The starburst-AGN connection: the role of stellar clusters in AGNs

Nuclear stellar clusters are a common phenomenon in spirals and in starburst galaxies, and they may be a natural consequence of the star formation processes in the central regions of galaxies. HST UV imaging of a few Seyfert 2 galaxies have resolved nuclear starbursts in Seyfert 2 revealing stellar clusters as the main components of the extended emission. However, we do not know whether stellar clusters are always associated with all types of nuclear activity. We present HST NUV and optical images to study the role that stellar clusters play in different types of AGNs. Also with these images, we study the circumnuclear dust morphology as a probe of the circumnuclear environment of AGNs.     

Rate coefficients for the chemi-ionization processes in sodium- and other alkali-metal geocosmical plasmas

The chemi-ionization processes in thermal and sub-thermal collisions of excited alkali atoms with atoms in ground and excited states were considered, with a particular accent to the applications for geocosmical plasma research. An improved version of semi-classical method for the rate coefficients calculation is presented. The method is applied to the cases of excited alkali atoms with the principal quantum numbers 5 ⩽ n ⩽ 25. The results of calculation of the considered chemi-ionization processes rate coefficients are compared to the existing experimental data. Their good agreement recommends to use the rate coefficients obtained for analysis of geocosmical plasma processes.     

The ``Great Debate': The case for AGNs

We summarize the evidence from multiwavelength observations that the dominant power source in the majority of ultraluminous infrared galaxies (ULIGs) may be an active galactic nucleus (AGN). In the broader context of the debate, we also show that –- 1. ULIGs are indeed a key stage in the transformation of merging gas-rich disks into ellipticals,2. ULIGs are plausibly the precursors of quasi-stellar objects (QSOs), and 3. ULIGs do appear to be local templates of the high luminosity tail of major gas-rich mergers at z 1–4.     

Coulomb collisional processes in space plasmas; relaxation of suprathermal particle distributions

Nonequilibrium distributions of space plasmas are often characterized by extended high-energy tails. This paper provides a detailed analysis of the relaxation of such isotropic nonequilibrium plasmas. We consider an energetic charged species dilutely dispersed in a fully ionized plasma, which acts as a heat bath at equilibrium. The minor constituent is referred to as a “test particle” and collisions between test particles are not included. We study the approach to equilibrium with a finite difference method of solution of the Fokker-Planck equation appropriate for collisions between charged particles. The solution of the Fokker-Planck equation is also presented formally as an expansion in the eigenfunctions of the Fokker-Planck operator. The main objective of the paper is the calculation of the energy-dependent relaxation times of the distribution function. A strong energy dependence for these relaxation times is anticipated since, for Coulomb collisions, the Rutherford cross-section varies with relative speed g as g⁻⁴. Analogous results for neutral species are presented for comparison in the following paper.     

Autoionization rate coefficients for some ions of astrophysical interest

Autoionization rate coefficients for some quadruply and quintuply ionized atoms have been computed in the temperature range 10⁵–10⁸ K. Typical temperature dependence has been found and simple expressions are given for their quick estimation.     

The broad emission-line region in AGNs and quasars: The impact of variability studies

Coordinated observations of variability of the continuum and the emission-line luminosities (reverberation mapping) in AGNs and quasars have fundamentally altered our understanding of the broad-line regions in active galaxies. The constraints these observations impose on the models of the BLRs have been demonstrated here by an attempt to model the BLR of NGC5548, the most intensively monitored AGN. Two models of a BLR, with one having a power law radial distribution of density and the other a gaussian radial distribution, are described and the modelled line luminosities and centroid of the line response functions are presented. A self-consistent model is presented for the change in theCiv/Lyα ratio as the continuum luminosity changes. It is shown that BLR gas must be composed of a mixture of optically thin and optically thick gas and the proportion of thick and thin gas alters with the luminosity of the ionizing continuum. The observed centroid or the lag of a line, can be a function of the continuum luminosity. The variability of the profile of theCiv line in the spectrum of NGC5548 is investigated. This profile is extremely robust and is not significantly affected by changes in the ionizing continuum. Future models of the kinematics of the BLR clouds will have to be based on very stable cloud motions and include anisotropic line emission.     

A New Approach for Estimating Kinetic Luminosity of Jet in AGNs

The Konigl inhomogeneous jet model can successfully reproduce most observational features of jets in active galactic nuclei （AGN）, when suitable physical parameters are adopted. We improve Konigl＇s calculations on the core emission from the jet with a small viewing angle θ0 - φ （φ is half opening angle of the conical jet）. The proper motion of the jet component provides a constraint on the jet kinematics. Based on the inhomogeneous jet model, we use the proper motion data of the jet component to calculate the minimal kinetic luminosity of the jet required to reproduce the core emission measured by the very-long- baseline interferometry （VLBI） for a sample of BL Lac objects. Our results show that the minimal kinetic luminosity is slightly higher than the bolometric luminosity for most sources in the sample, which implies that radiatively inefficient accretion flows （RIAFs） may be in those BL Lac objects, or/and the properties of their broad-line regions （BLRs） are significantly different from flat-spectrum radio-loud quasars.     

