The 2dF-SDSS LRG and QSO Survey: the star formation histories of luminous red galaxies

We present a detailed investigation into the recent star formation histories of 5697 luminous red galaxies (LRGs) based on the Hδ (4101 Å), and [O  ii ] (3727 Å) lines and the D4000 index. LRGs are luminous  ( L > 3 L *)  galaxies which have been selected to have photometric properties consistent with an old, passively evolving stellar population. For this study, we utilize LRGs from the recently completed 2dF-SDSS LRG and QSO Survey (2SLAQ). Equivalent widths of the Hδ and [O  ii ] lines are measured and used to define three spectral types, those with only strong Hδ absorption (k+a), those with strong [O  ii ] in emission (em) and those with both (em+a). All other LRGs are considered to have passive star formation histories. The vast majority of LRGs are found to be passive (∼80 per cent); however, significant numbers of k+a (2.7 per cent), em+a (1.2 per cent) and em LRGs (8.6 per cent) are identified. An investigation into the redshift dependence of the fractions is also performed. A sample of SDSS MAIN galaxies with colours and luminosities consistent with the 2SLAQ LRGs is selected to provide a low-redshift comparison. While the em and em+a fractions are consistent with the low-redshift SDSS sample, the fraction of k+a LRGs is found to increase significantly with redshift. This result is interpreted as an indication of an increasing amount of recent star formation activity in LRGs with redshift. By considering the expected lifetime of the k+a phase, the number of LRGs which will undergo a k+a phase can be estimated. A crude comparison of this estimate with the predictions from semi-analytic models of galaxy formation shows that the predicted level of k+a and em+a activities is not sufficient to reconcile the predicted mass growth for massive early types in a hierarchical merging scenario.     

Positional accuracy of the Google Earth terrain model derived from stratigraphic unconformities in the Big Bend region,Texas, USA

The Google Earth terrain model could prove beneficial for extraction of positional data in the future. At present, only an aging independent benchmark study (Potere, D., 2008. Horizontal position accuracy of Google Earth's high-resolution imagery archive. Sensors, 8, 7973–7981) provides constraints on positional accuracy for Google Earth imagery. In this investigation, we compared virtually traced positions against high-precision (<1 m) field measurements along three stratigraphic unconformity sub-sections in the Big Bend region to determine current positional accuracy for the Google Earth terrain model. A horizontal position accuracy of 2.64 m RMSE_r was determined for the Google Earth terrain model with mean offset distance being 6.95 m. A vertical position accuracy of 1.63 m RMSE_z with mean offset distance of 2.66 m was also calculated for the terrain model. Results suggest data extracted from the Google Earth terrain model could plausibly be used in future studies. However, we urge caution in using Google Earth data due to limited information disclosures by developers.     

Reference hydrologic networks I. The status and potential future directions of national reference hydrologic networks for detecting trends

Abstract

Identifying climate-driven trends in river flows on a global basis is hampered by a lack of long, quality time series data for rivers with relatively undisturbed regimes. This is a global problem compounded by the lack of support for essential long-term monitoring. Experience demonstrates that, with clear strategic objectives, and the support of sponsoring organizations, reference hydrologic networks can constitute an exceptionally valuable data source to effectively identify, quantify and interpret hydrological change—the speed and magnitude of which is expected to a be a primary driver of water management and flood alleviation strategies through the future—and for additional applications. Reference hydrologic networks have been developed in many countries in the past few decades. These collections of streamflow gauging stations, that are maintained and operated with the intention of observing how the hydrology of watersheds responds to variations in climate, are described. The status of networks under development is summarized. We suggest a plan of actions to make more effective use of this collection of networks.

Editor Z.W. Kundzewicz; Associate editor K. Hamed

Citation Whitfield, P.H., et al., 2012 Burn, D.H. 2012. Reference hydrologic networks, II. Using reference hydrologic networks to assess climate-driven changes in streamflow. Hydrological Sciences Journal, 57(8) (this issue)[Taylor & Francis Online] , [Google Scholar]. Reference hydrologic networks I. The status and potential future directions of national reference hydrologic networks for detecting trends. Hydrological Sciences Journal, 57 (8), 1562–1579.     

