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31.
Rolando Dünner reas Reisenegger rés Meza † Pablo A. Araya Hernán Quintana 《Monthly notices of the Royal Astronomical Society》2007,376(4):1577-1587
An exponentially expanding Universe, possibly governed by a cosmological constant, forces gravitationally bound structures to become more and more isolated, eventually becoming causally disconnected from each other and forming so-called 'island universes'. This new scenario reformulates the question about which will be the largest structures that will remain gravitationally bound, together with requiring a systematic tool that can be used to recognize the limits and mass of these structures from observational data, namely redshift surveys of galaxies. Here we present a method, based on the spherical collapse model and N -body simulations, by which we can estimate the limits of bound structures as observed in redshift space. The method is based on a theoretical criterion presented in a previous paper that determines the mean density contrast that a spherical shell must have in order to be marginally bound to the massive structure within it. Understanding the kinematics of the system, we translated the real-space limiting conditions of this 'critical' shell to redshift space, producing a projected velocity envelope that only depends on the density profile of the structure. From it we created a redshift-space version of the density contrast that we called 'density estimator', which can be calibrated from N -body simulations for a reasonable projected velocity envelope template, and used to estimate the limits and mass of a structure only from its redshift-space coordinates. 相似文献
32.
We use a comparison between the autocorrelation length value in 21-cm hi maps of Holmberg II and the autocorrelation length derived from simulated maps in order to constrain the physical processes that are responsible for the observed distribution of holes and shells in the interstellar medium (ISM) of the galaxy. Our results show that large scale turbulence coupled to a weak cooling can reproduce the value of the autocorrelation length which is observed. 相似文献
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34.
We use semi-analytical modelling of galaxy formation to predict the mix of elliptical galaxies with boxy and disky isophotes,
assuming they originated from major mergers of different mass ratios. Numerical simulations of merging spiral galaxies indicate
equal mass mergers leading to boxy and merger with a mass ratio of 3:1 to disky ellipticals. Assigning isophotal shapes to
elliptical galaxies in our model we find bright disky ellipticals being as frequent or more frequent as bright boxy ellipticals,
in contrast to observations which indicate that most of the bright ellipticals should be boxy. The precursors of bright ellipticals
in our model are mainly also ellipticals which merge with each other later. Assuming that the merger of two ellipticals results
in boxy ellipticals increases the fraction of bright boxy ellipticals. By defining a disky as a bulge dominated galaxy with
an additional disk mass of more than 20% the total baryonic mass, increases the fraction of low mass disky ellipticals and
reproduces the observed trend of a steep increase in the fraction of low mass disky ellipticals.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
35.
We use semi-analytical modelling of galaxy formation to predict the mix of elliptical galaxies with boxy and disky isophotes,
assuming they originated from major mergers of different mass ratios. Numerical simulations of merging spiral galaxies indicate
equal mass mergers leading to boxy and merger with a mass ratio of 3:1 to disky ellipticals. Assigning isophotal shapes to
elliptical galaxies in our model we find bright disky ellipticals being as frequent or more frequent as bright boxy ellipticals,
in contrast to observations which indicate that most of the bright ellipticals should be boxy. The precursors of bright ellipticals
in our model are mainly also ellipticals which merge with each other later. Assuming that the merger of two ellipticals results
in boxy ellipticals increases the fraction of bright boxy ellipticals. By defining a disky as a bulge dominated galaxy with
an additional disk mass of more than20% the total baryonic mass, increases the fraction of low mass disky ellipticals and
reproduces the observed trend of a steep increase in the fraction of low mass disky ellipticals.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
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37.
