The first galaxies: assembly, cooling and the onset of turbulence     

Thomas H. Greif †  Jarrett L. Johnson  Ralf S. Klessen  Volker Bromm 《Monthly notices of the Royal Astronomical Society》2008,387(3):1021-1036

We investigate the properties of the first galaxies at   z ≳ 10  with highly resolved numerical simulations, starting from cosmological initial conditions and taking into account all relevant primordial chemistry and cooling. A first galaxy is characterized by the onset of atomic hydrogen cooling, once the virial temperature exceeds  ≃10⁴ K  , and its ability to retain photoheated gas. We follow the complex accretion and star formation history of a  ≃5 × 10⁷ M_⊙  system by means of a detailed merger tree and derive an upper limit on the number of Population III (Pop III) stars formed prior to its assembly. We investigate the thermal and chemical evolution of infalling gas and find that partial ionization at temperatures  ≳10⁴ K  catalyses the formation of  H₂  and hydrogen deuteride, allowing the gas to cool to the temperature of the cosmic microwave background. Depending on the strength of radiative and chemical feedback, primordial star formation might be dominated by intermediate-mass Pop III stars formed during the assembly of the first galaxies. Accretion on to the nascent galaxy begins with hot accretion, where gas is accreted directly from the intergalactic medium and shock heated to the virial temperature, but is quickly accompanied by a phase of cold accretion, where the gas cools in filaments before flowing into the parent halo with high velocities. The latter drives supersonic turbulence at the centre of the galaxy and could lead to very efficient chemical mixing. The onset of turbulence in the first galaxies thus likely marks the transition to Pop II star formation.  相似文献   

Magnetorotational instability in protoplanetary discs: the effect of dust grains     

Raquel Salmeron  Mark Wardle 《Monthly notices of the Royal Astronomical Society》2008,388(3):1223-1238

  相似文献   

Testing the evolutionary link between submillimetre galaxies and quasars: CO observations of QSOs at z∼ 2     

K. E. K. Coppin  A. M. Swinbank  R. Neri  P. Cox  D. M. Alexander  Ian Smail  M. J. Page  J. A. Stevens  K. K. Knudsen  R. J. Ivison  A. Beelen  F. Bertoldi  A. Omont 《Monthly notices of the Royal Astronomical Society》2008,389(1):45-62

  相似文献   

Dynamical response to supernova-induced gas removal in spiral galaxies with dark matter halo     

Hiroko Koyama  Masahiro Nagashima  Takayuki Kakehata  Yuzuru Yoshii 《Monthly notices of the Royal Astronomical Society》2008,389(1):237-249

We investigate the dynamical response, in terms of disc size and rotation velocity, to mass loss by supernovae in the evolution of spiral galaxies. A thin baryonic disc having the Kuzmin density profile embedded in a spherical dark matter halo having a density profile proposed by Navarro, Frenk & White is considered. For the purpose of comparison, we also consider the homogeneous and   r ⁻¹  profiles for dark matter in a truncated spherical halo. Assuming for simplicity that the dark matter distribution is not affected by mass-loss from discs and the change of baryonic disc matter distribution is homologous, we evaluate the effects of dynamical response in the resulting discs. We found that the dynamical response only for an adiabatic approximation of mass-loss can simultaneously account for the rotation velocity and disc size as observed particularly in dwarf spiral galaxies, thus reproducing the Tully–Fisher relation and the size versus magnitude relation over the full range of magnitude. Furthermore, we found that the mean specific angular momentum in discs after the mass-loss becomes larger than that before the mass-loss, suggesting that the mass-loss would most likely occur from the central disc region where the specific angular momentum is low.  相似文献   

Ongoing assembly of massive galaxies by major merging in large groups and clusters from the SDSS     

Daniel H. McIntosh †  Yicheng Guo  Jen Hertzberg ‡  Neal Katz  H. J. Mo  Frank C. van den Bosch  Xiaohu Yang 《Monthly notices of the Royal Astronomical Society》2008,388(4):1537-1556

  相似文献   

Calculation of the Intensity Ratio and Intrinsic Redshift Ratio of the Cerenkov Iron K_α and K_β Lines and Its Possible Application in the Study of AGNs     