Editorial

Abstract

With the increase in volume of spatial data now available, more effective ways must be found of storing and processing these data. This paper presents a compacted version of the linear quadtree and a spatially-referenced index method that can significantly reduce the storage requirements of a set of images and the time taken to process spatial queries. The index acts as a high-level summary of a regular-sized portion of the underlying image and so can be used to avoid examining areas of the image where none of the required features is present. Some example results are given. A method for the optimization of spatial searches is presented which takes into account the area and distribution of features within an image. Finally, a method for directly associating the edges of features with the individual nodes of a quadtree is reported. This is important since the edges of objects are no longer explicitly present in linear quadtrees and so must be recalculated when they are required for part of a query. Recalculation of object edges or boundaries is expensive; it is best, therefore, to perform the operation once only, and then save the results.     

Eolian Transport of Geogenic Hexavalent Chromium to Ground Water

A conceptual model of eolian transport is proposed to address the widely distributed, high concentrations of hexavalent chromium (Cr⁺⁶) observed in ground water in the Emirate of Abu Dhabi, United Arab Emirates. Concentrations (30 to more than 1000 μg/L Cr⁺⁶) extend over thousands of square kilometers of ground water systems. It is hypothesized that the Cr is derived from weathering of chromium-rich pyroxenes and olivines present in ophiolite sequence of the adjacent Oman (Hajar) Mountains. Cr⁺³ in the minerals is oxidized to Cr⁺⁶ by reduction of manganese and is subsequently sorbed on iron and manganese oxide coatings of particles. When the surfaces of these particles are abraded in this arid environment, they release fine, micrometer-sized, coated particles that are easily transported over large distances by wind and subsequently deposited on the surface. During ground water recharge events, the readily soluble Cr⁺⁶ is mobilized by rain water and transported by advective flow into the underlying aquifer. Chromium analyses of ground water, rain, dust, and surface (soil) deposits are consistent with this model, as are electron probe analyses of clasts derived from the eroding Oman ophiolite sequence. Ground water recharge flux is proposed to exercise some control over Cr⁺⁶ concentration in the aquifer.     

A Note on Tree Deaths during the Current (2001–?) Drought in South-eastern Australia

Tree deaths over large areas during droughts are uncommon. In the semi-arid zone of New South Wales (NSW) such events were last reported during the prolonged droughts of 1895–1903 and 1939–45, though whether caused solely by drought is open to question. Anecdotal and quantitative evidence during the current prolonged drought (2001–?) in NSW's sub-humid zone (where deaths are localised) and semi-arid zone (where deaths are more extensive) suggest that climatic conditions are comparable to, and perhaps more severe than, those of the earlier prolonged droughts. It is suggested that lack of access to deep soil moisture due to inherent shallow rooting, or due to site conditions, is the most likely cause; and that this may be associated with higher than optimal tree densities in cohorts of trees that arose from previous regeneration events during periods of well above average rainfall. A more thorough examination of the extent and severity of this event is warranted.     

The 2dF-SDSS LRG and QSO Survey: the spectroscopic QSO catalogue

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep,  18 < g < 21.85  (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at   z ≲ 2.6  . The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg² and contains new spectra of 16 326 objects, of which 8764 are QSOs and 7623 are newly discovered [the remainder were previously identified by the 2dF QSO Redshift Survey (2QZ) and SDSS]. The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12 702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission-line galaxies with a median redshift of   z = 0.22  .
We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at   g ≃ 21.85  QSO colours are significantly affected by the presence of a host galaxy up to redshift   z ∼ 1  in the SDSS ugriz bands. In particular, we see a significant reddening of the objects in   g − i   towards the fainter g -band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2–3 Gyr.
The full catalogue, including completeness estimates, is available on-line at http://www.2slaq.info/ .