Radio spectra of objects in the RATAN-600 RZF catalog and a population analysis of faint radio sources

Results of centimeter-wavelength RATAN-600 observations of several hundred NVSS objects with a wide range of flux densities are presented. Changes in spectral-index distributions from strong-to faint-object populations are estimated. In the transition from strong to faint radio sources, the number of steep-and ultrasteep-spectrum objects decreases and the minimum between steep-and flat-spectrum objects, which was first discovered in the 1970s, becomes less distinct. A less certain increase in the fraction of inverted-spectrum objects is also found. Comparison with theoretical models of radio-source evolution suggests that the decrease in the number of steep-and ultrasteep-spectrum objects can be naturally explained by a sharp decrease in the fraction of classical FRII radio galaxies. The decrease in the sharpness of the minimum may be due to the growing contribution from the population with spectral indices 0.7–0.5, which is usually identified with starbust galaxies. The small increase in the fraction of inverted-spectrum radio sources can be attributed to the growing number of objects with active nuclei. Our spectrum-based conclusions are confirmed by the morphology of the FIRST Catalog images. The number of radio sources associated with supermassive black holes in the Early Universe is estimated using data from the SDSS Catalog.     

Relative variations of the physical parameters of variable extragalactic radio sources

A method for studying the physical conditions in compact components of extragalactic radio sources displaying variability on time scales of hundreds of days is proposed. The method can be used to estimate the relative variations of the magnetic-field strength and number density of relativistic electrons in superluminal jets from the cores of quasars and radio galaxies. Results are presented for the jets of the quasars 3C 120, 3C 273, 3C 279, and 3C 345. The energies of the magnetic field and relativistic particles in these objects are not in equipartition. As a rule, the magnetic-field strength decreases appreciably during the evolution of an expanding jet, while the number of relativistic electrons grows.     

Determining the spins of supermassive black holes in Active Galactic Nuclei based on spectropolarimetric observations

Strong constraints are obtained for the spins of supermassive black holes in a number of Active Galactic Nuclei. These estimates are based on spectropolarimetric data, obtained mainly on the 6-m telescope of the Special Astrophysical Observatory, as well as data on the kinetic power of relativistic jets. The magnetic fields at the innermost stable Keplerian orbit in the accretion disk and at the event horizon of the supermassive black hole are estimated. These data are used to place strong constraints on the spins of supermassive black holes in Active Galactic Nuclei.     

The supermassive black hole at the center of our galaxy: Determination of its main physical parameters

We have used two astrometric methods developed at the Main Astronomical Observatory of the Russian Academy of Sciences—the method of apparent-motion parameters (AMP) and a direct geometrical method (DGM)—to derive the orbit of the star S2 around the Galactic center, and thereby the mass of the supermassive black hole at the Galactic center. The AMP method, which is based on measurements of the curvature of a fairly short orbital arc, is efficient if observational data on the relative radial velocity are available. The mass of the supermassive black hole was also estimated using astrophysical methods, based on the empirical relation between the masses of the supermassive black holes at the centers of galaxies and quasars and the radio and X-ray luminosities of these regions. We estimate the magnetic-field strength near the event horizon of the supermassive black hole at the Galactic center using a synchrotron self-absorption model.     

Correlations between the development of a flare in the Blazar 3C 454.3 in the radio and optical

Radio and optical data are used to analyze the development of the flare in the blazar 3C 454.3 observed in 2004–2007. A detailed correspondance between the optical and radio flares is established, with a time delay that depends on the observing frequency. The variation of the delay of the radio flare relative to the optical flare is opposite to the dispersion delay expected for the propagation of radiation in the interstellar medium, testifying to an intrinsic origin for the observed outburst. Small-scale flux variations on time intervals of 5–10 days in the millimeter and optical are also correlated, with a time delay of about ten months. This may provide evidence for a single source generating the radiation at all wavelengths. Rapid flux fluctuations in the radio and optical that are correlated with the indicated time delays could be associated with inhomogeneities in the accretion disk. Detailed studies of the flux variations of Active Galactic Nuclei (AGN) can be used to analyze the structure of the accretion disk. A model for the energy release in AGN that is not associated purely with accretion onto supermassive black holes is proposed. As is the case for other active members of the AGN family, estimates of the lifetime of the binary black-hole system in 3C 454.3 suggest that this object is in a stage of its evolution that is fairly close to the coalescence of its black holes. The energy that is released as the companion of the central black hole loses orbital angular momentum is sufficient to explain the observed AGN phenomena. The source of primary energy release could be heating of the gas behind shock fronts that arise due to the friction between the companion black hole and the ambient gaseous medium. The orbit of the companion could be located at the periphery of the accretion disk of the central body at its apocenter and plunge more deeply into the accretion disk at its pericenter, inducing flares at all wavelengths. Energy-release parameters such as the temperature and density of the heated gas are estimated for 3C 454.3. The model considered assumes omnidirectional radiation of the medium in the presence of a magnetic field. The radiation corresponding to the minimum flux level (base level) could represent omnidirectional radiation due to the orbit of the moving companion. The fraction of the energy that is transferred to directed jets is small, comprising 1–2% of the total energy released due to the loss of orbital angular momentum by the companion.     

