Study of faint young open clusters as tracers of spiral features in our galaxy

Continuing the study of faint young open clusters as tracers of spiral features in our Galaxy, photoelectric and photographic photometry of 39 stars was done in the field of the faint open cluster NGC 2236 ≡ OCl 501 in the direction of Monoceros constellation. Out of these stars, a total of 22 down tom _v ≃ 15.4 mag have been found to be probable members. There is apparently a variable extinction across the field of the cluster with E(B - V) ranging between 0.84 mag and 0.68 mag. The median age of this cluster is estimated to be 7.6 × 10⁷ years and the cluster is thereby considered as belonging to the marginally old category. Thus, it cannot be specifically used as a spiral arm tracer in the study of our Galaxy. This cluster is located at a distance of 3.72 ± 0.13 kpc, which places it at the inner edge of the outer Perseus spiral feature of the Milky Way.     

Tidal torques and galactic warps

%We study the evolution of galactic disks subject to tidal torques motivated by cosmological N-body simulations using analytic and numerical techniques. We find that self-gravitating disks subject to these torques resemble observed warped galaxies. The warps develop at a local surface density of 70 M _⊙ pc^-2 and move out through the disk at a rate that depends on the surface density of the disk. This revised version was published online in August 2006 with corrections to the Cover Date.     

Isolated and mildly interacting spiral galaxies: Rotation curves and metallicities

We present the study of long slit spectra in the region ofHα emission line of a sample of 111 spiral galaxies with recognizable and well defined spiral morphology and with a well determined environmental status, ranging from isolation to non-disruptive interaction with satellites or companions. The form and properties of the rotation curves (RC) are considered as a function of the isolation degree, morphological type, t, and luminosity. The line ratios are used to estimate the metallicity of all the detected HII regions, thus producing a composite metallicity profile for different types of spirals. We have found that isolated galaxies (ISO) tend to be of later types and lower luminosity than the interacting galaxies (INT). The outer parts of the RC of ISO tend to be flatter than in INT, but they show similar relations between global parameters. The scatter of the Tully-Fisher (TF) relation defined by ISO is significantly lower than that of INT. The [NII]/Hα ratios, used as a metallicity indicator, show a clear trend between Z and t, with earlier spirals showing higher ratios; this trend is tighter when instead of t the gradient of the inner RC, G, is used; no trend is found with the change in interaction status. The Z-gradient of the disks depends on the type, being almost flat for early spirals, and increasing for later types. The [NII]/Hα ratios measured for disk HII regions of INT are higher than for normal/ISO objects, even if all the galaxy families present similar distributions of Hα Equivalent Width. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effective Sensitivity of A Non-uniform Phased Array of Short Dipoles

Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band.     

The dynamical evolution of substructure

The evolution of substructure embedded in non-dissipative dark haloes is studied through N -body simulations of isolated systems, both in and out of initial equilibrium, complementing cosmological simulations of the growth of structure. We determine by both analytic calculations and direct analysis of the N -body simulations the relative importance of various dynamical processes acting on the clumps, such as the removal of material by global tides, clump–clump heating, clump–clump merging and dynamical friction. The ratio of the internal clump velocity dispersion to that of the dark halo is an important parameter; as this ratio approaches a value of unity, heating by close encounters between clumps becomes less important, while the other dynamical processes continue to increase in importance. Our comparison between merging and disruption processes implies that spiral galaxies cannot be formed in a protosystem that contains a few large clumps, but can be formed through the accretion of many small clumps; elliptical galaxies form in a more clumpy environment than do spiral galaxies. Our results support the idea that the central cusp in the density profiles of dark haloes is the consequence of self-limiting merging of small, dense haloes. This implies that the collapse of a system of clumps/substructure is not sufficient to form a cD galaxy, with an extended envelope; plausibly, subsequent accretion of large galaxies is required. The post-collapse system is in general triaxial, with rounder systems resulting from fewer, but more massive, clumps. Persistent streams of material from disrupted clumps can be found in the outer regions of the final system, and at an overdensity of around 0.75, can cover 10 to 30 per cent of the sky.