We propose a statistical tool to compare the scaling behaviour of turbulence in pairs of molecular cloud maps. Using artificial maps with well defined spatial properties, we calibrate the method and test its limitations to ultimately apply it to a se
t of observed maps. We develop the wavelet-based weighted cross-correlation (WWCC) method to study the relative contribution of structures of different sizes and their degree of correlation in two maps as a function of spatial scale, and the mutual displacement of structures in the molecular cloud maps. We test the WWCC for circular structures having a single prominent scale and fractal structures showing a self-similar behavior without prominent scales. Observational noise and a finite map size limit the scales where the cross-correlation coefficients and displacement vectors can be reliably measured. For fractal maps containing many structures on all scales, the limitation from the observational noise is negligible for signal-to-noise ratios >5. (abbrev). Application of the WWCC to the observed line maps of the giant molecular cloud G333 allows to add specific scale information to the results obtained earlier using the principle component analysis. It confirms the chemical and excitation similarity of $^{13}$CO and C$^{18}$O on all scales, but shows a deviation of HCN at scales of up to 7 (~7 pc). This can be interpreted as a chemical transition scale. The largest structures also show a systematic offset along the filament, probably due to a large-scale density gradient. The WWCC can compare correlated structures in different maps of molecular clouds identifying scales that represent structural changes such as chemical and phase transitions and prominent or enhanced dimensions.
The empirical relations in the black hole-accretion disk-relativistic jet system and physical processes behind these relations are still poorly understood, partly because they operate close to the black hole within the central light year. Very long b
aseline array (VLBA) provides unparalleled resolution at 15 GHz with which to observe the jet components at sub-milliarcsecond scales, corresponding to sub-pc-scales for local blazars. We discuss the jet inner structure of blazars, location and radiation mechanisms operating in the innermost parsec-scale region of blazars, and evidence for jet-excited broad-line region (BLR) ouflowing downstream the jet. Outflowing BLR can provide necessary conditions for production of high energy emission along the jet between the base of the jet and the BLR and far beyond the BLR as evidenced by recent observations. Flat spectrum quasars and low synchrotron peaked sources are the most likely objects to host the outfllowing BLR. From the $gamma$-ray absorption arguments, we propose that the jet-excited region of the outflowing BLR in quasars is small and/or gas filling factor is low, and that the orientation and opening angle of the outflowing BLR can lead to relevant $gamma$-ray absorption features observed in quasars.
Context. Observations of polarized radio emission show that large-scale (regular) magnetic fields in spiral galaxies are not axisymmetric, but generally stronger in interarm regions. In some nearby galaxies such as NGC 6946 they are organized in narr
ow magnetic arms situated between the material spiral arms. Aims. The phenomenon of magnetic arms and their relation to the optical spiral arms (the material arms) call for an explanation in the framework of galactic dynamo theory. Several possibilities have been suggested but are not completely satisfactory; here we attempt a consistent investigation. Methods. We use a 2D mean-field dynamo model in the no-z approximation and add injections of small-scale magnetic field, taken to result from supernova explosions, to represent the effects of dynamo action on smaller scales. This injection of small scale field is situated along the spiral arms, where star-formation mostly occurs. Results. A straightforward explanation of magnetic arms as a result of modulation of the dynamo mechanism by material arms struggles to produce pronounced magnetic arms, at least with realistic parameters, without introducing new effects such as a time lag between Coriolis force and {alpha}-effect. In contrast, by taking into account explicitly the small-scale magnetic field that is injected into the arms by the action of the star forming regions that are concentrated there, we can obtain dynamo models with magnetic structures of various forms that can be compared with magnetic arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered as coherent magnetic structures generated by large-scale dynamo action, and associated with spatially modulated small-scale magnetic fluctuations, caused by enhanced star formation rates within the material arms.
Aims. We use a sample of 83 core-dominated active galactic nuclei (AGN) selected from the MOJAVE (Monitoring of Jets in AGN with VLBA Experiments) radio-flux-limited sample and detected with the Fermi Large Area Telescope (LAT) to study the relations
between non-simultaneous radio, optical, and gamma-ray measurements. Methods. We perform a multi-band statistical analysis to investigate the relations between the emissions in different bands and reproduce these relations by modeling of the spectral energy distributions of blazars. Results. There is a significant correlation between the gamma-ray luminosity and the optical nuclear and radio (15 GHz) luminosities of blazars. We report a well defined positive correlation between the gamma-ray luminosity and the radio-optical loudness for quasars and BL Lacertae type objects (BL Lacs). A strong positive correlation is found between the radio luminosity and the gamma-ray-optical loudness for quasars, while a negative correlation between the optical luminosity and the gamma-ray-radio loudness is present for BL Lacs. Modeling of these correlations with a simple leptonic jet model for blazars indicates that variations of the accretion disk luminosity (and hence the jet power) is able to reproduce the trends observed in most of the correlations. To reproduce all observed correlations, variations of several parameters, such as the accretion power, jet viewing angle, Lorentz factor, and magnetic field of the jet, are required.