The exact transformation from spherical harmonic to ellipsoidal harmonic coefficients for gravitational field modeling

The spherical and ellipsoidal harmonic series of the external gravitational potential for a given mass distribution are equivalent in their mutual region of uniform convergence. In an instructive case, the equality of the two series on the common coordinate surface of an infinitely large sphere reveals the exact correspondence between the spherical and ellipsoidal harmonic coefficients. The transformation between the two sets of coefficients can be accomplished via the numerical methods by Walter (Celest Mech 2:389–397, 1970) and Dechambre and Scheeres (Astron Astrophys 387:1114–1122, 2002), respectively. On the other hand, the harmonic coefficients are defined by the integrals of mass density moments in terms of the respective solid harmonics. This paper presents general algebraic formulas for expressing the solid ellipsoidal harmonics as a linear combination of the corresponding solid spherical harmonics. An exact transformation from spherical to ellipsoidal harmonic coefficients is found by incorporating these connecting expressions into the density integral. A computational procedure is proposed for the transformation. Numerical results based on the nearly ellipsoidal Martian moon, Phobos, are presented for validation of the method.     

A New Approach for Estimating Kinetic Luminosity of Jet in AGNs

The Konigl inhomogeneous jet model can successfully reproduce most observa-tional features of jets in active galactic nuclei (AGN), when suitable physical parameters are adopted. We improve Konigl's calculations on the core emission from the jet with a small viewing angle θ0～ψ (ψ is half opening angle of the conical jet). The proper motion of the jet component provides a constraint on the jet kinematics. Based on the inhomogeneous jet model, we use the proper motion data of the jet component to calculate the minimal kinetic luminosity of the jet required to reproduce the core emission measured by the very-long-baseline intefferometry (VLBI) for a sample of BL Lac objects. Our results show that the minimal kinetic luminosity is slightly higher than the bolometric luminosity for most sources in the sample, which implies that radiatively inefficient accretion flows (RIAFs) may be in those BL Lac objects, or/and the properties of their broad-line regions (BLRs) are signifi-cantly different from flat-spectrum radio-loud quasars.     

The relationship between the properties of PAHs and AGN activities in type-I AGNs

In order to explore the relationship between properties of Polycyclic Aromatic Hydrocarbons(PAHs) and AGN activities in Type-I AGNs, we compiled a sample of 47 Type-I AGNs with measured PAH 11.3 μm and 7.7 μm emission lines.The PAH emission and optical properties of these AGNs are taken from the literature.It is found that the equivalent width(EW) of 11.3 μm emission from PAHs shows a weak correlation with the ratio of the EWs of the Fe II complex between λ4434 andλ4684 to Hβ(RFe). The PAH 11.3/7.7 ratio is correlated with various elements defined by the first eigenvector space, which are RFe, [OIII]λ5007 luminosity and Hβasymmetry. These correlations infer that AGNs with high RFe, weak [OIII] emission and a strong Hβ blue-wing are likely to have low PAH ionization, and hence a larger amount of neutral PAHs.     

The role of suprathermal particle measurements in CrossScale studies of collisionless plasma processes

The CrossScale mission will advance our understanding of fundamental plasma processes in collisionless plasmas. It will exploit the excellent natural plasma laboratory provided by the Earth’s magnetosphere and the near-Earth solar wind and, in particular, carry out multi-scale studies that will strongly complement plasma studies in ground-based laboratories. Previous studies of collisionless plasmas in space environments across the solar system have shown the ubiquitous nature of suprathermal particles and that these particles exhibit a power-law energy spectrum. In this paper we discuss the great significance of these suprathermal particles for CrossScale studies. We show that the presence of these particles is a natural consequence of the collisionless regime as they can propagate across the heliosphere with little spectral change and are not thermalised by collisions. They are a key indicator of the non-equilibrium nature of collisionless plasmas and an important source of free energy that can drive plasma processes. We discuss how these suprathermal particles influence the overall properties of the plasma. In particular, the energy distribution of particles follows a Kappa, rather than Maxwellian, distribution and thus the plasma does not have a single thermodynamic temperature. We also discuss the importance of the suprathermal tail as a tool to diagnose the processes responsible for particle energisation in collisionless plasmas. Such energisation is a common feature in collisionless plasmas, especially in terms of the primary science targets for CrossScale: reconnection, shocks and turbulence. Finally we also touch on the value of using CrossScale studies to provide ground truth measurements for a number of astrophysical techniques that exploit the effects of energetic electrons in the distant universe. Throughout the paper, we stress that suprathermal (30 keV-1 MeV) measurements are essential to fully characterise particle distributions. We show that such measurements will benefit greatly from the improved spatial and temporal resolution (compared to Cluster) that is proposed for the HEP instrument on CrossScale.     

