

Following Toomre & Kalnajs (1991), local models of slightly dissipative self-gravitating disks show how inhomogeneous structures can be maintained over several galaxy rotations. Their basic physical ingredients are self-gravity, dissipation and differential rotation. In order to explore the structures resulting from these processes on the kpc scale, local simulation of self-gravitating disks are performed in this paper in 2D as well as in 3D. The third dimension becomes a priori important as soon as matter clumping causes a tight coupling of the 3D equations of motion. The physically simple and general framework of the model permits to make conclusions beyond the here considered scales. A time dependent affine coordinate system is used, allowing to calculate the gravitational forces via a particle-mesh FFT-method, increasing the performance with respect to previous direct force calculations. Persistent patterns, formed by transient structures, whose intensity and morphological characteristic depend on the dissipation rate are obtained and described. Some of our simulations reveal first signs of mass-size and velocity dispersion-size power-law relations, but a clear scale invariant behavior will require more powerful computer techniques.Comment: 28 pages, 32 figures. Accepted for publication in A&A. Full resolution paper available at http://obswww.unige.ch/Preprints/dyn_art.htm

Tracing spiral density waves in M81

We use SPITZER IRAC 3.6 and 4.5micron near infrared data from the Spitzer Infrared Nearby Galaxies Survey (SINGS), optical B, V and I and 2MASS Ks band data to produce mass surface density maps of M81. The IRAC 3.6 and 4.5micron data, whilst dominated by emission from old stellar populations, is corrected for small-scale contamination by young stars and PAH emission. The I band data are used to produce a mass surface density map by a B-V colour-correction, following the method of Bell and de Jong. We fit a bulge and exponential disc to each mass map, and subtract these components to reveal the non-axisymmetric mass surface density. From the residual mass maps we are able to extract the amplitude and phase of the density wave, using azimuthal profiles. The response of the gas is observed via dust emission in the 8micron IRAC band, allowing a comparison between the phase of the stellar density wave and gas shock. The relationship between this angular offset and radius suggests that the spiral structure is reasonably long lived and allows the position of corotation to be determined.Comment: 15 pages, 17 figures, accepted for publication in MNRA

Gas distribution, kinematics and star formation in faint dwarf galaxies

We compare the gas distribution, kinematics and the current star formation in a sample of 10 very faint (-13.37 < M_B < -9.55) dwarf galaxies. For 5 of these galaxies we present fresh, high sensitivity, GMRT HI 21cm observations. For all our galaxies we construct maps of the HI column density at a constant linear resolution of ~300 pc; this forms an excellent data set to check for the presence of a threshold column density for star formation. We find that while current star formation (as traced by Halpha emission) is confined to regions with relatively large (N_HI > (0.4 -1.7) X 10^{21} atoms cm^{-2}) HI column density, the morphology of the Halpha emission is in general not correlated with that of the high HI column density gas. Thus, while high column density gas may be necessary for star formation, in this sample at least, it is not sufficient to ensure that star formation does in fact occur. We examine the line profiles of the HI emission, but do not find a simple relation between regions with complex line profiles and those with on-going star formation. Finally, we examine the very fine scale (~20-100 pc) distribution of the HI gas, and find that at these scales the emission exhibits a variety of shell like, clumpy and filamentary features. The Halpha emission is sometimes associated with high density HI clumps, sometimes the Halpha emission lies inside a high density shell, and sometimes there is no correspondence between the Halpha emission and the HI clumps. In summary, the interplay between star formation and gas density in these galaxy does not seem to show the simple large scale patterns observed in brighter galaxies (abridged).Comment: 15 pages, 6 tables, 13 figures. Accepted for publication in MNRA

High Magnetic Shear Gain in a Liquid Sodium Stable Couette Flow Experiment; A Prelude to an alpha-Omega Dynamo

