Structure through colour: a pixel approach towards understanding galaxies

We present a study of pixel colour–magnitude diagrams (pCMDs) for a sample of 69 nearby galaxies chosen to span a wide range of Hubble types. Our goal is to determine how useful a pixel approach is for studying galaxies according to their stellar light distributions and content. The galaxy images were analysed on a pixel-by-pixel basis to reveal the structure of the individual pCMDs. We find that the average surface brightness (or projected mass density) in each pixel varies according to galaxy type. Early-type galaxies exhibit a clear 'prime sequence' and some pCMDs of face-on spirals reveal 'inverse-L' structures. We find that the colour dispersion at a given magnitude is found to be approximately constant in early-type galaxies but this quantity varies in the mid and late types. We investigate individual galaxies and find that the pCMDs can be used to pick out morphological features. We discuss the discovery of 'Red Hooks' in the pCMDs of six early-type galaxies and two spirals and postulate their origins. We develop quantitative methods to characterize the pCMDs, including measures of the blue-to-red light ratio and colour distributions of each galaxy and we organize these by morphological type. We compare the colours of the pixels in each galaxy with the stellar population models of Bruzual & Charlot to calculate star formation histories for each galaxy type and compare these to the stellar mass within each pixel. Maps of pixel stellar mass and mass-to-light ratio are compared to galaxy images. We apply the pCMD technique to three galaxies in the Hubble Ultra Deep Field to test the usefulness of the analysis at high redshift. We propose that these results can be used as part of a new system of automated classification of galaxies that can be applied at high redshift.     

Identification of some of the unidentified EIC-1 sources

We have identified 20 of the 75 unidentified Equatorial Infrared Catalogue Number-1 sources with stars in the BD Catalogue. Two of these are also found in the HD Catalogue.     

Kelvin-Helmholtz instability in an Alfvén resonant layer of a solar coronal loop

A Kelvin-Helmholtz instability has been identified numerically on an azimuthally symmetric Alfvén resonant layer in an axially bounded, straight cylindrical coronal loop. The physical model employed is an incompressible, reduced magnetohydrodynamic (MHD) model including resistivity, viscosity, and density variation. The set of equations is solved numerically as an initial value problem. The linear growth rate of this instability is shown to be approximately proportional to the Alfvén driving amplitude and inversely proportional to the width of the Alfvén resonant layer. It is also shown that the linear growth rate increases linearly with m - 1 up to a certain m, reaches its maximum value for the mode whose half wavelength is comparable to the Alfvén resonant layer width, and decreases at higher m's. (m is the azimuthal mode number.)     

Suspended matter in surface waters of the eastern Gulf of Guinea

Suspended matter in the surface waters of the eastern Gulf of Guinea was studied in relation to the prevailing oceanic currents and the sediment composition and source. A confused current system arises from the confluence of the Guinea Current and the South Equatorial Current in this area. Sediment-laden water is transported to the south of Fernando Póo into the Gulf in a south-westerly direction. The southeasterly flowing Guinea Current along the western edge of the Niger delta and the enormous sediment loads of the distributaries in this area contribute to a major lobe of suspended matter off the southwestern nose of the delta. Sediment concentration is inversely related to salinity in the northeastern Gulf. Sediment is also swept northwestward from the continental shelf of Gabon and the Congo Republic into the Gulf. Diatoms are the most abundant constituents with lesser amounts of organic aggregates and two varieties of fecal pellets.     

Comparison of parallel group velocity of ECH waves with electron resonant velocity: Implication for electron diffusion by ECH waves

We have investigated the role of group velocity in the calculation of pitch-angle diffusion coefficients by electron cyclotron harmonic (ECH) waves in planetary magnetospheres. The assumption which is generally made that the parallel group velocity can be neglected in comparison with particle parallel velocity is examined in detail. It is found that for lowest harmonic band this assumption is quite good. It is found that in general it is not possible to ignore the parallel group velocity. However, for lowest harmonic band this assumption is quite good at low electron temperatures.     

How can α 2-dynamos generate axisymmetric magnetic fields?

Inspired by a recently observed axisymmetric field in a fully convective star we investigate the influence of an anisotropic diffusivity on the dynamo. We find that with reasonable assumptions for the anisotropy of the diffusivity and the α -effect the preference of axisymmetric modes is achieved. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the nature of the z=0 X-ray absorbers: I. Clues from an external group

We describe the main features of the evolutionary code ATON 3.1 and its latest version, particularly deviced to be apt for follow up asteroseismology applications. An older version of the code including rotational evolution is also shortly described.