Eruptive history and magmatic evolution of the 1.9 kyr Plinian dacitic Chiltepe Tephra from Apoyeque volcano in west-central Nicaragua 总被引：1，自引：1，他引：0
The youngest dacitic Plinian eruption in west-central Nicaragua, forming the 18 km3 Chiltepe Tephra (CT), occurred about nineteen hundred years ago at Apoyeque stratovolcano, which dominates the Chiltepe volcanic
complex 15 km north of the capital Managua, where the CT is 2 m thick. We have traced the CT from its proximal facies at the
crater rim, through the medial facies in the lowlands around Apoyeque, and to the distal facies up to 550 km offshore in the
Pacific. While medial and distal facies consist of widespread Plinian fall deposits, the proximal facies reveals the complexity
of this eruption, which we divide into four phases (I–IV). Interaction of rising magma with a pre-existing crater lake generated
the phreatomagmatic opening phase I of the eruption, which produced ash fall with accretionary lapilli. Phase II marked a
rapid change to persistent magmatic activity that yielded several large Plinian eruptions, declining through a period of unstable
eruption conditions, followed by a short hiatus. Phase III began with unstable conditions, probably as a result of eastward
migration and widening of the vent, leading to a second period of Plinian eruptions with three major events reaching magma
discharge rates five times larger than those of phase II. Phase III again declined through unstable eruption conditions before
magmatic activity terminated. Numerous explosions in the shallow hydrothermal system during the final phase IV resulted in
the formation of a phreatic tuff ring on the rim of Apoyeque crater. The white, highly-vesicular, dacitic CT pumice contains
plagioclase (An45–68), orthopyroxene, clinopyroxene, and minor hornblende, apatite and titanomagnetite phenocrysts. A very subordinate fraction
of gray pumice has the highest crystal content, the least evolved bulk-rock, but the most evolved matrix-glass composition.
The CT dacite has two unusual compositional features: (1) all white dacite has the same melt (matrix-glass) composition such
that variations in bulk-rock compositions (64–68 wt% SiO2) simply reflect different phenocryst contents of 10–35%, interpreted as the result of gradual phenocryst settling in the
magma chamber. (2) Abundant olivine crystals with a bimodal distribution in Mg# (modes at Mg# = 0.75 and Mg# = 0.8) are dispersed
throughout the erupted dacite. These are clearly out of equilibrium with the dacitic melt and are interpreted as xenocrysts
derived from the basaltic Nejapa-Miraflores volcanic lineament that intersects the Chiltepe volcanic complex and was contemporaneously
active. Thermobarometric estimates place the dacitic CT magma reservoir in the upper crust (<250 MPa), with a temperature
of about 890°C and about 5 wt% water dissolved in the melt. Comparing water and chlorine contents with respective solubility
models suggests that volatile degassing began in the magma reservoir and triggered the CT eruption. From the vertical compositional
variation pattern of the CT we deduce that the conduit tapped the magma chamber not at the top but from the side, at some
deeper level, and that subsequent magma withdrawal was governed by both variations in discharge rate and possible upward migration
and/or widening of the conduit entrance. 相似文献
38.
Roland Jesseit Michele Cappellari Thorsten Naab Eric Emsellem Andreas Burkert 《Monthly notices of the Royal Astronomical Society》2009,397(3):1202-1214
We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the λR -parameter, which is a luminosity-weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early-type galaxies in so-called fast λR > 0.1 and slow rotators λR < 0.1 . Tests on merger remnants reveal that λR is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of λR values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in transforming fast rotators into slow rotators in a single binary merger, the latter being created mostly in an equal-mass merger. Slow rotators have a λR which does not vary with projection. The confusion rate with face-on fast rotators is very small. Mergers with a gas component form slow rotators with smaller ellipticities than collisionless merger remnants have, and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators, but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of λ R as high as our progenitor disc galaxies. 相似文献
39.
In view of the Turbulent Cooling Flows scenario we carry out several 3D axisymmetric calculations to follow the evolution of magnetically subcritical weakly ionized and rotating turbulent cloud cores. Turbulent Cooling Flows appear to pronounce the effects of ambipolar diffusion considerably, inducing thereby a runaway collapse of the core already on a diluted free-fall time scale. Ambipolar diffusion significantly weakens the efficiency of magnetic braking. This implies that most of the rotational energy is trapped into the dynamically collapsing core and that initiation of outflows is prevented at least in the early isothermal phases. The trapped rotational energy is found to enhance the formation of rings that may afterwards fragment. It is shown that the central region of a strongly ionized magnetically subcritical core is principally overdense, with central density up to one order of magnitude larger than the surroundings. These results confirm that large scale magnetic fields threading a cloud core relax the supersonic random motions on an Alfvén wave crossing time. Moreover, ambipolar diffusion enhances dissipation of supersonic turbulence even more. 相似文献
40.
Seb Oliver Robert G. Mann Ruth Carballo Alberto Franceschini Michael Rowan-Robinson Maria Kontizas Anastasios Dapergolas Evanghelos Kontizas Aprajita Verma David Elbaz Gian Luigi Granato Laura Silva Dimitra Rigopoulou J. Ignacio Gonzalez-Serrano Steve Serjeant reas Efstathiou Paul P. van der Werf 《Monthly notices of the Royal Astronomical Society》2002,332(3):536-548
We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory ( ISO ). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population. 相似文献