Centaurs from the Oort cloud and the origin of Jupiter-family comets     

V. V. Emel'yanenko  D. J. Asher  M. E. Bailey 《Monthly notices of the Royal Astronomical Society》2005,361(4):1345-1351

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Can the faint submillimetre galaxies be explained in the Λ cold dark matter model?     

C. M. Baugh  C. G. Lacey  C. S. Frenk  G. L. Granato  L. Silva  A. Bressan  A. J. Benson  S. Cole 《Monthly notices of the Royal Astronomical Society》2005,356(3):1191-1200

We present predictions for the abundance of submillimetre galaxies (SMGs) and Lyman-break galaxies (LBGs) in the Λ cold dark matter cosmology. A key feature of our model is the self-consistent calculation of the absorption and emission of radiation by dust. The new model successfully matches the LBG luminosity function, as well as reproducing the properties of the local galaxy population in the optical and infrared. The model can also explain the observed galaxy number counts at 850 μm, but only if we assume a top-heavy initial mass function for the stars formed in bursts. The predicted redshift distribution of SMGs depends relatively little on their flux over the range 1–10 mJy, with a median value of   z ≈ 2.0  at a flux of 5 mJy, in good agreement with the recent measurement by Chapman et al. The counts of SMGs are predicted to be dominated by ongoing starbursts. However, in the model these bursts are responsible for making only a few per cent of the stellar mass locked up in massive ellipticals at the present day.  相似文献   

The 60-μm extragalactic background radiation intensity, dust-enshrouded active galactic nuclei and the assembly of groups and clusters of galaxies     

A. W. Blain  T. G. Phillips 《Monthly notices of the Royal Astronomical Society》2002,333(1):222-230

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Metal Dispersal and the Number of Population III Stars     

M.S. Oey 《Astrophysics and Space Science》2002,281(1-2):483-486

Two fundamental constraints on the earliest star formation conditions in the Galaxy are an apparent empirical low-metallicity threshold of-4 ? [Fe/H], an and upper limit to the fraction of Population III halo stars of F _III < 4 × 10^-4. How do these observed constraints compare with predictions of simple models? This is investigated within the framework of element dispersal from clustered core-collapse SNe. Simple arguments considering turbulent mixing within multi-phase ISM suggest that the observed low-metallicity threshold is consistent with rough expected values. However, the observed limit on F _III is two orders of magnitude larger than predictions from this simple, one-zone inhomogeneous chemical evolution.  相似文献   

How the X-ray spectrum of a narrow-line Seyfert 1 galaxy may be reflection-dominated     

A. C.Fabian  D. R.Ballantyne  A.Merloni  S.Vaughan  K.Iwasawa  Th.Boller 《Monthly notices of the Royal Astronomical Society》2002,331(3):L35-L39

A model for the inner regions of accretion flows is presented where, owing to disc instabilities, cold and dense material is clumped into deep sheets or rings. Surrounding these density enhancements is hot, tenuous gas where coronal dissipation processes occur. We expect this situation to be most relevant when the accretion rate is close to Eddington and the disc is radiation-pressure-dominated, and so may apply to narrow-line Seyfert 1 (NLS1) galaxies. In this scenario, the hard X-ray source is obscured for most observers, and so the detected X-ray emission would be dominated by reflection off the walls of the sheets. A simple Comptonization calculation shows that the large photon-indices characteristic of NLS1s would be a natural outcome of two reprocessors closely surrounding the hard X-ray source. We test this model by fitting the XMM-Newton spectrum of the NLS1 1H  0707–495  between 0.5 and 11 keV with reflection-dominated ionized disc models. A very good fit is found with three different reflectors each subject to the same  Γ=2.35  power law. An iron overabundance is still required to fit the sharp drop in the spectrum at around 7 keV. We note that even a small corrugation of the accretion disc may result in  Γ>2  and a strong reflection component in the observed spectrum. Therefore, this model may also explain the strength and the variability characteristics of the MCG–6-30-15 Fe K α line. The idea needs to be tested with further broad-band XMM-Newton observations of NLS1s.  相似文献   

