Starburst galaxies and structure in the submillimetre background towards the Hubble Deep Field

We use an 850-μm SCUBA map of the Hubble Deep Field (HDF) to study the dust properties of optically-selected starburst galaxies at high redshift. The optical/infrared (IR) data in the HDF allow a photometric redshift to be estimated for each galaxy, together with an estimate of the visible star-formation rate. The 850-μm flux density of each source provides the complementary information: the amount of hidden, dust-enshrouded star formation activity. Although the 850-μm map does not allow detection of the majority of individual sources, we show that the galaxies with the highest UV star-formation rates are detected statistically, with a flux density of about S ₈₅₀=0.2 mJy for an apparent UV star-formation rate of 1  h ⁻² M_⊙ yr⁻¹. This level of submillimetre output indicates that the total star-forming activity is on average a factor of approximately 6 times larger than the rate inferred from the UV output of these galaxies. The general population of optical starbursts is then predicted to contribute at least 25 per cent of the 850-μm background. We carry out a power-spectrum analysis of the map, which yields some evidence for angular clustering of the background source population, but at a level lower than that seen in Lyman-break galaxies. Together with other lines of argument, particularly from the NICMOS HDF data, this suggests that the 850-μm background originates over an extremely wide range of redshifts – perhaps 1≲ z ≲6.     

A model of the macro-features of the surface layer of a cometary nucleus

The macro-features of the surface layer of a 'fresh' cometary nucleus are modelled by assuming that the dust and the snow particles of which it consists both have a mass distribution index of 1.65, and that the dust/gas mass ratio is 0.45. Conclusions are drawn as to how this model helps us to understand the cometary sublimation process and the cometary surface layer. The latter most probably consists of weak, low-density, friable, slightly dusty snow. Its ability effectively to support even the small weight of, say, the Rosetta landing probe is in considerable doubt.     

Simulating saturation‐excess surface run‐off in a semi‐distributed hydrological model

The level of complexity, and the number of parameters, to include in a hydrological model is a relatively contentious issue in hydrological modelling. However, it can be argued that explicitly representing important run‐off generation processes can improve the practical value of a model's outputs. This paper explores the benefits of including a new function into an existing semi‐distributed hydrological model (the Pitman model) that is widely used in the sub‐Saharan Africa region. The new function was designed to represent saturation‐excess surface run‐off processes at subcatchment scales and was motivated by the evidence of dambo (low topography riparian areas) type features in many sub‐Saharan river basins. The results for uncertainty versions of the model, with and without the new function, were compared for 25 catchments, which were divided up into those where evidence of dambos exists and those where there is no such evidence. The results suggest that the new function certainly improves the model results for the catchments where dambos exist, but not in situations where saturation‐excess surface run‐off is not expected to occur. The overall conclusion is therefore that the addition of the new function is justified.     

Hillslope response to knickpoint migration in the Southern Appalachians: implications for the evolution of post‐orogenic landscapes

The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for landscape evolution in a post‐orogenic landscape remain enigmatic. The non‐glaciated Cullasaja River basin of south‐western North Carolina, with uniform lithology, frequent debris flows, and the availability of high‐resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post‐orogenic landscape through the lens of hillslope–channel coupling. This investigation is limited to channels with upslope contributing areas >2.7 km², a conservative estimate of the transition from fluvial to debris‐flow dominated channel processes. Values of normalized hypsometry, hypsometric integral, and mean slope vs elevation are used for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. The transient effect of knickpoint‐driven channel incision on basin hillslopes is captured by measuring the relief, mean slope steepness, and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the Cullasaja River. A conceptual model of area–elevation and slope distributions is presented that may be representative of post‐orogenic landscape evolution in analogous geologic settings. Importantly, the model explains how knickpoint migration and channel–hillslope coupling is an important factor in tectonically‐inactive (i.e. post‐orogenic) orogens for the maintenance of significant relief, steep slopes, and weathering‐limited hillslopes. Copyright © 2011 John Wiley & Sons, Ltd.     

A palaeolimnological meta-database for assessing the ecological status of lakes

We describe a palaeolimnological meta-database (“LakeCores”) that includes information for lakes in Europe that have been cored for studies of recent environmental change. Only cores with intact mud-water interfaces and good chronologies for the last ~100–200 years are currently included. The information provided in the database includes the general characteristics of the lakes cored, the methods used for dating and analysis and the bibliographic source of the information selected. At present the database contains information on 1847 studies from 975 sites. Here we use the database to identify cores for which diatom-based pH and total phosphorus (TP) reconstructions have been derived and, from an inspection of the relevant papers, we compile summary data for the timing and extent of acidification and eutrophication of lakes across Europe. We show that there were few cases of acidification and eutrophication in Europe prior to about 1850, and therefore that conditions prior to 1850 can be used for reference with respect to current attempts to restore surface waters to “good ecological status”. We also show the extent to which pH and TP levels have changed across the continent both for all lakes, and, in the case of TP, for lakes of different types covering a range of alkalinities, altitudes, water depths and surface areas.