Future radio observations with the SKA and its precursors will be sensitive to trace spiral galaxies and their magnetic field configurations up to redshift $zapprox3$. We suggest an evolutionary model for the magnetic configuration in star-forming di
sk galaxies and simulate the magnetic field distribution, the total and polarized synchrotron emission, and the Faraday rotation measures for disk galaxies at $zla 3$. Since details of dynamo action in young galaxies are quite uncertain, we model the dynamo action heuristically relying only on well-established ideas of the form and evolution of magnetic fields produced by the mean-field dynamo in a thin disk. We assume a small-scale seed field which is then amplified by the small-scale turbulent dynamo up to energy equipartition with kinetic energy of turbulence. The large-scale galactic dynamo starts from seed fields of 100 pc and an averaged regular field strength of 0.02,$mu$G, which then evolves to a spotty magnetic field configuration in about 0.8,Gyr with scales of about one kpc and an averaged regular field strength of 0.6,$mu$G. The evolution of these magnetic spots is simulated under the influence of star formation, dynamo action, stretching by differential rotation of the disk, and turbulent diffusion. The evolution of the regular magnetic field in a disk of a spiral galaxy, as well as the expected total intensity, linear polarization and Faraday rotation are simulated in the rest frame of a galaxy at 5,GHz and 150,MHz and in the rest frame of the observer at 150,MHz. We present the corresponding maps for several epochs after disk formation. (abridged)
Polarised radio synchrotron emission from interstellar, intracluster and intergalactic magnetic fields is affected by frequency-dependent Faraday depolarisation. The maximum polarised intensity depends on the physical properties of the depolarising m
edium. New-generation radio telescopes like LOFAR, SKA and its precursors need a wide range of frequencies to cover the full range of objects. The optimum frequency of maximum polarised intensity (PI) is computed for the cases of depolarisation in magneto-ionic media by regular magnetic fields (differential Faraday rotation) or by turbulent magnetic fields (internal or external Faraday dispersion), assuming that the Faraday spectrum of the medium is dominated by one component or that the medium is turbulent. Polarised emission from bright galaxy disks, spiral arms and cores of galaxy clusters are best observed at wavelengths below a few centimeters (at frequencies beyond about 10 GHz), halos of galaxies and clusters around decimeter wavelengths (at frequencies below about 2 GHz). Intergalactic filaments need observations at meter wavelengths (frequencies below 300 MHz). Sources with extremely large intrinsic $|RM|$ or RM dispersion can be searched with mm-wave telescopes. Measurement of the PI spectrum allows us to derive the average Faraday rotation measure $|RM|$ or the Faraday dispersion within the source, as demonstrated for the case of the spiral galaxy NGC 6946. Periodic fluctuations in PI at low frequencies are a signature of differential Faraday rotation. Internal and external Faraday dispersion can be distinguished by the different slopes of the PI spectrum at low frequencies. A wide band around the optimum frequency is important to distinguish between varieties of depolarisation effects.
We perform a multi-band statistical analysis of core-dominated superluminal active galactic nuclei (AGN) detected with Fermi Large Area Telescope (LAT). The detection rate of $gamma$-ray jets is found to be high for optically bright AGN. There is a s
ignificant correlation between the $gamma$-ray luminosity and the optical nuclear and radio (15 GHz) luminosities of AGN. We report a well defined positive correlation between the $gamma$-ray luminosity and the radio-loudness for quasars and BL Lacertae type objects (BL Lacs). The slope of the best-fit line is significantly different for quasars and BL Lacs. The relations between the optical and radio luminosities and the $gamma$-ray loudness are also examined, showing a different behavior for the populations of quasars and BL Lacs. Statistical results suggest that the $gamma$-ray, optical and radio emission is generated at different locations and velocity regimes along the parsec-scale jet.
(abridged) We investigate the possibility to recognize the magnetic field structures in nearby galaxies and to test the cosmological evolution of their large- and small-scale magnetic fields with the SKA and its precursors. We estimate the required d
ensity of the background polarized sources detected with the SKA for reliable reconstruction and reconstruction of magnetic field structures in nearby spiral galaxies. The dynamo theory is applied to distant galaxies to explore the evolution of magnetic fields in distant galaxies in the context of a hierarchical dark matter cosmology. Under favorite conditions, a emph{recognition} of large-scale magnetic structures in local star-forming disk galaxies (at a distance $la 100$ Mpc) is possible from $ga 10$ RMs towards background polarized sources. Galaxies with strong turbulence or small inclination need more polarized sources for a statistically reliable recognition. A reliable emph{reconstruction} of the field structure without precognition needs at least 20 RM values on a cut along the projected minor axis which translates to $approx1200$ sources towards the galaxy. We demonstrate that early regular fields are already in place at $z sim 4$ (approximately 1.5 Gyr after the disk formation) in massive gas-rich galaxies ($>10^9$ M$_{sun}$) which then evolve to Milky-Way type galaxies. Major and minor mergers influence the star formation rate and geometry of the disk which has an effect of shifting the generation of regular fields in disks to later epochs. Predictions of the evolutionary model of regular fields, simulations of the evolution of turbulent and large-scale regular fields, total and polarized radio emission of disk galaxies, as well as future observational tests with the SKA are discussed.
The future new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Hundred and thousands polarized background sources can be measured towards nearby galaxies thu
s allowing their detailed magnetic field mapping by means of Faraday rotation measures (RM). We aim to estimate the required density of the background polarized sources detected with the SKA for reliable recognition and reconstruction of the magnetic field structure in nearby spiral galaxies. We construct a galaxy model which includes the ionized gas and magnetic field patterns of different azimuthal symmetry (axisymmetric (ASS), bisymmetric (BSS) and quadrisymmetric spiral (QSS), and superpositions) plus a halo magnetic field. RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Recognition of magnetic structures is possible from RM towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. Under favourite conditions, about a few dozens of polarized sources are sufficient for a reliable recognition. Reconstruction of the field structure without precognition becomes possible for a large number of background sources. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis which translates to approximately 1200 sources towards the galaxy. Radio telescopes operating at low frequencies (LOFAR, ASKAP and the low-frequency SKA array) may also be useful instruments for field recognition or reconstruction with the help of RM, if background sources are still significantly polarized at low frequencies (abriged).
