The formation of ultracompact dwarf galaxies

Recent spectroscopic observations of galaxies in the Fornax Cluster reveal nearly unresolved 'star-like' objects with redshifts appropriate to the Fornax Cluster. These objects have intrinsic sizes of ≈100 pc and absolute B -band magnitudes in the range  −14< M _B<−11.5 mag  and lower limits for the central surface brightness   μ _B≳23 mag arcsec⁻²  , and so appear to constitute a new population of ultracompact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar superclusters (by Kroupa) , which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of superclusters in a tidal field. The YMCs merge on a few supercluster crossing times. Superclusters that are initially as concentrated and massive as knot S in the interacting Antennae galaxies evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. Less massive superclusters resembling knot 430 in the Antennae may evolve to ω Cen-type systems. Low-concentration superclusters are disrupted by the tidal field, dispersing their surviving star clusters while the remaining merger objects rapidly evolve into the   μ _B− M _B  region populated by low-mass Milky Way dSph satellites.     

Near-infrared and optical emission-line structure of the Keyhole Nebula in NGC 3372

Narrow-band infrared and optical images of the Keyhole Nebula in NGC 3372 reveal which structures are caused by extinction, and show the underlying morphology of photoionized and shock-excited gas. Dark clouds conspire with ionized gas to create the apparent keyhole shape, which is prominent at blue wavelengths and less apparent in the infrared. The  Pa β /H α   line ratio shows the spatial distribution of foreground extinction. The wavelength dependence of this extinction indicates a reddening law with   R ≈4.8  , different from the normal interstellar medium. This confirms previous estimates of reddening toward the Carina Nebula determined from stellar photometry, and reveals that the anomalous extinction is patchy and within the H  ii region. The morphology of the ionized gas is different from the extinction clouds; it shows an edge-on ionization front running NE to SW, with a limb-brightened indentation that forms the upper outline of the keyhole shape. A fast polar wind from η Carinae may have punctured the ionization front, since the indentation is directly along a projection of the polar axis of the star. This is supported by the morphology of shock-excited gas revealed by a high  [S  ii ]/H α   ratio. High-excitation gas emitting [O  iii ] and He  i has a smoother distribution. Molecular clumps in the region are also discussed.     

Morphological observation of Epithemia ocellata (Ehr.) Kütz. (Bacillariophyceae) from China

The species of Epithemia ocellata (Ehr.) Kütz. from Northeastern China were observed by scanning electron microscopy (SEM). The studies on silica valve formation in different stages were made. At the early stage, a Y-shaped raphe sternum was found. Areolae were made from virgae and vimines. In immature valves, the canal formation before the raphe fissure is visible. The raphe system, virgae and vimines (areolae) might be the early siliceous elements of the valve during morphogenesis. The formations of virgae and vimines are similar to that of Gephyria media W. Arnott. Rows of silica papillae formed gradually on the face of the valve. After the areolae were covered with silica and form rows of papillae, the outer valve surface could become mature. Details of the internal costae were observed in mature valves.     

The star-forming content of the W3 giant molecular cloud

We have surveyed a ∼0.9 square degree area of the W3 giant molecular cloud (GMC) and star-forming region in the 850-μm continuum, using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope. A complete sample of 316 dense clumps were detected with a mass range from around 13 to  2500 M_⊙  . Part of the W3 GMC is subject to an interaction with the H  ii region and fast stellar winds generated by the nearby W4 OB association. We find that the fraction of total gas mass in dense, 850-μm traced structures is significantly altered by this interaction, being around 5–13 per cent in the undisturbed cloud but ∼25–37 per cent in the feedback-affected region. The mass distribution in the detected clump sample depends somewhat on assumptions of dust temperature and is not a simple, single power law but contains significant structure at intermediate masses. This structure is likely to be due to crowding of sources near or below the spatial resolution of the observations. There is little evidence of any difference between the index of the high-mass end of the clump mass function in the compressed region and in the unaffected cloud. The consequences of these results are discussed in terms of current models of triggered star formation.     

Magnetic fields in massive star-forming regions

We present the largest sample of high-mass star-forming regions observed using submillimetre imaging polarimetry. The data were taken using the Submillimetre Common User Bolometer Array (SCUBA) in conjunction with the polarimeter on the James Clerk Maxwell Telescope (JCMT) in Hawaii. In total, 16 star-forming regions were observed, although some of these contain multiple cores. The polarimetry implies a variety of magnetic field morphologies, with some very ordered fields. We see a decrease in polarization percentage for seven of the cores. The magnetic field strengths estimated for 14 of the cores, using the corrected Chandrasekhar and Fermi (CF) method, range from <0.1 mG to almost 6 mG. These magnetic fields are weaker on these large scales when compared to previous Zeeman measurements from maser emission, implying the role of the magnetic field in star formation increases in importance on smaller scales. Analysis of the alignment of the mean field direction and the outflow directions reveals no relation for the whole sample, although direct comparison of the polarimetry maps suggests good alignment (to at least one outflow direction per source) in seven out of the 15 sources with outflows.