arXiv subjects

E. M. de Gouveia Dal Pino

Publications and source records attributed to E. M. de Gouveia Dal Pino.

At least 55 records · Page 3Linked to original sources

arXiv2014

Planck intermediate results. XIX. An overview of the polarized thermal emission from Galactic dust

This paper presents the large-scale polarized sky as seen by Planck HFI at 353 GHz, which is the most sensitive Planck channel for dust polarization. We construct and analyse large-scale maps of dust polarization fraction and polarization direction, while taking account of noise bias and possible systematic effects. We find that the maximum observed dust polarization fraction is high (pmax > 18%), in particular in some of the intermediate dust column density (AV < 1mag) regions. There is a systematic decrease in the dust polarization fraction with increasing dust column density, and we interpret the features of this correlation in light of both radiative grain alignment predictions and fluctuations in the magnetic field orientation. We also characterize the spatial structure of the polarization angle using the angle dispersion function and find that, in nearby fields at intermediate latitudes, the polarization angle is ordered over extended areas that are separated by filamentary structures, which appear as interfaces where the magnetic field sky projection rotates abruptly without apparent variations in the dust column density. The polarization fraction is found to be anti-correlated with the dispersion of the polarization angle, implying that the variations are likely due to fluctuations in the 3D magnetic field orientation along the line of sight sampling the diffuse interstellar medium.We also compare the dust emission with the polarized synchrotron emission measured with the Planck LFI, with low-frequency radio data, and with Faraday rotation measurements of extragalactic sources. The two polarized components are globally similar in structure along the plane and notably in the Fan and North Polar Spur regions. A detailed comparison of these three tracers shows, however, that dust and cosmic rays generally sample different parts of the line of sight and confirms that much of the variation observed in the Planck data is due to the 3D structure of the magnetic field.

astro-ph.GA↗

arXiv2013

The role of fast magnetic reconnection in acceleration zones of microquasars and AGNs

Fast magnetic reconnection events can be a very powerful mechanism operating at the jet launching region of microquasars and AGNs. We have recently found that the power released by reconnection between the magnetic field lines of the coronal inner disk region and the lines anchored into the black hole is able to accelerate relativistic particles through a first-order Fermi process and produce the observed radio luminosity from both microquasars and low luminous AGNs (LLAGNs). We also found that the observed correlation between the radio luminosity and the mass of these sources, spanning 10^9 orders of magnitude in mass, is naturally explained by this process. In this work, assuming that the gamma-ray emission is probably originated in the same acceleration zones that produce the radio emission, we have applied the scenario above to investigate the origin of the high energy outcomes from an extensive number of sources including high (HLAGNs) and LLAGNs, microquasars and GRBs. We find correlation of our model with the gamma emission only for microquasars and a few LLAGNs, while none of the HLAGNs or GRBs are fitted, neither in radio nor in gamma. We attribute the lack of correlation of the gamma emission for most of the LLAGNs to the fact that this processed emission doesn't depend only on the local magnetic field activity around the source/accretion disk, but also on other environmental factors like the photon and density fields. We conclude that the emission from the LLAGNs and microquasars comes from the nuclear region of their sources and therefore, can be driven by nuclear magnetic activity. However, in the case of the HLAGNs and GRBs, the nuclear emission is blocked by the surrounding density and photon fields and therefore, we can only see the jet emission further out.