The $\Omega$-phase of the liquid sodium $\alpha$-$\Omega$ dynamo experiment at NMIMT in cooperation with LANL has successfully demonstrated the production of a high toroidal field, $B_{\phi} \simeq 8\times B_r$ from the radial component of an applied poloidal magnetic field, $B_r$. This enhanced toroidal field is produced by rotational shear in stable Couette flow within liquid sodium at $Rm \simeq 120$. The small turbulence in stable Taylor-Couette flow is caused by Ekman flow where $(\delta v/v)^2 \sim 10^{-3}$. This high $\Omega$-gain in low turbulence flow contrasts with a smaller $\Omega$-gain in higher turbulence, Helmholtz-unstable shear flows. This result supports the ansatz that large scale astrophysical magnetic fields are created within semi-coherent large scale motions in which turbulence plays only a smaller diffusive role that enables magnetic flux linkage.Comment: 5 pages, 5 figures, submitted PRL revised version: add one author, minor typo'

Lorentz Covariant Theory of Light Propagation in Gravitational Fields of Arbitrary-Moving Bodies

The Lorentz covariant theory of propagation of light in the (weak) gravitational fields of N-body systems consisting of arbitrarily moving point-like bodies with constant masses is constructed. The theory is based on the Lienard-Wiechert presentation of the metric tensor. A new approach for integrating the equations of motion of light particles depending on the retarded time argument is applied. In an approximation which is linear with respect to the universal gravitational constant, G, the equations of light propagation are integrated by quadratures and, moreover, an expression for the tangent vector to the perturbed trajectory of light ray is found in terms of instanteneous functions of the retarded time. General expressions for the relativistic time delay, the angle of light deflection, and gravitational red shift are derived. They generalize previously known results for the case of static or uniformly moving bodies. The most important applications of the theory are given. They include a discussion of the velocity dependent terms in the gravitational lens equation, the Shapiro time delay in binary pulsars, and a precise theoretical formulation of the general relativistic algorithm of data processing of radio and optical astrometric measurements in the non-stationary gravitational field of the solar system. Finally, proposals for future theoretical work being important for astrophysical applications are formulated.Comment: 77 pages, 7 figures, list of references is updated, to be published in Phys. Rev. D6

Constraining corotation from shocks in tightly-wound spiral galaxies

We present a new method for estimating the corotation radius in tightly wound spiral galaxies, through analysis of the radial variation of the offset between arms traced by the potential (P-arms) and those traced by dust (D-arms). We have verified the predictions of semi-analytical theory through hydrodynamical simulations and have examined the uniqueness of the galactic parameters that can be deduced by this method. We find that if the range of angular offsets measured at different radii in a galaxy is greater than around pi/4, it is possible to locate the radius of corotation to within ~ 25%. We argue that the relative location of the P- and D-arms provides more robust constraints on the galactic parameters than can be inferred from regions of enhanced star formation (SF-arms), since interpretation of the latter involves uncertainties due to reddening and the assumed star formation law. We thus stress the importance of K-band studies of spiral galaxies.Comment: Accepted for publication in MNRAS. 15 pages, 23 figure

Spin and quadrupole contributions to the motion of astrophysical binaries

Compact objects in general relativity approximately move along geodesics of spacetime. It is shown that the corrections to geodesic motion due to spin (dipole), quadrupole, and higher multipoles can be modeled by an extension of the point mass action. The quadrupole contributions are discussed in detail for astrophysical objects like neutron stars or black holes. Implications for binaries are analyzed for a small mass ratio situation. There quadrupole effects can encode information about the internal structure of the compact object, e.g., in principle they allow a distinction between black holes and neutron stars, and also different equations of state for the latter. Furthermore, a connection between the relativistic oscillation modes of the object and a dynamical quadrupole evolution is established.Comment: 43 pages. Proceedings of the 524. WE-Heraeus-Seminar "Equations of Motion in Relativistic Gravity". v2: fixed reference. v3: corrected typos in eqs. (1), (57), (85