3,871 research outputs found
The Hubble Sequence in Groups: The Birth of the Early-Type Galaxies
The physical mechanisms and timescales that determine the morphological
signatures and the quenching of star formation of typical (~L*) elliptical
galaxies are not well understood. To address this issue, we have simulated the
formation of a group of galaxies with sufficient resolution to track the
evolution of gas and stars inside about a dozen galaxy group members over
cosmic history. Galaxy groups, which harbor many elliptical galaxies in the
universe, are a particularly promising environment to investigate morphological
transformation and star formation quenching, due to their high galaxy density,
their relatively low velocity dispersion, and the presence of a hot intragroup
medium. Our simulation reproduces galaxies with different Hubble morphologies
and, consequently, enables us to study when and where the morphological
transformation of galaxies takes place. The simulation does not include
feedback from active galactic nuclei showing that it is not an essential
ingredient for producing quiescent, red elliptical galaxies in galaxy groups.
Ellipticals form, as suspected, through galaxy mergers. In contrast with what
has often been speculated, however, these mergers occur at z>1, before the
merging progenitors enter the virial radius of the group and before the group
is fully assembled. The simulation also shows that quenching of star formation
in the still star-forming elliptical galaxies lags behind their morphological
transformation, but, once started, is taking less than a billion years to
complete. As long envisaged the star formation quenching happens as the
galaxies approach and enter the finally assembled group, due to quenching of
gas accretion and (to a lesser degree) stripping. A similar sort is followed by
unmerged, disk galaxies, which, as they join the group, are turned into the
red-and-dead disks that abound in these environments.Comment: 12 pages, 12 Figures, 1 Table, accepted for publication in AP
Vector meson - mixing and their form factors in light-cone quark model
The vector meson - mixing is studied in two alternative
scenarios with different numbers of mixing angles, i.e., the one-mixing-angle
scenario and the two-mixing-angle scenario, in both the octect-singlet mixing
scheme and the quark flavor mixing scheme. Concerning the reproduction of
experimental data and the behavior of transition form factors,
one-mixing-angle scenario in the quark flavor scheme performs better than that
in the octet-singlet scheme, while the two-mixing-angle scenario works well for
both mixing schemes. The difference between the two mixing angles in the
octet-singlet scheme is bigger than that in the quark flavor scheme.Comment: 16 pages, 7 figures, final version to appear in PR
Preventing Trafficking by Protecting Refugees
An inherent tension underlies the duty to prevent trafficking. On the one hand, nation-states are required to take border control measures aimed at preventing trafficking. At the same time, such measures must respect international obligations toward asylum-seekers and other migrants relating to the free movement of people. In the past twenty years, countries such as the United States have developed increasingly sophisticated systems designed to regulate and restrict the movement of people across borders. However, the same period has seen an increasing disregard for the human rights of the very people who are crossing those borders. In order to fully meet the duty to prevent trafficking, states must come to recognize the importance of involving victims of this crime in the solution, which will never happen if countries demonize all migrants as criminals and traffickers. In short, states that seek to lead the fight against human trafficking need to work with victims (including foreign national victims in the state’s territory) and other partners (such as non-governmental organizations and victims’ attorneys) to ensure that their rhetoric more closely matches reality
Changes in vegetation spectra with deterioration of leaves under two methods of preservation
An experiment to measure changes in leaf spectra under different methods of preservation over time was conducted. The spectral measurements were made by a three band hand held radiometer which simulated three Thematic Mapper (TM) bands: TM3, TM4, and TM5. Daily spectral measurements of white oak leaves under three preservation treatments were made. The spectral readings over three treatments (fresh, bottled, and bagged vegetation) were indistinguishable in bands TM3 and TM5 for up to 4 days after collection. After that time bagged and bottled samples showed significant increases in reflected energy caused by loss of chlorophyll from and dehydration of the vegetation. No significant variation in the reflectance values from TM4 over preservation type for the experimental period was observed
Geochemistry and metallogeny of Neoproterozoic pyrite in oxic and anoxic sediments
The Neoproterozoic Dalradian Supergroup contains widespread diagenetic sulphides present as pyrite. The sulphides occur in both carbonaceous shales and glacial diamictites, that were deposited in relatively reducing and oxidising conditions respectively. The trace element compositions of the pyrite, and consequently the whole rock compositions, contrast between the two lithologies. The highest concentrations of selenium, tellurium and gold are all found in diamictite-hosted pyrite. The data suggest that increased mobility of these elements in oxidising conditions led to greater uptake when pyrite was precipitated. As one model for the formation of orogenic gold ore deposits assumes a sulphide-rich protolith, pyrite ultimately formed during relatively oxidising conditions could make a contribution, including the widespread pyrite precipitated during the Neoproterozoic ‘Snowball Earth’ glaciations
A black shale protolith for gold-tellurium mineralisation in the Dalradian Supergroup (Neoproterozoic) of Britain and Ireland
The Dalradian Supergroup of Britain and Ireland is mineralised by gold-tellurium vein deposits. The host succession includes carbonaceous, pyritic shales (pelites) which were a source of trace elements, including gold and tellurium. LA-ICP-MS mapping of pyrite crystals shows that late stages are enriched in gold, tellurium and lead, representing concentration of these elements during metamorphism and related hydrothermal activity. The sulphur isotope composition of the pyrite varies with stratigraphic position, reflecting an origin for the pyrite in the depositional environment through microbial sulphate reduction. Where pyrite was converted to pyrrhotite, trace element contents are much lower, indicating element liberation during metamorphism. These observations are consistent with a model of black shale protoliths for orogenic gold deposits
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