Spectrophotometric discrimination of river dissolved organic matter

There is a need to be able to differentiate the dissolved organic matter (DOM) fraction in river waters. Research in the 1970s and 1980s has attempted to utilize both absorbance and fluorescence to distinguish between DOM fractions in river waters, but both were limited by the available technology. Total organic carbon content has, therefore, been widely used as a standard method of measuring DOM concentration, although it has little power to differentiate DOM fractions. Recent advances in fluorescence spectrophotometry enable rapid and optically precise analysis of DOM. Here, we show how a combination of both fluorescence and absorbance can be used to discriminate statistically between spatial variations of DOM in tributaries in a small catchment of the Ouseburn, NE England. The results of the discriminant analysis suggest that about 70% of the samples can be correctly classified to its tributary. Discriminant function 1 explains 60·8% of the variance in the data and the fulvic‐like fluorescence intensity has the largest absolute correlation within this function; discriminant function 2 explains a further 21·5% of the variance and the fulvic‐like fluorescence emission wavelength has the largest absolute correlation within this function. The discriminant analysis does not correctly classify all tributaries every time, and successfully discriminates between the different tributaries 70% of the time. Occasions when the tributary waters are less well discriminated are due to either episodic pollution events (at two sites) or due to tributaries that have strong seasonal trends in spectrophotometric parameters, which allows the sites to be misclassified. Results suggest that spectrophotometric techniques have considerable potential in the discrimination of DOM in rivers. Copyright © 2002 John Wiley & Sons, Ltd.     

Short-wavelength infrared (1.3-2.6 μm) observations of the nucleus of Comet 19P/Borrelly

During the last two minutes before closest approach of Deep Space 1 to Comet 19P/Borrelly, a long exposure was made with the short-wavelength infrared (SWIR) imaging spectrometer. The observation yielded 46 spectra covering 1.3-2.6 μm; the footprint of each spectrum was ∼160 m × width of the nucleus. Borrelly's highly variegated and extremely dark 8-km-long nucleus exhibits a strong red slope in its short-wavelength infrared reflection spectrum. This slope is equivalent to J-K and H-K colors of ∼0.82 and ∼0.43, respectively. Between 2.3-2.6 μm thermal emission is clearly detectable in most of the spectra. These data show the nucleus surface to be hot and dry; no trace of H₂O ice was detected. The surface temperature ranged continuously across the nucleus from ?300 K near the terminator to a maximum of ∼340 K, the expected sub-solar equilibrium temperature for a slowly rotating body. A single absorption band at ∼2.39 μm is quite evident in all of the spectra and resembles features seen in nitrogen-bearing organic molecules that are reasonable candidates for compositional components of cometary nuclei. However as of yet the source of this band is unknown.     

Multi-object spectroscopy of the field surrounding PKS 2126−158: discovery of a z= 0.66 galaxy group

The high-redshift radio-loud quasar PKS 2126−158 is found to have a large number of red galaxies in close apparent proximity. We use the Gemini Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for a large fraction of these sources. We show that there is a group of galaxies at   z ∼ 0.66  , coincident with a metal-line absorption system seen in the quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us to measure redshifts of many foreground galaxies in the field surrounding the quasar.
The galaxy group has five confirmed members, and a further four fainter galaxies are possibly associated. All confirmed members exhibit early-type galaxy spectra, a rare situation for a Mg  ii absorbing system. We discuss the relationship of this group to the absorbing gas, and the possibility of gravitational lensing of the quasar due to the intervening galaxies.     

Latitudinal variation of Saturn photochemistry deduced from spatially-resolved ultraviolet spectra

We obtained spatially-resolved ultraviolet spectra of Saturn in 1994 with the Faint Object Spectrometer and Goddard High Resolution Spectrograph of the Hubble Space Telescope. We observed four areas on the planet at 15° N, 33° S, 41° S, and 52° S, with a field-of-view of less than 2 × 2 arcsec², compared to the 16-arcsec planet diameter. The wavelength range, 1550-2300 Å, encompasses absorption from major hydrocarbons (C₂H₆, C₂H₄, C₂H₂, CH₃C₂H, C₄H₂) and water. We find global hydrocarbon abundances and a C₂H₂ vertical distribution compatible with infrared observations, in contrast with previous analyses of ultraviolet spectra. The stratospheric haze opacity decreases from polar region to the equator. Saturn mid-latitudes are photochemically distinct from the rest of the planet. At 33° S, the spectrum requires either (1) a distinctly different C₂H₂ vertical distribution or (2) a locally enhanced water abundance. At 41° S, the hydrocarbon abundance exhibits a local minimum, within a global trend of increasing abundance from equator to pole. This global trend may result from an increased abundance of short-lived hydrocarbons such as C₄H₂. Photochemical models predict a depletion of hydrocarbon molecules in the presence of stratospheric water [Moses et al., 2000. Icarus 143, 166-202]. These results are consistent with a localized influx of water, in the form of high charge to mass ratio particles, flowing into Saturn's atmosphere at latitudes magnetically linked to the rings.     

Active Galaxies in the UV

In this article we present different aspects of AGN studies demonstrating the importance of the UV spectral range. Most important diagnostic lines for studying the general physical conditions as well as the metalicities in the central broad line region in AGN are emitted in the UV. The UV/FUV continuum in AGN excites not only the emission lines in the immediate surrounding but it is responsible for the ionization of the intergalactic medium in the early stages of the universe. Variability studies of the emission line profiles of AGN in the UV give us information on the structure and kinematics of the immediate surrounding of the central supermassive black hole as well as on its mass itself.     

Evidence for a large population of shocked interstellar clouds

A 21 cm absorption measurement over a long path length free of the effects of differential galactic rotation indicates the existence of two distinct cloud populations in the plane. One of them consisting of cold, dense clouds has been well studied before. The newly found hot clouds appear to be at least five times more numerous. They have a spin temperature of ~ 300 K, an rms velocity of ~ 35 km s^-1, twice the total mass, and hundred times the kinetic energy of the cold clouds. Over long path lengths, the hot clouds haveN _H/kpc ~ 2 X 10²¹ cm^-2 Kpc^-1, and are estimated to have individual column densities ≤ 10²⁰ cm^-2. We propose that they are shocked clouds found only within supernova bubbles and that the cold clouds are found in the regions in-between old remnants, immersed in an intercloud medium. We conclude that the solar neighbourhood must be located between old supernova remnants rather than within one.