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NGC 6391



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Optical and Near-Infrared Color Profiles in Nearby Early-Type Galaxies and the Implied Age and Metallicity Gradients
We present results of an age and metallicity gradient analysis inferredfrom both optical and near-infrared surface photometry. The analysis isbased on a sample of 36 nearby early-type galaxies, obtained from theEarly Data Release of the Sloan Digital Sky Survey and the Two MicronAll Sky Survey. Surface brightness profiles were derived in each bandand used to study the color gradients of the galaxies. Using simplestellar population models with both optical and near-infrared colors, wemay interpret the color gradients in terms of age and metallicitygradients of galaxies. UsinggZ≡dlogZmet/dlogR andgA=dlog(age)/dlogR to represent the metallicity and agegradients, we found a median value of gZ=-0.25+/-0.03 for themetallicity gradient, with a dispersionσgZ=0.19+/-0.02. The corresponding valuesfor the age gradients were gA=0.02+/-0.04 andσgA=0.25+/-0.03. These results are in goodagreement with recent observational results, as well as with recentsimulations that suggest that both monolithic collapse and major mergershave played important roles in the formation of early-type galaxies. Ourresults demonstrate the potential of using multi-wave band colorsobtained from current and future optical and infrared surveys inconstraining the age and metallicity gradients of early-type galaxies.

Magnetic collimation by accretion discs of quasars and stars
After an historical introduction on the relationships between quasars,accretion discs and jets, the paper concentrates on the problem ofcollimation. When the central upward magnetic flux from an accretiondisc is twisted relative to the returning downward flux, the force-freefield splays out at a semi-angle of 60 deg and all the flux reaches outto infinity after only half a turn. However, when the accretion discitself lies within a medium with an ambient pressure the field cannotsplay out in this way because too much work is needed to push the mediumaway. Whereas the field initially splays out, it ceases to do so whenB^2/8pi balances the ambient pressure. Thereafter it assumes a verticalcylindrical geometry in which each additional twist of the fieldproduces an equal increment in the height of the cylinder. After manytwists the pitch of the field becomes no greater but the cylinderbecomes very tall and thin. It is suggested that these structures arethe collimators that lead to the remarkably narrow jets seen in quasarsand radio galaxies, and associated with the accretion discs of youngstars leading to Herbig-Haro objects, etc. Taken to the extreme, thejets are primarily these growing towers of twisted magnetic fieldtogether with the currents that they carry. A simple analytic model ofsuch a cylindrical tower is built from force-free fields and it is shownto have the properties of a stabilized pinch. A modificationdemonstrates how dynamical expansion will affect the model.

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Observation and Astrometry data

Right ascension:17h28m48.90s
Aparent dimensions:0.724′ × 0.501′

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 6391

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