活动星系核的非热致电子级联模型研究及其进展：纯康普顿机制冷却情形

活动星系核的高能辐射机制研究对于揭示其结构和演化特征以及中央驱动之迹具有其重要的意义。近十年来，随着活动星系统结构模型研究以及其高能连续谱观测的发展，解释活动星系统高能辐的非热致电子级联模型得以产生、发展和完善起来。     

Letter to the Editor: The collisional broadening of carbon radio recombination lines

The density matrix equations of highly excited carbon are analytically solved as functions of line number and temperature. Allowing for the series of the impact approximation theory and quadrupole interaction potential, the line widths are found with the theoretical error less than 0.06 in the range of temperatures T _e = 25 − 100 K. To determine the medium temperature and density by using the experimental values of line widths, the Doppler, radiational and impact broadening are calculated for the lines with the principal quantum numbers n > 300. This revised version was published online in July 2006 with corrections to the Cover Date.     

The role of proton injection processes in the nonlinear evolution of “pearls”

Given a constant number of resonant protons in a field tube the unstable wave amplitude reaches a steady value rapidly due to nonlinear effects. Observations show that the pearl amplitudes grow gradually for a long time. Consistency with observations is obtained by taking into account the processes of proton injection.     

On invisible plasma content in radio-loud AGNs: the case of TeV blazar Markarian 421

Invisible plasma content in blazar jets such as protons and/or thermal electron–positron ( e ^±) pairs is explored through combined arguments of dynamical and radiative processes. By comparing physical quantities required by the internal shock model with those obtained through the observed broad-band spectra for Mrk 421, we obtain that the ratio of the Lorentz factors of a pair of cold shells resides in about 2 ∼ 20, which implies that the shocks are at most mildly relativistic. Using the obtained Lorentz factors, the total mass density ρ in the shocked shells is investigated. The upper limit of ρ is obtained from the condition that thermal bremsstrahlung emission should not exceed the observed γ-ray luminosity, whilst the lower limit is constrained from the condition that the energy density of non-thermal electrons is smaller than that of the total plasma. Then, we find ρ is 10²–10³ times heavier than that of non-thermal electrons for pure   e ^±  pairs, while 10²–10⁶ times heavier for pure electron–proton ( e / p ) content, implying the existence of a large amount of invisible plasma. The origin of the continuous blazar sequence is briefly discussed and we speculate that the total mass density and/or the blending ratio of e ^± pairs and e / p plasma could be new key quantities for the origin of the sequence.     

Rushing to equilibrium: a simple model for the collisional evolution of asteroids

A simple mathematical model for the evolution of a system of collisionally interacting bodies—such as the asteroid population—consists of two coupled, nonlinear, first-order differential equations for the abundances of “small” and “big” bodies. The model easily allows us to recover Dohnanyi's value for the exponent of the equilibrium mass distribution. Moreover, the model shows that any initial value for the ratio of “big” to “small” bodies rapidly relaxes to the equilibrium ratio, corresponding to the exponent, and that integrating the evolution equations backward in time—an attractive possibility to investigate the mass distribution of primordial planetesimals—leads to strong numerical instability.     

The coupling coefficients of pulsation for radiative stellar models

The second order theory of coupling is discussed regarding the radial pulsation of stellar models which are constructed ignoring convection. The formula including the nonadiabatic effect is presented. Numerical values given for model classical cepheids are considerably greater than the adiabatic values.     

The role of photochemical processes in evolution of the isotopic composition of the atmosphere of Titan

Molecular nitrogen, the main component of the modern atmosphere of Titan, may have formed without significant changes in the nitrogen and hydrogen isotopic composition from the clathrate hydrate of ammonia NH₃ · H₂O_SLD, which is the main accreted form of nitrogen. The most preferable transformation mechanism of NH₃ · H₂O_SLD into atmospheric N₂ is its thermal decomposition in the interior of Titan rather than the photochemical decomposition of ammonia in the upper atmosphere of early Titan. The photolysis of ammonia does not lead to a change in the isotopic composition of nitrogen, as all the nitrogen remains in Titan’s atmosphere. The photolysis of NH does not lead to a change in the isotopic composition of nitrogen in Titan’s atmosphere. Fractionation of hydrogen and nitrogen isotopes during the impacts of comets with Titan does not seem to be significant either. It will be possible to determine the dissociative fractionation factor, the original ratio ¹⁴N/¹⁵N, and the mass of Titan’s original atmosphere when fractionation of nitrogen isotopes in Titan’s atmosphere is examined in additional theoretical and experimental studies that take into account processes occurring during the formation of a system of Saturn’s satellites.