268 research outputs found
A Sample of Quasars with Strong Nitrogen Emission Lines from the Sloan Digital Sky Survey
We report on 293 quasars with strong NIV] lambda 1486 or NIII] lambda 1750
emission lines (rest-frame equivalent width > 3 \AA) at 1.7 < z < 4.0 selected
from the Sloan Digital Sky Survey (SDSS) Fifth Data Release. These
nitrogen-rich (N-rich) objects comprise ~1.1% of the SDSS quasars. The
comparison between the N-rich quasars and other quasars shows that the two
quasar subsets share many common properties. We also confirm previous results
that N-rich quasars have much stronger Lya and NV lambda 1240 emission lines.
Strong nitrogen emission in all ionization states indicates high overall
nitrogen abundances in these objects. We find evidence that the nitrogen
abundance is closely related to quasar radio properties. The radio-loud
fraction in the NIII]-rich quasars is 26% and in the NIV]-rich quasars is 69%,
significantly higher than ~8% measured in other quasars with similar redshift
and luminosity. Therefore, the high nitrogen abundance in N-rich quasars could
be an indicator of a special quasar evolution stage, in which the radio
activity is also strong.Comment: 8 pages, 4 figures; accepted by ApJ (ApJ June 10, 2008, v680 n1
issue
Discovery of Eight z ~ 6 Quasars in the Sloan Digital Sky Survey Overlap Regions
We present the discovery of eight quasars at z~6 identified in the Sloan
Digital Sky Survey (SDSS) overlap regions. Individual SDSS imaging runs have
some overlap with each other, leading to repeat observations over an area
spanning >4000 deg^2 (more than 1/4 of the total footprint). These overlap
regions provide a unique dataset that allows us to select high-redshift quasars
more than 0.5 mag fainter in the z band than those found with the SDSS
single-epoch data. Our quasar candidates were first selected as i-band dropout
objects in the SDSS imaging database. We then carried out a series of follow-up
observations in the optical and near-IR to improve photometry, remove
contaminants, and identify quasars. The eight quasars reported here were
discovered in a pilot study utilizing the overlap regions at high galactic
latitude (|b|>30 deg). These quasars span a redshift range of 5.86<z<6.06 and a
flux range of 19.3<z_AB<20.6 mag. Five of them are fainter than z_AB=20 mag,
the typical magnitude limit of z~6 quasars used for the SDSS single-epoch
images. In addition, we recover eight previously known quasars at z~6 that are
located in the overlap regions. These results validate our procedure for
selecting quasar candidates from the overlap regions and confirming them with
follow-up observations, and provide guidance to a future systematic survey over
all SDSS imaging regions with repeat observations.Comment: AJ in press (8 pages
Application of Moringa oleifera seeds and Musa cavendish as coagulants for lead, nickel and cadmium removal from drinking water
Contamination of drinking water sources by heavy metals in many South Asian countries has become a major public health concern. As conventional chemical treatment of the contaminated water may not be feasible for many remote communities in the region due to technical and financial constraints, some low-cost, native and abundantly available natural materials have emerged as a potential alternative to the expensive water treatment chemicals. Plant-based materials 'Moringa oleifera' (MO) and 'Musa cavendish' (MC) were investigated in this study as the coagulants for removing lead (Pb), nickel (Ni) and cadmium (Cd) from groundwater containing metal ions at their typical concentrations found in selected South Asian countries. Coagulation tests were conducted with the individual coagulants and their combinations on synthetic and real groundwater samples. The best removal efficiencies for Ni (77%) and Cd (67%) were achieved by the combined coagulants dosed in a mixing manner at the optimum coagulant dosages of 200+200 mg/L from the initial concentration of 31 mug/L and 5 mug/L, respectively. This was attributed to the presence of additional binding sites with different active functional groups, leading to higher removal efficiency compared to the individual coagulant. However, MO alone showed the best removal for Pb (87%) at the coagulant dose of 300 mg/L from the initial concentration of 19 mg/L; suggesting that MO had a good affinity towards Pb. The study demonstrated that MO and MC have the potential to remove Pb, Ni and Cd from drinking water to meet the World Health Organisation (WHO) drinking water standards
The impact of wastewater characteristics, algal species selection and immobilisation on simultaneous nitrogen and phosphorus removal