astro-ph.HE↗

arXiv2013

Collapse of Turbulent Cores and Reconnection Diffusion

For a molecular cloud clump to form stars some transport of magnetic flux is required from the denser, inner regions to the outer regions of the cloud, otherwise this can prevent the collapse. Fast magnetic reconnection which takes place in the presence of turbulence can induce a process of reconnection diffusion (RD). Extending earlier numerical studies of reconnection diffusion in cylindrical clouds, we consider more realistic clouds with spherical gravitational potentials and also account for the effects of the gas self-gravity. We demonstrate that within our setup RD is efficient. We have also identified the conditions under which RD becomes strong enough to make an initially subcritical cloud clump supercritical and induce its collapse. Our results indicate that the formation of a supercritical core is regulated by a complex interplay between gravity, self-gravity, the magnetic field strength and nearly transonic and trans-Alfvénic turbulence, confirming that RD is able to remove magnetic flux from collapsing clumps, but only a few of them become nearly critical or supercritical, sub-Alfvénic cores, which is consistent with the observations. Besides, we have found that the supercritical cores built up in our simulations develop a predominantly helical magnetic field geometry which is also consistent with observations. Finally, we have evaluated the effective values of the turbulent reconnection diffusion coefficient and found that they are much larger than the numerical diffusion, especially for initially trans-Alfvénic clouds, ensuring that the detected magnetic flux removal is due to to the action of the RD rather than to numerical diffusivity.

astro-ph.HE↗

arXiv2013

Magnetic field amplification and evolution in turbulent collisionless MHD: an application to the ICM

The amplification and maintenance of the observed magnetic fields in the ICM are usually attributed to the turbulent dynamo action. This is generally derived employing a collisional MHD model. However, in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. Unlike collisional MHD simulations, our study uses an anisotropic plasma pressure with respect to the direction of the local magnetic field, which brings the plasma within a parameter space where collisionless instabilities should take place. Within the adopted model these instabilities are contained at bay through the relaxation term of the pressure anisotropy which simulates the feedback of the mirror and firehose instabilities. This relaxation acts to get the plasma distribution function consistent with the empirical studies of collisionless plasmas. Our 3D numerical simulations of forced transonic turbulence motivated by modeling of the turbulent ICM are performed for different initial values of the magnetic field intensity, and different relaxation rates of the pressure anisotropy. We found that in the high beta plasma regime corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results which are similar to the collisional-MHD model as far as the statistical properties of the turbulence are concerned. Also, the amplification of seed magnetic fields due to the turbulent dynamo action is similar to the collisional-MHD model. Our simulations that do not employ the anisotropy relaxation deviate significantly from the collisional-MHD results, and show more power at the small-scale fluctuations of both density and velocity representing the results of the instabilities. For these simulations the large scale fluctuations in the magnetic field are mostly suppressed and the turbulent dynamo fails in amplifying seed magnetic fields.

astro-ph.CO↗

arXiv2013

Turbulence and Dynamo Interlinks

The role of turbulence in astrophysical environments and its interplay with magnetic fields is still highly debated. In this lecture, we will discuss this issue in the framework of dynamo processes. We will first present a very brief summary of turbulent dynamo theories, then will focus on small scale turbulent dynamos and their particular relevance on the origin and maintenance of magnetic fields in the intra-cluster media (ICM) of galaxies. In these environments, the very low density of the flow requires a collisionless-MHD treatment. We will show the implications of this approach in the turbulent amplification of the magnetic fields in these environments. To finalize, we will also briefly address the connection between MHD turbulence and fast magnetic reconnection and its possible implications in the diffusion of magnetic flux in the dynamo process.

astro-ph.SR↗

arXiv2013

Evolution of M82-like starburst winds revisited: 3D radiative cooling hydrodynamical simulations

In this study we present three-dimensional radiative cooling hydrodynamical simulations of galactic winds generated particularly in M82-like starburst galaxies. We have considered intermittent winds induced by SNe explosions within super star clusters randomly distributed in the central region of the galaxy and were able to reproduce the observed M82 wind conditions with its complex morphological outflow structure. We have found that the environmental conditions in the disk in nearly recent past are crucial to determine whether the wind will develop a large scale rich filamentary structure, as in M82 wind, or not. Also, the numerical evolution of the SN ejecta have allowed us to obtain the abundance distribution over the first 3 kpc extension of the wind and we have found that the SNe explosions change significantly the metallicity only of the hot, low-density wind component. Moreover, we have found that the SN-driven wind transports to outside the disk large amounts of energy, momentum and gas, but the more massive high-density component reaches only intermediate altitudes smaller than 1.5 kpc. Therefore, no significant amounts of gas mass are lost to the IGM and the mass evolution of the galaxy is not much affected by the starburst events occurring in the nuclear region.

astro-ph.CO↗

arXiv2012

Disc formation in turbulent cloud cores: is magnetic flux loss necessary to stop the magnetic braking catastrophe or not?