Dang-bo Liu  Lei Chen  Jun-han You 《Chinese Astronomy and Astrophysics》2008

We have pointed out that, when thermal relativistic electrons with an isotropic velocity distribution move through a region of dense gas or impinge upon its surface, the Cerenkov effect will produce a particular kind of atomic or ionic emission line, which we have called the “Cerenkov line-like radiation”. This prediction for the optical wavebands has been verified by laboratory experiments. In this paper, we extend this line-like radiation theory to the X-ray waveband and give the basic formulae for calculating the intensity ratio and intrinsic redshift ratio of the Cerenkov iron K_α and K_β lines, for different ionization orders of iron. Potential application of this calculated result may be found in the study of AGNs. The recent observations of NGC3783 show that besides the iron K_α line at ∼6.4 keV, there is also a very strong iron Kβ line at ∼7.0 keV, and that the ratio of equivalent widths between the two is anomalous: EWK_α/EWK_β ≈ 3.43. It is difficult to explain this by the conventional mechanism of “photoelectric absorption-fluorescence emission”. We suggest that the mechanism of Cerenkov line-like radiation can provides a way of solving this puzzle. Besides, we expect that the calculated intrinsic redshift ratio of Cerenkov iron K_α and K_β lines could be tested in future observations. If our suggestion is further supported by observations, then our view on the physical environment around the central massive black hole of an AGN will be greatly modified: the activity becomes more energetic and violent, and the gas, much denser than previously thought.  相似文献   

Detection of CO Outflow in Rotating Cores     

Xin Guan  Yue-Fang Wu 《中国天文和天体物理学报》2008,8(2):230-236

We investigate the effect of bulk motion on the detection of molecular outflows in the sources S 146, GGD27, and IRAS 22566 5830. The traditional techniques do allow for bulk motions or systematic VLSR shifts of the core emissions, which may cause contamination of the high velocity gas emissions, and outflows may either fail to be detected or have their properties miscalculated. We used a program to follow the systematic shift of VLSR and better results have been obtained.  相似文献   

The Effect of AGN and SNe Feedback on Star Formation, Reionization and the Near Infrared Background     

Lei Wangt Ji-Rong Mao Shou-Ping Xiang Ye-Fei Yuan 《中国天文和天体物理学报》2008,8(6)

Feedback from supernovae (SNe) and from active galactic nuclei (AGN) accom-panies the history of star formation and galaxy evolution. We present an analytic model to explain how and when the SNe and AGN exert their feedback effects on the star formation and galaxy evolution processes. By using SNe and AGN kinetic feedback mechanisms based on the Lambda Cold Dark Matter (LCDM) model, we explore how these feedback mecha-nisms affect the star formation history (SFH), the Near-Infrared Background (NIRB) flux and the cosmological reionization. We find the values of the feedback strengths, ∈AGN =1.0+0.50.3and ∈SN=0.04+0.02-0.02, can provide a reasonable explanation of most of the observational re-suits, and that the AGN feedback effect on star formation history is quite different from the SNe feedback at high redshifts. Our conclusions manifest quantitatively that these feedback effects decrease star formation rate density (SFRD) and the NIRB flux (in 1.4 - 4.0μm), and postpone the time of completion of the cosmological reionization.  相似文献   

Static pressure of hot gas: Its effect on the gas disks of galaxies     

A. V. Zasov  A. V. Khoperskov 《Astronomy Letters》2008,34(11):739-744

The static pressure of the hot gas that fills clusters and groups of galaxies can affect significantly the volume density and thickness of the gas disks in galaxies. In combination with the dynamic pressure, the static pressure allows several observed peculiarities of spiral galaxies surrounded by a hot medium to be explained.  相似文献   

The formation of the Oort cloud in open cluster environments     

Nathan A. Kaib  Thomas Quinn 《Icarus》2008,197(1):221-238

We study the influence of an open cluster environment on the formation and current structure of the Oort cloud. To do this, we have run 19 different simulations of the formation of the Oort cloud for 4.5 Gyrs. In each simulation, the Solar System spends its first 100 Myrs in a different open cluster environment before transitioning to its current field environment. We find that, compared to forming in the field environment, the inner Oort cloud is preferentially loaded with comets while the Sun resides in the open cluster and that most of this material remains locked in the interior of the cloud for the next 4.4 Gyrs. In addition, the outer Oort cloud trapping efficiencies we observe in our simulations are lower than previous formation models by about a factor of 2, possibly implying an even more massive early planetesimal disk. Furthermore, some of our simulations reproduce the orbits of observed extended scattered disk objects, which may serve as an observational constraint on the Sun's early environment. Depending on the particular open cluster environment, the properties of the inner Oort cloud and extended scattered disk can vary widely. On the other hand, the outer portions of the Oort cloud in each of our simulations are all similar.  相似文献