No-z Approximation and rz-Model for Studying Magnetic Fields in Astrophysical Objects

Astronomy Reports - Different astrophysical objects, such as the Sun, another stars, galaxies and accretion discs have regular magnetic field structures. Their generation is usually connected with...     

Spectral distribution of the polarized radiation from standard accretion disks in Active Galactic Nuclei: Observational analysis

Spectropolarimetric observations of a number of Active Galactic Nuclei obtained using the SCORPIO-2 aperture focal reducer installed on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences are used to estimate physical parameters of these objects. The measured polarization and its wavelength dependence are consistent with the expectations of a standard accretion-disk model taking into account the effect of Faraday depolarization over the mean free path of the emitted photons. Estimates of the magnetic field in the accretion disk near the innermost stable orbit and the spin of the accreting central black hole are obtained. It is concluded that supermassive black holes with standard accretion disks and equal magnetic and radiative pressures are primarily Kerr black holes.     

Non-uniform model for the synchrotron radiation of Sgr A* and other low-luminosity galactic nuclei

A model for non-uniform source of synchrotron radiation with a power-law radial distribution of the magnetic field and relativistic-electron density along one-or two-sided jets is described. Non-relativistic jets with both constant cross sections (collimated jets) and cross sections that are proportional to distance (conical jets) are considered. Formulas that can be used to determine source parameters from the spectral index, source size, and index of the relativistic-electron energy spectrum based on multi-frequency observations are obtained. In the case of a conical jet, these formulas coincide with the analogous formulas for a spherical source obtained by A.P. Marscher. Relations that can be used to estimate the magnetic-field strength from the brightness temperature in the self-absorbed region are also obtained. As examples, the inhomogeneous-source model is applied to the compact radio sources at the centers of the Milky Way, Sgr A*, and the low-luminosity galactic nuclei M81* and M87*, which are associated with supermassive black holes. The inner radius of the radiation region is determined. For Sgr A*, this distance turns out to be comparable to the gravitational radius, smaller than the radius of the last stable orbit for a non-rotating black hole, and consistent with the radius of the last stable orbit expected for a rotating black hole. The inner radii in M81* and M87* are ～15 R _S , an order of magnitude larger than for Sgr A*. Estimates of the magnetic field at the inner radius are 400 G for M81*, 0.65–5.3 kG for Sgr A*, and 20–100 kG for M87*. These magnetic fields and the Blandford-Znajek model for the radiation of a rotating black hole are used to estimate the rotational speed of the black holes, which are in agreement with the characteristic variability time scales for these three objects. However, the accuracy of these estimates is modest, and is limited primarily by the accuracy of interferometric measurements at millimeter wavelengths.     

The estimation of black-hole masses in distant radio galaxies

We have estimated the masses of the central supermassive black holes of 2442 radio galaxies froma catalog compiled using data from the NED, SDSS, and CATS databases. Mass estimates based on optical photometry and radio data are compared. Relationships between the mass of the central black hole M _p ^bh and the redshift z _p are constructed for both wavelength ranges. The distribution of the galaxies in these diagrams and systematic effects influencing estimation of the black-hole parameters are discussed. Upperenvelope cubic regression fits are obtained using the maximum estimates of the black-hole masses. The optical and radio upper envelopes show similar behavior, and have very similar peaks in position, z _p ≅ 1.9, and amplitude, log M _p/bh = 9.4. This is consistent with a model in which the growth of the supermassive black holes is self-regulating, with this redshift corresponding to the epoch when the accretion-flow phase begins to end and the nuclear activity falls off.     

