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Search Results: 1 - 10 of 3564 matches for " Jackson Neal "
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The Hubble Constant
Jackson Neal
Living Reviews in Relativity , 2007,
Abstract: I review the current state of determinations of the Hubble constant, which gives the length scale of the Universe by relating the expansion velocity of objects to their distance. In the last 20 years, much progress has been made and estimates now range between 60 and 75 km s^-1 Mpc^-1, with most now between 70 and 75 km s^-1 Mpc^-1, a huge improvement over the factor-of-2 uncertainty which used to prevail. Further improvements which gave a generally agreed margin of error of a few percent rather than the current 10% would be vital input to much other interesting cosmology. There are several programmes which are likely to lead us to this point in the next 10 years.
The faintest radio source yet: EVLA observations of the gravitational lens SDSS J1004+4112
Neal Jackson
Physics , 2011, DOI: 10.1088/2041-8205/739/1/L28
Abstract: We present new radio observations of the large-separation gravitationally-lensed quasar SDSS J1004+4112, taken in a total of 6 hours of observations with the Extended Very Large Array (EVLA). The maps reach a thermal noise level of approximately 4microJy. We detect four of the five lensed images at the 15-35microJy level, representing a source of intrinsic flux density, after allowing for lensing magnification, of about 1microJy, intrinsically probably the faintest radio source yet detected. This reinforces the utility of gravitational lensing in potentially allowing us to study nanoJy-level sources before the advent of the SKA. In an optical observation taken three months after the radio observation, image C is the brightest image, whereas the radio map shows flux density ratios consistent with previous optical observations. Future observations separated by a time delay will give the intrinsic flux ratios of the images in this source.
Quasar lensing
Neal Jackson
Physics , 2013,
Abstract: I review the observations of gravitationally lensed quasars. These systems are important because they allow us to probe the properties of the lensing galaxies at various scales, and they also allow insights into the structures of the quasars themselves. Samples of quasar lenses also have the potential to act as cosmographic probes. These areas are described, together with observational and scientific prospects for the future.
Expansion/Facemask Treatment of an Adult Class III Malocclusion
Gregory W. Jackson,Neal D. Kravitz
Case Reports in Dentistry , 2014, DOI: 10.1155/2014/270257
Abstract: The orthodontic treatment of class III malocclusion with a maxillary deficiency is often treated with maxillary protraction with or without expansion. Skeletal and dental changes have been documented which have combined for the protraction of the maxilla and the correction of the class III malocclusion. Concerning the ideal time to treat a developing class III malocclusion, studies have reported that, although early treatment may be the most effective, face mask therapy can provide a viable option for older children as well. But what about young adults? Can the skeletal and dental changes seen in expansion/facemask therapy in children and adolescents be demonstrated in this age group as well, possibly eliminating the need for orthodontic dental camouflage treatment or orthognathic surgery? A case report is presented of an adult class III malocclusion with a Class III skeletal pattern and maxillary retrusion. Treatment was with nonextraction, comprehensive edgewise mechanics with slow maxillary expansion with a bonded expander and protraction facemask. 1. Introduction The orthodontic treatment of Class III malocclusion with a maxillary deficiency is often treated with maxillary protraction either with or without maxillary expansion [1–4]. Studies on both humans and experimental animals have demonstrated the orthopedic advancement of the maxilla. These studies have shown that a significant component of skeletal class III malocclusion includes maxillary retrusion in combination with a normal or mildly prognathic mandible [5–17]. Skeletal and dental changes have been documented in these studies which have combined for the protraction of the maxilla and the correction of the class III malocclusion. Is there an ideal time to treat a developing class III malocclusion? Just a few studies have examined the effect of age on maxillary protraction therapy. Takada et al. [9] examined 61 Japanese female patients with class III malocclusion, divided into three groups (7 to 10 years, 10 to 12 years, and 12 to 15 years). They concluded that a greater orthopedic effect was observed when therapy was applied before or during the pubertal growth spurt (7 to 12 years). Baik [14] studied maxillary expansion and protraction in 47 Korean subjects, divided into three groups (<10 years, 10 to 12 years, and 12 years or older). He concluded that age did not show any statistically significant difference in treatment effects of expansion/facemask therapy. Braun [18] studied 63 subjects aged 4–13 and found that expansion/facemask therapy produces dentofacial changes that combine to