Recent numerical analysis of Keplerian disk formation in turbulent, magnetized cloud cores by Santos-Lima, de Gouveia Dal Pino, & Lazarian (2012) demonstrated that reconnection diffusion is an efficient process to remove the magnetic flux excess during the build up of a rotationally supported disk. This process is induced by fast reconnection of the magnetic fields in a turbulent flow. In a similar numerical study, Seifried et al. (2012) concluded that reconnection diffusion or any other non-ideal MHD effects would not be necessary and turbulence shear alone would provide a natural way to build up a rotating disk without requiring magnetic flux loss. Their conclusion was based on the fact that the mean mass-to-flux ratio (μ) evaluated over a spherical region with a radius much larger than the disk is nearly constant in their models. In this letter we compare the two sets of simulations and show that this averaging over large scales can mask significant real increases of μ in the inner regions where the disk is built up. We demonstrate that turbulence-induced reconnection diffusion of the magnetic field happens in the initial stages of the disk formation in the turbulent envelope material that is accreting. Our analysis is suggestive that reconnection diffusion is present in both sets of simulations and provides a simple solution for the "magnetic braking catastrophe" which is discussed in the literature in relation to the formation of protostellar accretion disks.

astro-ph.GA↗

arXiv2012

Characterizing the magnetic field in the intracluster medium

During structure formation, energetic events and random motions of the hot gas residing inside galaxy clusters (the intracluster medium, ICM) generate turbulent motions. Radio diffuse emission probes the presence of magnetic fields and relativistic particles in the ICM, being a key ingredient for understanding the physical processes at work in clusters of galaxies. In this work, we present results from numerical simulations of magnetic field turbulence in magnetohydrodynamics (MHD) and kinetic MHD (KMHD) frameworks. We characterize magnetic field structures through the imprints left on polarization maps.

astro-ph.CO↗

arXiv2012

The role of pressure anisotropy in the turbulent intracluster medium

In low-density plasma environments, such as the intracluster medium (ICM), the Larmour frequency is much larger than the ion-ion collision frequency. In such a case, the thermal pressure becomes anisotropic with respect to the magnetic field orientation and the evolution of the turbulent gas is more correctly described by a kinetic approach. A possible description of these collisionless scenarios is given by the so-called kinetic magnetohydrodynamic (KMHD) formalism, in which particles freely stream along the field lines, while moving with the field lines in the perpendicular direction. In this way a fluid-like behavior in the perpendicular plane is restored. In this work, we study fast growing magnetic fluctuations in the smallest scales which operate in the collisionless plasma that fills the ICM. In particular, we focus on the impact of a particular evolution of the pressure anisotropy and its implications for the turbulent dynamics of observables under the conditions prevailing in the ICM. We present results from numerical simulations and compare the results which those obtained using an MHD formalism.

astro-ph.CO↗

arXiv2011

The role of turbulent magnetic reconnection on the formation of rotationally supported protostellar disks

The formation of protostellar disks out of molecular cloud cores is still not fully understood. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low mass stars. This has been known as the magnetic braking problem and the most investigated mechanism to alleviate this problem and help removing the excess of magnetic flux during the star formation process, the so called ambipolar diffusion (AD), has been shown to be not sufficient to weaken the magnetic braking at least at this stage of the disk formation. In this work, motivated by recent progress in the understanding of magnetic reconnection in turbulent environments, we appeal to the diffusion of magnetic field mediated by magnetic reconnection as an alternative mechanism for removing magnetic flux. We investigate numerically this mechanism during the later phases of the protostellar disk formation and show its high efficiency. By means 3D MHD simulations, we show that this mechanism is able to transport magnetic flux to the outskirts of the disk progenitor at time scales compatible with the collapse, allowing the formation of a rotationally supported disk around the protostar of dimensions ~100 AU. Since MHD turbulence is expected to be present in protostellar disks, this is a natural mechanism for removing magnetic flux excess and allowing the formation of these disks. This mechanism dismiss the necessity of postulating a hypothetical increase of the Ohmic resistivity as discussed in the literature. Together with our earlier work which showed that magnetic flux removal from molecular cloud cores is very efficient, this work calls for reconsidering the relative role of AD for the processes of star and planet formation.

