arXiv subjects

Takuya Akahori

Publications and source records attributed to Takuya Akahori.

70 records · Page 4Linked to original sources

arXiv2014

Faraday dispersion functions of galaxies

The Faraday dispersion function (FDF), which can be derived from an observed polarization spec- trum by Faraday rotation measure synthesis, is a profile of polarized emissions as a function of Faraday depth. We study intrinsic FDFs along sight lines through face-on, Milky-Way-like galaxies by means of a sophisticated galactic model incorporating 3D MHD turbulence, and investigate how much the FDF contains information intrinsically. Since the FDF reflects distributions of thermal and cosmic- ray electrons as well as magnetic fields, it has been expected that the FDF could be a new probe to examine internal structures of galaxies. We, however, find that an intrinsic FDF along a sight line through a galaxy is very complicated, depending significantly on actual configurations of turbulence. We perform 800 realizations of turbulence, and find no universal shape of the FDF even if we fix the global parameters of the model. We calculate the probability distribution functions of the standard deviation, skewness, and kurtosis of FDFs and compare them for models with different global pa- rameters. Our models predict that the presence of vertical magnetic fields and large scale-height of cosmic-ray electrons tend to make the standard deviation relatively large. Contrastingly, differences in skewness and kurtosis are relatively less significant.

astro-ph.GA↗

arXiv2014

Statistical Techniques for Detecting the Intergalactic Magnetic Field from Large Samples of Extragalactic Faraday Rotation Data

Rotation measure (RM) grids of extragalactic radio sources have been widely used for studying cosmic magnetism. But their potential for exploring the intergalactic magnetic field (IGMF) in filaments of galaxies is unclear, since other Faraday-rotation media such as the radio source itself, intervening galaxies, and the interstellar medium of our Galaxy are all significant contributors. We study statistical techniques for discriminating the Faraday rotation of filaments from other sources of Faraday rotation in future large-scale surveys of radio polarization. We consider a 30 degree times 30 degree field-of-view toward the south Galactic pole, while varying the number of sources detected in both present and future observations. We select sources located at high redshifts and toward which depolarization and optical absorption systems are not observed, so as to reduce the RM contributions from the sources and intervening galaxies. It is found that a high-pass filter can satisfactorily reduce the RM contribution from the Galaxy, since the angular scale of this component toward high Galactic latitudes would be much larger than that expected for the IGMF. Present observations do not yet provide a sufficient source density to be able to estimate the RM of filaments. However, from the proposed approach with forthcoming surveys, we predict significant residuals of RM that should be ascribable to filaments. The predicted structure of the IGMF down to scales of 0.1 degree should be observable with data from the SKA, if we achieve selections of sources toward which sight lines do not contain intervening galaxies and RM errors are less than a few rad m-2.

astro-ph.CO↗

arXiv2014

Properties of Intrinsic Polarization Angle Ambiguities in Faraday Tomography

Faraday tomography is a powerful method to diagnose polarizations and Faraday rotations along the line of sight. The quality of Faraday tomography is, however, limited by several conditions. Recently, it is reported that Faraday tomography indicates false signals in some specific situations. In this paper, we systematically investigate the condition of the appearance of false signals in Faraday tomography. We study this by pseudo-observing two sources within a beam, and change in the intrinsic polarization angles, rotation measures, intensities, and frequency coverage. We find that false signals arise when rotation measure between the sources is less than 1.5 times the full width at half maximum of the rotation measure spread function. False signals also depend on the intensity ratio between the sources and are reduced for large ratio. On the other hand, the appearance of false signals does not depend on frequency coverage, meaning that the uncertainty should be correctly understood and taken into consideration even with future wide-band observations such as Square Kilometer Array (SKA).

astro-ph.IM↗

arXiv2014

Exploring the Intergalactic Magnetic Field by Means of Faraday Tomography

Unveiling the intergalactic magnetic field (IGMF) in filaments of galaxies is a very important and challenging subject in modern astronomy. In order to probe the IGMF from rotation measures (RMs) of extragalactic radio sources, we need to separate RMs due to other origins such as the source, intervening galaxies, and our Galaxy. In this paper, we discuss observational strategies for the separation by means of Faraday tomography (Faraday RM Synthesis). We consider an observation of a single radio source such as a radio galaxy or a quasar viewed through the Galaxy and the cosmic web. We then compare the observation with another observation of a neighbor source with a small angular separation. Our simulations with simple models of the sources suggest that it would be not easy to detect the RM due to the IGMF of order ~ 1 rad/m/m, an expected value for the IGMF through a single filament. Contrary to it, we find that the RM of at least ~10 rad/m/m could be detected with the SKA or its pathfinders/precursors, if we achieve selections of ideal sources. These results would be improved if we incorporate decomposition techniques such as RMCLEAN and QU-fitting. We discuss feasibility of the strategies for cases with complex Galactic emissions as well as with effects of observational noise and radio frequency interferences.

astro-ph.CO↗

arXiv2013

Multi-Wavelength Studies of Spectacular Ram Pressure Stripping of a Galaxy: Discovery of an X-ray Absorption Feature

