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1.
Hierarchical clustering of dark matter halos is thought to describe well the large scale structure of the universe. The baryonic component of the halos is shock heated to the virial temperature while a small fraction of the energy flux through the shocks may be energized through the first order Fermi process to relativistic energy per particle. It has been proposed that the electrons accelerated in this way may upscatter the photons of the universal microwave background to gamma ray energies and indeed generate a diffuse background of gamma rays that compares well to the observations. In this paper, we calculate the spectra of the particles accelerated at the merger shocks and re-evaluate the contribution of structure formation to the extragalactic diffuse gamma ray background (EDGRB), concluding that this contribution adds up to at most 10% of the observed EDGRB.  相似文献   

2.
The evolution of the Star Formation Rate (SFR) density of the Universe as a function of look-back time is a fundamental parameter in order to understand the formation and evolution of galaxies. The current picture, only outlined in the last years, is that the global SFR density has dropped by about an order of magnitude from a redshift of z∼1.5 to the current value at z=0. Because these SFR density studies are now extended to the whole range in redshift, it becomes mandatory to combine data from different SFR tracers. At low redshifts, optical emission lines are the most widely used. Using Hα as current-SFR tracer, the Universidad Complutense de Madrid (UCM) Survey provided the first estimation of the global SFR density in the Local Universe. The Hα flux in emission is directly related to the number of ionizing photons and, modulo IMF, to the total mass of stars formed. Metallic lines like [OII]λ3727 and [OIII]λ5007 are affected by metallicity and excitation. Beyond redshifts z∼0.4, Hα is not observable in the optical and [OII]λ3727 or UV luminosities have to be used. The UCM galaxy sample has been used to obtain a calibration between [OII]λ3727 luminosity and SFR specially suitable for the different types of star-forming galaxies found by deep spectroscopic surveys in redshifts up to z∼1.5. These calibrations, when applied to recent deep redshift surveys confirm the drop of the SFR density of the Universe since z∼1 previously infered in the UV. However, the fundamental parameter that determines galactic evolution is mass, not luminosity. The mass function for local star-forming galaxies is critical for any future comparison with other galaxy populations of different evolutionary status. Hα velocity-widths for UCM galaxies indicate that besides a small fraction of 1010-1011 M⊙ starburst nuclei spirals, the majority have dynamical masses in the ∼109 M⊙ range. A comparison with published data for faint blue galaxies suggests that star-forming galaxies at z∼1 would have SFR per unit mass and burst strengths similar to those at z=0, but being intrinsically more massive. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

3.
Current cold dark matter models of structure formation make a clear prediction for cosmic structures in the Dark Ages. We discuss the formation and nature of the first collapsed and first luminous objects in the universe arising in these theories. The first virialized objects are dark matter halos at the free streaming length which depends on the mass and nature of the assumed weakly interacting massive particle. The first objects that also contain significant fractions of gas have masses of the cosmological Jeans scale ∼ 104M ⊙ at the redshifts of interest (z ∼ 30). The first pre-galactic objects that host stars have masses of 106 M ⊙. This mass scale is given by the requirement of a sufficiently high virial temperature to enable the chemical reactions necessary to form molecular hydrogen which subsequently allows the gas to dissipate its gravitational energy and to collapse to form a star. An individual massive star is formed per such object and explodes in a supernova within a few Myrs. All these stages of the formation of the first objects are illustrated by fully resolved three dimensional cosmological hydrodynamic simulations. This revised version was published online in September 2006 with corrections to the Cover Date.  相似文献   

4.
We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk. Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 1010 cm from the neutron star. At a distance R ∼ 1011 cm, where the total surface density in the disk reaches Σ0 ∼ 20 g cm−2, X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence of an extended atmospheric layer with a temperature T atm ≈ (2–3) × 106 K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range of X-ray photon energies E < 20 keV.  相似文献   

5.
We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a −4. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.  相似文献   

6.
Due to the Hubble redshift, photon energy, chiefly in the form of CMBR photons, is currently disappearing from the universe at the rate of nearly 1055 erg s−1. An ongoing problem in cosmology concerns the fate of this energy. In one interpretation it is irretrievably lost, i.e., energy is not conserved on the cosmic scale. Here we consider a different possibility which retains universal energy conservation. Treating gravitational potential energy conventionally as ‘negative’, it has earlier been proposed that the Hubble shift flips positive energy (photons) to negative energy (gravitons) and vice versa. The lost photon energy would thus be directed towards gravitation, making gravitational energy wells more negative. Conversely, within astrophysical bodies, the flipping of gravitons to photons would give rise to a ‘Hubble luminosity’ of magnitude −UH 0, where U is the internal gravitational potential energy of the object. Preliminary evidence of such an energy release is presented in bodies ranging from planets, white dwarfs and neutron stars to supermassive black holes and the visible universe.  相似文献   