A new gravitational lens from the MUSCLES survey: ULAS J082016.1+081216
Neal Jackson,Eran O. Ofek,Masamune Oguri
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15198.x
Abstract: We present observations of a new double-image gravitational lens system, ULAS J082016.1+081216, of image separation 2.3" and high (~6) flux ratio. The system is selected from the Sloan Digital Sky Survey spectroscopic quasar list using new high-quality images from the UKIRT Deep Sky Survey (UKIDSS). The lensed quasar has a source redshift of 2.024, and we identify the lens galaxy as a faint red object of redshift 0.803+/-0.001. Three other objects from the UKIDSS survey, selected in the same way, were found not to be lens systems. Together with the earlier lens found using this method, the SDSS-UKIDSS lenses have the potential to significantly increase the number of quasar lenses found in SDSS, to extend the survey to higher flux ratios and lower separations, and to give greater completeness which is important for statistical purposes.
Quasar emission lines, radio structures and radio unification
Neal Jackson,I. W. A. Browne
Physics , 2012, DOI: 10.1093/mnras/sts468
Abstract: Unified schemes of radio sources, which account for different types of radio AGN in terms of anisotropic radio and optical emission, together with different orientations of the ejection axis to the line of sight, have been invoked for many years. Recently, large samples of optical quasars, mainly from the Sloan Digital Sky Survey, together with large radio samples, such as FIRST, have become available. These hold the promise of providing more stringent tests of unified schemes but, compared to previous samples, lack high resolution radio maps. Nevertheless they have been used to investigate unified schemes, in some cases yielding results which appear inconsistent with such theories. Here we investigate using simulations how the selection effects to which such investigations are subject can influence the conclusions drawn. In particular, we find that the effects of limited resolution do not allow core-dominated radio sources to be fully represented in the samples, that the effects of limited sensitivity systematically exclude some classes of sources and the lack of deep radio data make it difficult to decide to what extent closely separated radio sources are associated. Nevertheless, we conclude that relativistic unified schemes are entirely compatible with the current observational data. For a sample selected from SDSS and FIRST which includes weak-cored triples we find that the equivalent width of the [OIII] emission line decreases as core-dominance increases, as expected, and also that core-dominated quasars are optically brighter than weak-cored quasars.
New lensed quasars from the MUSCLES survey
Neal Jackson,Hayden Rampadarath,Eran O. Ofek,Masamune Oguri,Min-Su Shin
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19857.x
Abstract: Gravitational lens systems containing lensed quasars are important as cosmological probes, as diagnostics of structural properties of the lensing galaxies and as tools to study the quasars themselves. The largest lensed quasar sample is the SDSS Quasar Lens Search, drawn from the Sloan Digital Sky Survey (SDSS). We are attempting to extend this survey using observations of lens candidates selected from a combination of the quasar sample from the SDSS and the UKIRT Infrared Deep Sky Survey (UKIDSS). This adds somewhat higher image quality together with a wider range of wavelength for the selection process. In previous pilot surveys we observed 5 objects, finding 2 lenses; here we present further observations of 20 objects in which we find 4 lenses, of which 2 are independently discovered in SQLS (in preparation). Following earlier work on the combination of these two surveys, we have refined our method and find that use of a colour-separation diagnostic, where we select for separations between components which appear to decrease in wavelength, is an efficient method to find lensed quasars and may be useful in ongoing and future large-scale strong lensing surveys with instruments such as Pan-STARRS and LSST. The new lenses have mostly high flux ratios, with faint secondaries buried in the lensing galaxy and typically 6-10 times less bright than the primary. Our survey brings the total number of lenses discovered in the SDSS quasar sample to 46, plus 13 lenses already known. This is likely to be up to 60-70% of the total number of lensed quasars; we briefly discuss strategies by which the rest might be found.