Giant radio galaxies: Old long-lived quasars?

The properties of giant radio sources (GRS’s) are considered with the aim of identifying conditions contributing to their formation, using data from the literature, the Sloan Digital Sky Survey (SDSS), and the APM catalog. The optical and radio properties of normal-size radio sources, (≤1 Mpc), are compared. The following conclusions are reached. (1) The fraction of objects with broad emission lines among GRS’s with high-excitation spectra is the same as for isotropic samples of radio sources; in the framework of the “unified scheme,” this testifies to an isotropic distribution of angles between the radio jets of GRS’s and the line of sight, i.e., GRS’s do not represent a population of objects whose radio jets are in the plane of the sky. (2) Giant radio sources do not differ from normal radio sources in the distributions of various asymmetry parameters for their extended radio components; in the unified scheme, the similarity of the asymmetry distributions for giant radio galaxies and giant radio quasars suggests that the origin of the asymmetry of their extended radio components is inhomogeneity of the external conditions. (3) The observed powers of the radio jets of giant and normal radio sources do not differ, making it unlikely that the large sizes of the GRS’s are due to this factor. (4) The richness and character of the environments of giant and normal radio sources do not differ: giant host galaxies are found in both isolated fields and in clusters of up to Abell class 1 in richness. This argues against the idea that a low density of the environment is the only origin of GRS’s. (5) The relatively large fraction of radio sources with two pairs of extended radio components (so-called double-double radio sources) among GRS’s testifies that the lifetimes of GRS’s are approximately an order of magnitude longer than those of normal radio sources.Given the equal spatial densities of nearby (z < 0.1) GRS’s and FR II radio sources with powers P _{1.4 MHz} > 10²⁵ W/Hz, this indicates that ∼10% of FR II radio sources have lifetimes an order of magnitude longer, and evolve into GRS’s. (6) The small (∼0.1) ratio of the number of known GRS’s to the number of normal FR II radio sources, together with the observed spatial density of GRS’s at z ∼ 0.6, which is an order of magnitude lower than the predicted value, suggests that a considerable number of GRS’s were missed by surveys at z > 0.1, possibly due to observational selection effects because of their relatively low radio powers and radio surface brightnesses. (7) The absence of “double-double” giant quasars suggests that these objects have a shorter activity time scale than GRS’s. In an evolutionary scenario that is an alternative to the unified scheme uniting “radio loud” quasars and radio galaxies, radio quasars evolve with time into radio galaxies, and the observed relative number of radio quasars among the GRS’s (∼10%) can be interpreted as reflecting the existence of a long-lived population of “radio loud” quasars comprising ∼10% of all radio quasars, with such a population of long-lived radio quasars being the parent population for giant radio galaxies.     

Search for differences in the velocities and directions of the kiloparsec-scale jets of quasars with and without X-ray emission

The X-ray emission of the kiloparsec-scale jets of core-dominant quasars is usually interpreted as inverse Compton scattering on the cosmic microwave background (CMB) emission (Sample I). By analogy with the situation on parsec scales, ultrarelativistic motion along a jet oriented at a small angle to the line of sight is usually invoked to explain the X-ray emission while also satisfying the condition of equipartition between the energies associated with the relativistic particles and the magnetic field on kiloparsec scales. This leads to an increase in the energy flux of the CMB radiation in the rest frame of the kiloparsec-scale jets. Consequently, the intensity of the CMB radiation is enhanced to the level required for detectable X-ray emission. This suggests that kiloparsec jets of quasars with similar extents and radio flux densities that are not detected in the X-ray (Sample II) could have subrelativistic speeds and larger angles to the line of sight, due to deceleration and bending of the jet between parsec and kiloparsec scales. This suggests the possible presence of differences in the distributions of the difference between the position angle for the parsec-scale and kiloparsec-scale jets for these two groups of quasars; this is not confirmed by a statistical analysis of the data for Samples I and II. It is deduced that most of the sources considered exhibit bending of their jets by less than about 1.5 times the angle of the parsec-scale jet to the line of sight. This suggests that the X-ray emission is generated by other mechanisms that there is no equipartition.     