astro-ph.GA↗

arXiv2011

Magnetohydrodynamic Simulations of Reconnection and Particle Acceleration: Three-Dimensional Effects

The magnetic fields can change their topology through a process known as magnetic reconnection. This process in not only important for understanding the origin and evolution of the large-scale magnetic field, but is seen as a possibly efficient particle accelerator producing cosmic rays mainly through the first order Fermi process. In this work we study the properties of particle acceleration in reconnection zones and show that the velocity component parallel to the magnetic field of test particles inserted in nearly non-resistive magnetohydrodynamic (MHD) domains of reconnection without including kinetic effects, such as pressure anisotropy, the Hall term, or anomalous effects, increases exponentially. Also, the acceleration of the perpendicular component is always possible in such models. We have found that within contracting magnetic islands or current sheets the particles accelerate predominantly through the first order Fermi process, as previously described, while outside the current sheets and islands the particles experience mostly drift acceleration due to magnetic fields gradients. Considering two dimensional MHD models without a guide field, we find that the parallel acceleration stops at some level. This saturation effect is however removed in the presence of an out-of-plane guide field or in three dimensional models. Therefore, we stress the importance of the guide field and fully three dimensional studies for a complete understanding of the process of particle acceleration in astrophysical reconnection environments.

astro-ph.HE↗

arXiv2011

MHD turbulence-Star Formation Connection: from pc to kpc scales

The transport of magnetic flux to outside of collapsing molecular clouds is a required step to allow the formation of stars. Although ambipolar diffusion is often regarded as a key mechanism for that, it has been recently argued that it may not be efficient enough. In this review, we discuss the role that MHD turbulence plays in the transport of magnetic flux in star forming flows. In particular, based on recent advances in the theory of fast magnetic reconnection in turbulent flows, we will show results of three-dimensional numerical simulations that indicate that the diffusion of magnetic field induced by turbulent reconnection can be a very efficient mechanism, especially in the early stages of cloud collapse and star formation. To conclude, we will also briefly discuss the turbulence-star formation connection and feedback in different astrophysical environments: from galactic to cluster of galaxy scales.

astro-ph.SR↗

arXiv2011

Dynamo in the Intra-Cluster Medium: Simulation of CGL-MHD Turbulent Dynamo

The standard magnetohydrodynamic (MHD) description of the plasma in the hot, magnetized gas of the intra-cluster (ICM) medium is not adequate because it is weakly collisional. In such collisionless magnetized gas, the microscopic velocity distribution of the particles is not isotropic, giving rise to kinetic effects on the dynamical scales. These kinetic effects could be important in understanding the turbulence, as so as the amplification and maintenance of the magnetic fields in the ICM. It is possible to formulate fluid models for collisonless or weakly collisional gas by introducing modifications in the MHD equations. These models are often referred as kinetic MHD (KMHD). Using a KMHD model based on the CGL-closure, which allows the adiabatic evolution of the two components of the pressure tensor (the parallel and perpendicular components with respect to the local magnetic field), we performed 3D numerical simulations of forced turbulence in order to study the amplification of an initially weak seed magnetic field. We found that the growth rate of the magnetic energy is comparable to that of the ordinary MHD turbulent dynamo, but the magnetic energy saturates in a level smaller than of the MHD case. We also found that a necessary condition for the dynamo works is to impose limits to the anisotropy of the pressure.