A search for distant radio-loud quasars in the CLASS survey: three new radio-selected quasars at z>4
Ignas Snellen,Richard McMahon,Jane Dennett-Thorpe,Neal Jackson,Karl-Heinz Mack,Emily Xanthopoulos
Physics , 2001, DOI: 10.1046/j.1365-8711.2001.04528.x
Abstract: We report on the search for distant radio-loud quasars in the Cosmic Lens All Sky Survey (CLASS) of flat spectrum radio sources with S_5GHz>30 mJy. Unresolved optical counterparts were selected from APM scans of POSS-I plates, with e<19.0 and red o-e>2.0 colours, in an effective area of ~6400 deg^2. Four sources were found to be quasars with z>4, of which one was previously known. This sample bridges the gap between the strong radio surveys with S_5GHz>200 mJy and the samples of radio-weak quasars that can be generated via radio observations of optically selected quasars. In addition, 4 new quasars at z>3 have been found. The selection criteria result in a success-rate of ~1:7 for radio-loud quasars at z>4, which is a significant improvement over previous studies. This search yields a surface density of 1 per 1600 deg^2, which is about a factor of ~15 lower than that found in a similar search for radio-quiet quasars at z>4. The study presented here is strongly biased against quasars beyond z>4.5, since the e-passband of the POSS-I only samples the spectra shortward of 1200 Angstrom at these redshifts.
Observations of radio-quiet quasars at 10mas resolution by use of gravitational lensing
Neal Jackson,Amitpal S. Tagore,Carl Roberts,Dominique Sluse,Hannah Stacey,Hector Vives-Arias,Olaf Wucknitz,Filomena Volino
Physics , 2015, DOI: 10.1093/mnras/stv1982
Abstract: We present VLA detections of radio emission in four four-image gravitational lens systems with quasar sources: HS0810+2554, RXJ0911+0511, HE0435$-$1223 and SDSSJ0924+0219, and e-MERLIN observations of two of the systems. The first three are detected at a high level of significance, and SDSS J0924+0219 is detected. HS0810+2554 is resolved, allowing us for the first time to achieve 10-mas resolution of the source frame in the structure of a radio quiet quasar. The others are unresolved or marginally resolved. All four objects are among the faintest radio sources yet detected, with intrinsic flux densities in the range 1-5$\mu$Jy; such radio objects, if unlensed, will only be observable routinely with the Square Kilometre Array. The observations of HS0810+2554, which is also detected with e-MERLIN, strongly suggest the presence of a mini-AGN, with a radio core and milliarcsecond scale jet. The flux densities of the lensed images in all but HE0435-1223 are consistent with smooth galaxy lens models without the requirement for smaller-scale substructure in the model, although some interesting anomalies are seen between optical and radio flux densities. These are probably due to microlensing effects in the optical.
Use of Multiple Intelligence Modalities to Convey Genetic and Genomic Concepts in African American College Biology Students  [PDF]
Jabril Johnson, Fatimah Jackson
Natural Science (NS) , 2015, DOI: 10.4236/ns.2015.76033
Abstract: Correct conceptualizations of genetics and genomics are central to understand many aspects of the STEM disciplines as they provide the foundational building blocks for later work in the life sciences. Our study of 435 African American college students investigated the use of culturally- relevant memes transmitted using multiple intelligence (MI) modalities to convey core genetic and genomic information as an alternative to the traditional teaching approaches. We observed that this approach appears to optimize the transmission and retention of core genetics concepts, identify and correct misconceptions, and serve as a conduit to increased African American (AA) access to further studies in STEM disciplines. A review of the relevant literature and specific examples of our interventions and their MI links are provided.
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