Polarization effects in the radiation of magnetized envelopes and extended accretion structures

We have calculated the degree and position angle of the polarization of radiation scattered in a magnetized, optically thin or optically thick envelope around a central source, taking into account Faraday rotation of the plane of polarization during the propagation of the scattered radiation and the finite size of the radiation source. The wavelength dependence of the degree of polarization can be used to estimate the magnetic field of the source (a star, the region around a neutron star, or a black hole), and we have used our calculations to estimate the magnetic fields in a number of individual objects: several hot O and Wolf-Rayet stars, compact objects in X-ray close binaries with black holes (SS 433, Cyg X-1), and supernovae. The spectrum of the linear polarization can be used to determine the magnetic field in the vicinity of a central supermassive black hole, where the polarized optical radiation is generated. In a real physical model, this value can be extrapolated to the region of the last stable orbit. In the future, the proposed technique will make it possible to directly estimate the magnetic field in the region of the last stable orbit of a supermassive black hole using X-ray polarimetry.     

The dynamical evolution of galaxy clusters taking into account the deceleration of disk galaxies by the gaseous component of the cluster

Numerical simulations of the dynamical evolution of a galaxy cluster in the framework of the N-body problem taking into account dark matter are presented. These simulations are aimed at studying the role of intergalactic gas in the cluster (the ICM) in the formation of a central, supermassive cD galaxy. The numerical models indicate that deceleration of the galaxies by intergalactic gas supports the observed high temperature of this gas, and accelerates the formation of a supermassive cD galaxy in the cluster core. The accretion of interstellar gas by the cluster core can support a high accretion rate by the central, supermassive black hole associated with the nucleus of the cD galaxy. As a result, this nucleus harbors a bright quasar. The mass of the black hole can grow with time to values 10¹⁰ M _⊙, as are observed for the brightest quasars.     

The radio sources CTA 21 and OF+247: The hot spots of radio galaxies

The physical conditions in the radio sources CTA 21 and OF+247 are studied assuming that the low-frequency spectral turnovers are due to synchrotron self-absorption. The physical parameters of the radio sources are estimated using a technique based on a nonuniform synchrotron source model. It is shown that the magnetic-field distributions in the dominant compact components of these radio sources are strongly inhomogeneous. The magnetic fields at the center of the sources are B ～ 10^?1 G, and the fields are two to three orders of magnitude weaker at the periphery. The magnetic field averaged over the compact component is B ～ 10^?3 G, and the density of relativistic electrons is n _e ～ 10^?3 cm^?3. Assuming that there is equipartition of the energies of the magnetic field and relativistic particles, averaged over the source, 〈E _H 〉 = 〈E _e 〉 ～ 10^?7–10^?6 erg cm^?3. The energy density of the magnetic field exceeds that of the relativistic electrons at the centers of the radio sources. The derived parameters of CTA 21 and OF+247 are close to those of the hot spots in the radio galaxy Cygnus A. On this basis, it is suggested that CTA 21 and OF+247 are radio galaxies at an early stage of their evolution, when the hot spots (dominant compact radio components) have appeared, and the radio lobes (weak extended components) are still being formed.     

Phenomenological model for the evolution of radio galaxies such as Cygnus A

A phenomenological model for the evolution of classical radio galaxies such as Cygnus A is presented. An activity cycle of the host galaxy in the radio begins with the birth of radio jets, which correspond to shocks on scales ～1 pc (the radio galaxy B0108+388). In the following stage of the evolution, the radio emission comes predominantly from formations on scales of 10–100 pc, whose physical parameters are close to those of the hot spots of Cygnus A (this corresponds to GHz-peaked spectrum radio sources). Further, the hot spots create radio lobes on scales of 10³–10⁴ pc (compact steep-spectrum radio sources). The fully formed radio galaxies have radio jets, hot spots, and giant radio lobes; the direction of the jets can vary in a discrete steps with time, creating new hot spots and inflating the radio lobes (as in Cygnus A). In the final stage of the evolutionary cycle, first the radio jets disappear, then the hot spots, and finally the radio lobes (similar to the giant radio galaxies DA 240 and 3C 236). A large fraction of radio galaxies with repeating activity cycles is observed. The close connection between Cygnus A-type radio galaxies and optical quasars is noted, as well as similarity in the cosmological evolution of powerful radio galaxies and optical quasars.     