7.
We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443, is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4 keV band of 2.9×1045 erg s − 1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The 1400 MHz radio luminosity (L r) of the halo is ∼2 × 1025 W Hz − 1, in good agreement with the value expected from the L x − L r correlation for cluster halos.  相似文献   

8.
Black holes are the most compact objects in the universe. Therefore, matter accreting onto them is likely to radiate photons of energy comparable to very high gravitational potential energy. We discuss the nature of the emitted radiation in X-rays and gamma-rays from black hole candidates. We present theoretical solutions, which comprise both Keplerian and sub-Keplerian components and suggest that shocks in accretion and outflows may play a major role in producing these spectra.  相似文献   

9.
A composite sample of NIR-selected galaxies having extended multicolor coverage has been used to probe the cosmological evolution of the blue luminosity function and of the stellar mass function. The bright fraction of the sample has spectroscopic redshifts, and the remaining fraction well-calibrated photometric redshifts. The resulting blue luminosity function shows an increasing brightening with redshift respect to the local luminosity function. Hierarchical CDM models predictions are in agreement only at low and intermediate redshifts but fail to reproduce the observed brightening at high redshifts (z ∼ 2–3). This brightening marks the epoch where starburst activity triggered by galaxy interactions could be an important physical mechanism for the galaxy evolution. At the same time the NIR galaxy sample has been used to trace the evolution of the cosmological stellar mass density up to ∼3. A clear decrease of the average mass density is apparent with a fraction ∼15% of the local value at z ∼ 3. UV bright star-forming galaxies are substancial contributors to the evolution of the stellar mass density. Although these results are globally consistent with Λ–CDM scenarios, they tend to underestimate the mass density produced by more massive galaxies present at z > 2.  相似文献   

10.
In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with ΛCDM and ΛDGP. We focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the f(R)-DGP scenario is stable if one consider the curvature fluid as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up to the parameters values adopted in this paper, the extended F(R,ϕ)-DGP scenario is closer to the ΛCDM scenario than other proposed models. All of these scenarios explain the late-time cosmic speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different: while the ΛDGP model transits to the accelerating phase much earlier, the F(R,ϕ)-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper, the age of the universe in the f(R)-DGP model is larger than ΛCDM, but this age in F(G,ϕ)-DGP is smaller than ΛCDM.  相似文献   

11.
The Doppler parameter distribution of Lyα absorption lines is calculated for a set of numerical simulations with different re-ionization histories. The differences in temperature between different re-ionization histories are as large as a factor of 3–4 depending on the spectrum of the ionizing sources and the redshift of helium re-ionization. These temperature differences result in observable differences in the Doppler parameter distribution. The best agreement with the observed Doppler parameter distribution between redshifts of 2 and 4 is found if hydrogen and helium are re-ionized simultaneously at or before a redshift of 5 with a quasar-like spectrum.  相似文献   

12.
The dynamical masses of dwarf-spheroidals, spiral and elliptical galaxies, dwarf irregular binaries, groups of galaxies and clusters are shown to lie in a band about the M ∼ ρR3 line. The value of ρ is approximately the same as that estimated for unseen matter in the solar neighbourhood. The clusters themselves lie about theM ∼ R -3 line derived for a self-gravitating neutrino gas; their masses are distributed around the maximum Jeans-mass, MJmax. corresponding to mv - 10 eV in an expanding universe. The present day length scales of clusters and the dispersion in the velocities observed within them are understood in terms of a 100-fold expansion subsequent to the initial growth of the fluctuations at MJmax. These systematics on theR-M plane imply that the initial condensations in the expanding universe are on the scale of the rich clusters of galaxies, these condensations were triggered dominantly by the gravitation of the neutrinos and the constant density of al systems arises naturally due to the embedding of these systems in the large scale neutrino condensations. If the neutrino density falls off asr -2 beyond the cluster edge till the distributions from different clusters overlap, then the mean density of the neutrinos approximately equals the closure density of the universe.  相似文献   

13.
I present a model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new three-dimensional chemo-dynamical code, including dark matter, stars and a multi-phase ISM. We follow the evolution from redshift z= 4.85 until the present epoch. The energy release by massive stars and supernovae prevents a rapid collapse of the baryonic matter and delays the maximum star formation until redshift z ≈ 1. The galaxy forms radially from inside-out and vertically from top-to-bottom. Correspondingly, the inner halo is the oldest component, followed by the outer halo, the bar/bulge, the thick and the thin disk. The bulge in the model consists of at least two stellar subpopulations, an early collapse population and a population that formed later in the bar. This revised version was published online in August 2006 with corrections to the Cover Date.  相似文献   