astro-ph.CO↗

arXiv2011

Precessing AGN Jets, Bubbles and Cooling Flows

Several galaxy clusters are known to present multiple and misaligned pairs of cavities seen in X-rays, as well as twisted kiloparsec-scale jets at radio wavelengths. It suggests that the AGN precessing jets play a role in the formation of the misaligned bubbles. Also, X-ray spectra reveal that typically these systems are also able to supress cooling flows, predicted theoretically. The absence of cooling flows in galaxy clusters has been a mistery for many years since numerical simulations and analytical studies suggest that AGN jets are highly energetic, but are unable to redistribute it at all directions. We performed 3D hydrodynamical simulations of the interaction between a precessing AGN jet and the warm intracluster medium plasma, which dynamics is coupled to a NFW dark matter gravitational potential. Radiative cooling has been taken into account and the cooling flow problem was studied. We found that precession is responsible for multiple pairs of bubbles, as observed. The misaligned bubbles rise up to scales of tens of kiloparsecs, where the thermal energy released by the jets are redistributed. After $\sim 150$ Myrs, the temperature of the gas within the cavities is kept of order of $\sim 10^7$ K, while the denser plasma of the intracluster medium at the central regions reaches $T \sim 10^5$ K. The existence of multiple bubbles, at diferent directions, result in an integrated temperature along the line of sight much larger than the simulations of non-precessing jets. This result is in agreement with the observations. The simulations reveal that the cooling flows cessed $\sim 50 - 70$ Myr after the AGN jets are started.

astro-ph.GA↗

arXiv2010

The role of magnetic reconnection on jet/accretion disk systems

It was proposed earlier that the relativistic ejections observed in microquasars could be produced by violent magnetic reconnection episodes at the inner disk coronal region. Here we revisit this model, which employs a standard accretion disk description and fast magnetic reconnection theory, and discuss the role of magnetic reconnection and associated heating and particle acceleration in different jet/disk accretion systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

astro-ph.HE↗

arXiv2010

Diffusion of magnetic field and removal of magnetic flux from clouds via turbulent reconnection

The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence is reassuring that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. In the presence of gravity and turbulence, our 3D simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the saturated final state of the simulations, supporting the notion that the reconnection-enabled diffusivity relaxes the magnetic field + gas system in the gravitational field to its minimal energy state. This effect is expected to play an important role in star formation, from its initial stages of concentrating interstellar gas to the final stages of the accretion to the forming protostar.

astro-ph.GA↗

arXiv2010

Precessing jets and X-ray bubbles from NGC1275 (3C84) in the Perseus galaxy cluster: a view from 3D numerical simulations

The Perseus galaxy cluster is known to present multiple and misaligned pairs of cavities seen in X-rays, as well as twisted kiloparsec-scale jets at radio wavelengths; both morphologies suggest that the AGN jet is subject to precession. In this work we performed 3D hydrodynamical simulations of the interaction between a precessing AGN jet and the warm intracluster medium plasma, which dynamics is coupled to a NFW dark matter gravitational potential. The AGN jet inflates cavities that become buoyantly unstable and rise up out of the cluster core. We found that under certain circumstances precession can originate multiple pairs of bubbles. For the physical conditions in the Perseus cluster, multiple pairs of bubbles are obtained for a jet precession opening angle > 40 degrees acting for at least three precession periods, reproducing well both radio and X-ray maps. Based on such conditions, assuming that the Bardeen-Peterson effect is dominant, we studied the evolution of the precession opening angle of this system. We were able to constrain the ratio between the accretion disc and black hole angular momenta as 0.7 - 1.4. We were also able to constrain the present precession angle to 30 - 40 degrees, as well as the approximate age of the inflated bubbles to 100 - 150 Myrs.

astro-ph.CO↗

arXiv2010

Magnetic Field Effects near the launching region of Astrophysical Jets

One of the fundamental properties of astrophysical magnetic fields is their ability to change topology through reconnection and in doing so, to release magnetic energy, sometimes violently. In this work, we review recent results on the role of magnetic reconnection and associated heating and particle acceleration in jet/accretion disk systems, namely young stellar objects (YSOs), microquasars, and active galactic nuclei (AGNs).

astro-ph.HE↗