Method for Estimating the Doppler Factors of Relativistic Radio Jets

An expression for the intensity of synchrotron emission from a radio source (in the optically thin regime) in terms of the energy densities in the magnetic field and particles is obtained, based on a definition of a relativistic electron related to its rest energy. A relationship is obtained between the energy densities in particles E_e and the magnetic field E_H for a physical system containing a magnetic field and relativistic electrons in a minimum-energy state. A method for estimating the Doppler factors of the relativistic electrons has been developed. This method does not requires that all radio sources have the same radiation energies (brightness temperatures): it is sufficient that the energies of the magnetic fields and relativistic particles in the source be approximately equal. The method yields Doppler-factor estimates with reasonably good accuracy, even when there are modest deviations from energy equipartition in the radio source,making it applicable to many radio sources. The method is used to estimate the Doppler factor of the radio jet in CTA 21.

     

Kinematics of disk galaxies with known masses of their supermassive black holes. Observations

This is the first paper in a project aimed at analyzing relations between the masses of supermassive black holes or nuclear clusters in galaxies and the kinematic features of the host galaxies. We present long-slit spectroscopic observations of galaxies obtained on the 6-m telescope of the Special Astrophysical Observatory using the SCORPIO focal reducer. Radial profiles of the line-of-sight velocities and velocity dispersions of the stellar populations were obtained for seven galaxies with known masses of their supermassive black holes (Mkn 79, Mkn 279, NGC 2787, NGC 3245, NGC 3516, NGC 7457, and NGC 7469), and also for one galaxy with a nuclear cluster (NGC 428). Velocity profiles of the emitting gas were obtained for some of these galaxies as well. We present preliminary galactic rotation curves derived from these data.     

Spectroscopy of RC sources

The results of spectroscopic observations of the host galaxies of objects in the RC catalog (the “Big Trio” program) obtained using the new SCORPIO spectrograph of the Special Astrophysical Observatory are presented. The spectroscopic redshifts of the objects are compared with their photometric color redshifts, and the errors in the latter are estimated. Based on BV RI observations obtained on the 6-m telescope of the SAO, the errors for the population of powerful radio galaxies are close to those found previously for radio quiet galaxies (about 10–20%). The detection of Ly α in the B filter in RC 1626+0448 is confirmed. This object is the second spectrally studied FR II radio source from the RC catalog to have a redshift z>2.5. Star formation in its host galaxy began at a redshift z>3.3. This first use of the new SCORPIO spectrograph demonstrates its promise for studies of very distant steep-spectrum radio galaxies brighter than 23^m–24^m in V.     

Study of the physical conditions in active galactic nuclei. Physical conditions in the cores of two nearby radio galaxies

We have carried out a search for compact radio sources in the cores of 16 nearby radio galaxies. We detected compact components in four radio galaxies, and found upper limits for the flux density in compact components in ten radio galaxies. VLBI observations enabled the detection of a turnover in the spectra of the two nearby radio galaxies 3C 111 and 3C 465. Using a method based on an inhomogeneous model for a synchrotron source, we estimate the magnetic-field strength and the energy densities in the magnetic field and relativistic electrons in the cores of these radio galaxies. Strong inhomogeneity in the distribution of the magnetic fields in the cores of 3C 111 and 3C 465 is implied by our analysis. The magnetic-field strengths in the central regions of these galactic nuclei, on scales of ～0.1 pc, exceed the mean strength by four to five orders of magnitude, and lie in the range 10² G < H < 10⁴ G.     

Possible origins of the diversity of types of Active Galactic Nuclei

The observed variety of types of Active Galactic Nuclei (AGN) in galaxies with various masses and morphologies is considered. It is proposed that this diversity is related to the evolutionary development of their host galaxies and central regions, including their bulges, massive black holes, and accretion disks. A possible evolutionary scenario explaining the relationship between various types of AGN is proposed, in which, in addition to differences in the accretion states of the circumnuclear disks, the type of host galaxy is also taken into consideration. Special attention is given to the qualitative similarity of the accretion and radiative processes occurring in active objects with stellar-mass black holes (microquasars) and in AGN, in spite of the huge differences in the masses of their black holes. Studies of variability of the emission of microquasars taking this similarity into account can be used to make predictions about the behavior of the emission of AGN (with scaling in accordance with the black-hole mass).