14.
The brightnesses of supernovae are commonly understood to indicate that cosmological expansion is accelerating due to dark energy. However the entire discussion presumes a perfectly transparent universe because no effects of reddening associated with the interstellar extinction law are seen. We note that with two kinds of dark matter (baryonic and nonbaryonic) strongly dominating the known mass of the universe, it is seriously premature to assume that these dark matter components have not reduced the transmission of the universe for cosmological sources. We show that the long‐known Lyα clouds, if nucleated by the population of baryonic dark matter primordial planetoids indicated by quasar microlensing, would act as spherical lenses and achromatically fade cosmologically distant sources. We attempt to estimate the amount of this cosmological fading, but ultimately the calculation is limited by lack of a satisfactory model for the tenuous outer parts of a primordial planetoid. We also consider the effects of such cosmological fading on the light of quasars. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
We fit the spectra of Cyg X-1 using two component advective flows with Keplerian accretion disks on the equatorial plane surrounded by sub-Keplerian disks when standing shocks are present. The soft photons generated by the bremsstrahlung and synchrotron processes in the sub-Keplerian flow, as well as the multi-colour black body emission from the Keplerian disk are Comptonized by the thermal and non-thermal electrons. By varying Keplerian and sub-Keplerian rates we are able to reproduce the observed soft and hard states as far as X-ray region is concerned and ‘low γ-ray intensity’ and ‘high γ-ray intensity’ states as far as the soft γ-ray region is concerned. We also find two pivotal points where the spectra intersect as is observed in Cyg X-1.   相似文献   

16.
In the framework of ‘microscopic’ theory of black holes (J. Phys. Soc. Jpn. Suppl. B 70, 84, 2001; Astrophys. USSR 4, 659, 1996; 35, 335, 1991, 33, 143, 1990, 31, 345, 1989a; Astrophys. Space Sci. 1, 1992; Dokl. Akad. Nauk USSR 309, 97, 1989b), and references therein, we address the ‘pre-radiation time’ (PRT) of neutrinos from black holes, which implies the lapse of time from black hole’s birth till radiation of an extremely high energy neutrinos. For post-PRT lifetime, the black hole no longer holds as a region of spacetime that cannot communicate with the external universe. We study main features of spherical accretion onto central BH and infer a mass accretion rate onto it, and, further, calculate the resulting PRT versus bolometric luminosity due to accretion onto black hole. We estimate the PRTs of AGN black holes, with the well-determined masses and bolometric luminosities, collected from the literature by Woo Jong-Hak and Urry (Astrophys. J. 579, 530, 2002) on which this paper is partially based. The simulations for the black holes of masses M BH ≃(1.1⋅106 ÷4.2⋅109) M ⊙ give the values of PRTs varying in the range of about T BH ≃(4.3⋅105 ÷5.6⋅1011) yr. The derived PRTs for the 60 AGN black holes are longer than the age of the universe (∼13.7 Gyr) favored today. At present, some of remaining 174 BHs may radiate neutrinos. However, these results would be underestimated if the reservoir of gas for accretion in the galaxy center is quite modest, and no obvious way to feed the BHs with substantial accretion.  相似文献   

17.
Chandra X-ray observations of rich, dynamically relaxed galaxy clusters allow the properties of the X-ray gas and the total gravitating mass to be determined precisely. Here, we discuss how Chandra observations may be used as a powerful tool for cosmological studies. By combining Chandra X-ray results on the X-ray gas mass fractions in clusters with independent measurements of the Hubble constant and the mean baryonic matter density of the universe, we obtain a tight constraint on the mean total matter density of the universe, Οm, and an interesting constraint on the cosmological constant, ΟΛ. Using these results, together with the observed local X-ray luminosity function of the most X-ray luminous galaxy clusters, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we obtain a precise constraint on the normalization of the power spectrum of density fluctuations in the nearby universe,σ8. We compare our results with those obtained from other, independent methods. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

18.
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19.
We analyze the spectropolarimetric observations of 12 candidates for quasars from the spectroscopic database of the SDSS Catalog. The magnetic fields of these objects are estimated in the context of a theory that includes the Faraday rotation of the polarization plane on the mean free path of a photon in the outflow from an accretion disk. As a result, we have determined the column density in the outflow, N H ∼ 6 × 1023 cm−2, and the radial, B ∼ 1 G, and toroidal, B ∼ 600 G, magnetic fields.  相似文献   

20.
We present results of multicolour photometry of the nova-like binary MV Lyr in the years 2002–2003, corresponding to the transition of the star from its low (V = 17.8) to high (V = 12.3) brightness state. The transition lasted at most 200 days. During the first ∼50 days MV Lyr brightened at the rate 0.06 mag/day, and during the next ∼150 days at the rate 0.01 mag/day. The brightening was accompanied by a blue shift from 0.45 to 0.05 in V−R, interpreted as an increase of the accretion disk contribution to the total light. During the transition MV Lyr displayed well-known ‘quasi-orbital’ light variations and fast quasi-periodic oscillations with a typical time scale of tens of minutes.  相似文献   

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