University of Hawaii

About: University of Hawaii is a education organization based out in Honolulu, Hawaii, United States. It is known for research contribution in the topics: Galaxy & Population. The organization has 17971 authors who have published 36102 publications receiving 1620196 citations.

The Space Density of Compton-Thick Active Galactic Nucleus and the X-Ray Background

Abstract: We constrain the number density and evolution of Compton-thick (CT) active galactic nuclei (AGNs). In the local universe, we use the wide-area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on a combination of X-ray and mid-infrared (mid-IR) parameters. We find a significantly lower space density of CT AGNs in the local universe than expected from published AGN population synthesis models to explain the X-ray background (XRB). This can be explained by the numerous degeneracies in the parameters of those models; we use the high-energy surveys described here to remove those degeneracies. We show that only direct observations of CT AGNs can currently constrain the number of heavily obscured supermassive black holes. At high redshift, the inclusion of IR-selected CT AGN candidates leads to a much higher space density, implying (1) a different (steeper) evolution for these sources compared to less-obscured AGNs, (2) that the IR selection includes a large number of interlopers, and/or (3) that there is a large number of reflection-dominated AGNs missed in the INTEGRAL and Swift observations. The contribution of CT AGN to the XRB is small, ~9%, with a comparable contribution to the total cosmic accretion, unless reflection-dominated CT AGNs significantly outnumber transmission-dominated CT AGNs, in which case their contribution can be much higher. Using estimates derived here for the accretion luminosity over cosmic time, we estimate the local mass density in supermassive black holes and find a good agreement with available constraints for an accretion efficiency of ~10%. Transmission-dominated CT AGNs contribute only ~8% to total black hole growth.

Biochemistry of the violaxanthin cycle in higher plants

Abstract: The biochemistry of the violaxanthin cycle in relationship to photosynthesis is reviewed. The cycle is a component of the thylakoid and consists of a reaction sequence in which violaxanthin is converted to zeaxanthin (de-epoxidation) and then regenerated (epoxidation) through separate reaction mechanisms. The arrangement of the cycle in the thylakoid is transmembranous with the de-epoxidation system situated on the loculus side and epoxidation on the outer side of the membrane. Photosynthetic activities affect turnover of the cycle but the cycle itself consists entirely of dark reactions. Light has at least two roles in de-epoxidation. It establishes through the proton pump the acidic pH in the loculus that is required for de-epoxidase activity and it induces a presumed conformational change in the inner membrane surface which determines the fraction of violaxanthin in the membrane that enters the cycle. De-epoxidation, which requires ascorbate, is presumed to proceed by a reductive-dehydration mechanism. Non-cyclic electron transport can provide the required reducing potential through the dehydroascorbate-ascorbate couple. Whether ascorbate reduces the de-epoxidase system directly or through an intermediate has not been settled. Epoxidation requires NADPH and O2 which suggests a reductive mechanism. In contrast with de-epoxidation, it has a pH optimum near neutrality. The coupling of photosynthetically generated NADPH to epoxidation has been shown. Turnover of the cycle under optimal conditions is estimated to be about two orders of magnitude below optimal electron transport rates. This low rate appears to exclude a direct role of the cycle in photosynthesis or a role in significantly affecting photosynthate levels in a back reaction. The fact that the cycle is sensitive to events both before and after Photosystem I suggests a regulatory role, possibly through effects on membrane properties. A model showing the various relationships of the cycle to photosynthesis is presented. The contrasting view that the cycle can participate directly in photosynthesis, such as in oxygen evolution, is discussed. Violaxanthin de-epoxidase has been purified. It is a lipoprotein which contains monogalactosyldiglyceride (MG) exclusively. The enzyme is a mono-de-epoxidase which is specific for 3-OH, 5–6-epoxy carotenoids that are in a 3R , 5S , 6R configuration. In addition, the polyene chain must be all- trans . A model has been presented which depicts enzymic MG in a receptor role and the stereospecific active center situated in a narrow well-like depression that can accommodate only the all- trans structure.

Salts on Europa's Surface Detected by Galileo's Near Infrared Mapping Spectrometer

Abstract: Reflectance spectra in the 1- to 2.5-micrometer wavelength region of the surface of Europa obtained by Galileo's Near Infrared Mapping Spectrometer exhibit distorted water absorption bands that indicate the presence of hydrated minerals. The laboratory spectra of hydrated salt minerals such as magnesium sulfates and sodium carbonates and mixtures of these minerals provide a close match to the Europa spectra. The distorted bands are only observed in the optically darker areas of Europa, including the lineaments, and may represent evaporite deposits formed by water, rich in dissolved salts, reaching the surface from a water-rich layer underlying an ice crust.

Mars methane detection and variability at Gale crater

Abstract: Reports of plumes or patches of methane in the Martian atmosphere that vary over monthly timescales have defied explanation to date. From in situ measurements made over a 20-month period by the Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars (SAM) instrument suite on Curiosity at Gale Crater, we report detection of background levels of atmospheric methane of mean value 0.69 ± 0.25 ppbv at the 95% confidence interval (CI). This abundance is lower than model estimates of ultraviolet (UV) degradation of accreted interplanetary dust particles (IDP’s) or carbonaceous chondrite material. Additionally, in four sequential measurements spanning a 60-sol period, we observed elevated levels of methane of 7.2 ± 2.1 (95% CI) ppbv implying that Mars is episodically producing methane from an additional unknown source.