Nutrient removal from wastewater reduces the environmental impact of its discharge and provides opportunity for water reclamation. Algae can accomplish simultaneous nitrogen and phosphorus removal while also adding value to the wastewater treatment process through resource recovery. The application of algae to wastewater treatment has been limited by a low rate of nutrient removal and difficulty in recovering the algal biomass. Immobilising the algal cells can aid in overcoming both these issues and so improve the feasibility of algal wastewater treatment. Trends for nutrient removal by algal systems over different wastewater characteristics and physical conditions are reviewed. The impact that the selection of algal species and immobilisation has on simultaneous nutrient removal as well as the interdependence of nitrogen and phosphorus are established. Understanding these behaviours will allow the performance of algal wastewater treatment systems to be predicted, assist in their optimisation, and help to identify directions for future research
The Extremely Luminous Quasar Survey (ELQS) in the SDSS footprint I.: Infrared Based Candidate Selection
Studies of the most luminous quasars at high redshift directly probe the
evolution of the most massive black holes in the early Universe and their
connection to massive galaxy formation. However, extremely luminous quasars at
high redshift are very rare objects. Only wide area surveys have a chance to
constrain their population. The Sloan Digital Sky Survey (SDSS) has so far
provided the most widely adopted measurements of the quasar luminosity function
(QLF) at . However, a careful re-examination of the SDSS quasar sample
revealed that the SDSS quasar selection is in fact missing a significant
fraction of quasars at the brightest end. We have identified the
purely optical color selection of SDSS, where quasars at these redshifts are
strongly contaminated by late-type dwarfs, and the spectroscopic incompleteness
of the SDSS footprint as the main reasons. Therefore we have designed the
Extremely Luminous Quasar Survey (ELQS), based on a novel near-infrared JKW2
color cut using WISE AllWISE and 2MASS all-sky photometry, to yield high
completeness for very bright () quasars in the redshift
range of . It effectively uses random forest machine-learning
algorithms on SDSS and WISE photometry for quasar-star classification and
photometric redshift estimation. The ELQS will spectroscopically follow-up
new quasar candidates in an area of in the
SDSS footprint, to obtain a well-defined and complete quasars sample for an
accurate measurement of the bright-end quasar luminosity function at . In this paper we present the quasar selection algorithm and the
quasar candidate catalog.Comment: 16 pages, 8 figures, 9 tables; ApJ in pres
Gemini Near-infrared Spectroscopy of Luminous z~6 Quasars: Chemical Abundances, Black Hole Masses, and MgII Absorption
We present Gemini near-infrared spectroscopic observations of six luminous
quasars at z=5.86.3. Five of them were observed using Gemini-South/GNIRS,
which provides a simultaneous wavelength coverage of 0.9--2.5 m in cross
dispersion mode. The other source was observed in K band with
Gemini-North/NIRI. We calculate line strengths for all detected emission lines
and use their ratios to estimate gas metallicity in the broad-line regions of
the quasars. The metallicity is found to be supersolar with a typical value of
4 Z_{\sun}, and a comparison with low-redshift observations shows no
strong evolution in metallicity up to z6. The FeII/MgII ratio of the
quasars is 4.9+/-1.4, consistent with low-redshift measurements. We estimate
central BH masses of 10^9 to 10^{10} M_{\sun} and Eddington luminosity ratios
of order unity. We identify two MgII 2796,2803 absorbers with
rest equivalent width W_0^{\lambda2796}>1 \AA at 2.2<z<3 and three MgII
absorbers with W_0^{\lambda2796}>1.5 \AA at z>3 in the spectra, with the two
most distant absorbers at z=4.8668 and 4.8823, respectively. The redshift
number densities (dN/dz) of MgII absorbers with W_0^{\lambda2796}>1.5 \AA are
consistent with no cosmic evolution up to z>4.Comment: 33 pages (including 7 figures and 6 tables), AJ in pres
Energy-balanced multi-hop-aware cooperative geographic routing for wireless ad hoc networks
Since the cooperative communication can reduce the transmitted power and extend the transmission coverage, minimum energy routing protocols are considered to reduce the total energy consumption in a multi-hop wireless Ad Hoc network. In this paper, an Energy-balanced Multi-hop-aware Cooperative Geographic Routing (EMCGR) algorithm is proposed. We firstly formulate the outage probability and construct the minimum power route in Multi-hop-aware Cooperative Transmission (MCT) mode. The MCT mode can fully exploit the merit of the relay broadcasting characteristics to achieve the aim of saving the total transmitted power. Then an improved Energy-Balanced Geographic Routing (EBGR) algorithm is designed. The EBGR algorithm selects the next hop forwarding node by combining the geographic position information and energy information. The goal of this strategy is to balance the energy consumption among nodes so that the lifetime of the whole network can be prolonged. The route of the proposed EMCGR algorithm is based on EBGR algorithm. Simulation results show that in the same computer simulation scene, the power saving of the EMCGR algorithm with respect to the MPCR algorithm and EBGR algorithm can achieve 15.2% and 67.1%, respectively. Besides, the EMCGR algorithm does well in balancing the energy consumption among nodes in the wireless Ad Hoc network
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