Relative velocities among accreting planetesimals in binary systems: The circumprimary case     

P. Thébault  F. Marzari 《Icarus》2006,183(1):193-206

We investigate classical planetesimal accretion in a binary star system of separation ab?50 AU by numerical simulations, with particular focus on the region at a distance of 1 AU from the primary. The planetesimals orbit the primary, are perturbed by the companion and are in addition subjected to a gas drag force. We concentrate on the problem of relative velocities Δv among planetesimals of different sizes. For various stellar mass ratios and binary orbital parameters we determine regions where Δv exceed planetesimal escape velocities vesc (thus preventing runaway accretion) or even the threshold velocity vero for which erosion dominates accretion. Gaseous friction has two crucial effects on the velocity distribution: it damps secular perturbations by forcing periastron alignment of orbits, but at the same time the size-dependence of this orbital alignment induces a significant Δv increase between bodies of different sizes. This differential phasing effect proves very efficient and almost always increases Δv to values preventing runaway accretion, except in a narrow eb?0 domain. The erosion threshold Δv>vero is reached in a wide (ab,eb) space for small <10-km planetesimals, but in a much more limited region for bigger ?50-km objects. In the intermediate vesc<Δv<vero domain, a possible growth mode would be the type II runaway growth identified by Kortenkamp et al. [Kortenkamp, S., Wetherill, G., Inaba, S., 2001. Science 293, 1127-1129].  相似文献   

SCUBA observations of the Horsehead nebula – what did the horse swallow?     

D. Ward-Thompson  D. Nutter  S. Bontemps  A. Whitworth  R. Attwood 《Monthly notices of the Royal Astronomical Society》2006,369(3):1201-1210

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星系形成的半解析模型研究进展     

康熙 《天文学进展》2005,23(2):135-143

基于目前流行的冷暗物质等级成团理论的星系形成的半解析模型取得了很大进展．在引入一系列合理的关于重子物质物理过程的假设，如气体冷却、恒星形成率、超新星能量反馈以后，结合N体数值模拟，星系形成的半解析模型一方面能较成功地预言许多与观测符合的近邻星系的性质，如光度函数、星系的两点相关函数、Thlly-Fisher关系、星系团中椭圆星系的颜色一星等关系等；另一方面还能较成功地预言宇宙的恒星形成历史，以及一部分高红移星系的性质，如Lyman-Break星系的数目随红移的分布．但是最近一些观测表明，半解析模型在高红移预言的大质量星系较少，且红星系数目也较观测少．重点讨论半解析模型成功与不足之处，并指出解决这些问题可能的途径。  相似文献   

The formation of ultracompact dwarf galaxies     

MichaelFellhauer  PavelKroupa 《Monthly notices of the Royal Astronomical Society》2002,330(3):642-650

Recent spectroscopic observations of galaxies in the Fornax Cluster reveal nearly unresolved 'star-like' objects with redshifts appropriate to the Fornax Cluster. These objects have intrinsic sizes of ≈100 pc and absolute B -band magnitudes in the range  −14< M _B<−11.5 mag  and lower limits for the central surface brightness   μ _B≳23 mag arcsec⁻²  , and so appear to constitute a new population of ultracompact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar superclusters (by Kroupa) , which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of superclusters in a tidal field. The YMCs merge on a few supercluster crossing times. Superclusters that are initially as concentrated and massive as knot S in the interacting Antennae galaxies evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. Less massive superclusters resembling knot 430 in the Antennae may evolve to ω Cen-type systems. Low-concentration superclusters are disrupted by the tidal field, dispersing their surviving star clusters while the remaining merger objects rapidly evolve into the   μ _B− M _B  region populated by low-mass Milky Way dSph satellites.  相似文献   

On the origin of isophotal shapes in elliptical galaxies     

S. Khochfar  A. Burkert 《Monthly notices of the Royal Astronomical Society》2005,359(4):1379-1385

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