We report the detection of an X-ray absorption feature near the galaxy M86 in the Virgo cluster. The absorber has a column density of 2-3 E20 /cm^2, and its position coincides with the peak of an intracluster HI cloud which was removed from the galaxy NGC 4388 presumably by ram pressure. These results indicate that the HI cloud is located in front of M86 along the line-of-sight, and suggest that the stripping was primarily created by an interaction between NGC 4388 and the hot plasmas of the Virgo cluster, not the M86 halo. By calculating an X-ray temperature map, we further detected an X-ray counterpart of the HI cloud up to about 3' south of M86. It has a temperature of 0.89 keV and a mass of ~4.5 E8 solar mass, exceeding the estimated HI gas mass. The high hot-to-cold gas ratio in the cloud indicates a significant evaporation of the HI gas, probably by thermal conduction from the hotter cluster plasma with a sub-Spitzer rate.

astro-ph.HE↗

arXiv2013

Multi-Wavelength Studies of Spectacular Ram Pressure Stripping of a Galaxy. II. Star Formation in the Tail

With multiband photometric data in public archives, we detected four intracluster star-forming regions in the Virgo cluster. Two of them were at a projected distance of 35 kpc away from NGC4388, and the other two were 66 kpc away. Our new spectroscopic observation revealed that their recession velocities were comparable to the ram-pressure-stripped tail of NGC4388 and confirmed their association. The stellar mass of the star-forming regions ranged from 10^4 - 10^4.5 M_sun except for that of the faintest one which would be <10^3 M_sun. The metallicity was comparable to the solar abundance, and the age of the stars was ~ 10^6.8 years. Their young stellar age meant that the star formation should have started after the gas was stripped from NGC4388. This implied in situ condensation of the stripped gas. We also found that two star-forming regions lay near the leading edge of a filamentary dark cloud. The extinction of the filament was smaller than that derived from the Balmer decrement of the star-forming regions. It implied that the dust in the filament would be locally dense around the star-forming regions.

astro-ph.CO↗

arXiv2013

Fisher Analysis on Wide-Band Polarimetry for Probing the Intergalactic Magnetic Field

We investigate the capability of ongoing radio telescopes for probing Faraday rotation measure (RM) due to the intergalactic magnetic field (IGMF) in the large-scale structure of the universe which is expected to be of order $O(1) {\rm rad/m^2}$. We consider polarization observations of a compact radio source such as quasars behind a diffuse source such as the Galaxy, and calculate Stokes parameters $Q$ and $U$ assuming a simple model of the Faraday dispersion functions with Gaussian shape. Then, we perform the Fisher analysis to estimate the expected errors in the model parameters from QU-fitting of polarization intensity, accounting for sensitivities and frequency bands of Australian Square Kilometer Array Pathfinder, Low Frequency Array, and the Giant Meterwave Radio Telescope. Finally, we examine the condition on the source intensities which are required to detect the IGMF. Our analysis indicates that the QU-fitting is promising for forthcoming wideband polarimetry to explore RM due to the IGMF in filaments of galaxies.

astro-ph.CO↗

arXiv2013

Simulated Faraday Rotation Measures toward High Galactic Latitudes

We study the Faraday rotation measure (RM) due to the Galactic magnetic field (GMF) toward high Galactic latitudes. The RM arises from the global, regular component as well as from the turbulent, random component of the GMF. We model the former based on observations and the latter using the data of magnetohydrodynamic turbulence simulations. For a large number of different GMF models, we produce mock RM maps around the Galactic poles and calculate various statistical quantities with the RM maps. We find that the observed medians of RMs toward the north and south Galactic poles, $\sim 0.0\pm 0.5 {\rm rad m^{-2}}$ and $\sim +6.3\pm 0.5 {\rm rad m^{-2}}$, are difficult to explain with any of our many alternate GMF models. The standard deviation of observed RMs, $\sim 9 {\rm rad m^{-2}}$, is clearly larger than that of simulated RMs. The second-order structure function of observed RMs is substantially larger than that of simulated RMs, especially at small angular scales. We discuss other possible contributions to RM toward high Galactic latitudes. Besides observational errors and the intrinsic RM of background radio sources against which RM is observed, we suggest that the RM due to the intergalactic magnetic field may account for a substantial fraction of the observed RM. Finally we note that reproducing the observed medians may require additional components or/and structures of the GMF that are not present in our models.

astro-ph.GA↗

arXiv2013

Dynamo activities driven by magneto-rotational instability and Parker instability in galactic gaseous disk

We carried out global three-dimensional magneto-hydrodynamic simulations of dynamo activities in galactic gaseous disks without assuming equatorial symmetry. Numerical results indicate the growth of azimuthal magnetic fields non-symmetric to the equatorial plane. As magneto-rotational instability (MRI) grows, the mean strength of magnetic fields is amplified until the magnetic pressure becomes as large as 10% of the gas pressure. When the local plasma $β$ ($ = p_{\rm gas}/p_{\rm mag}$) becomes less than 5 near the disk surface, magnetic flux escapes from the disk by Parker instability within one rotation period of the disk. The buoyant escape of coherent magnetic fields drives dynamo activities by generating disk magnetic fields with opposite polarity to satisfy the magnetic flux conservation. The flotation of the azimuthal magnetic flux from the disk and the subsequent amplification of disk magnetic field by MRI drive quasi-periodic reversal of azimuthal magnetic fields in timescale of 10 rotation period. Since the rotation speed decreases with radius, the interval between the reversal of azimuthal magnetic fields increases with radius. The rotation measure computed from the numerical results shows symmetry corresponding to a dipole field.

astro-ph.GA↗

arXiv2011

Non-Equilibrium Ionization State and Two-Temperature Structure in the Bullet Cluster 1E0657-56

We investigate a non-equilibrium ionization state and an electron-ion two-temperature structure of the intracluster medium in the merging galaxy cluster, 1E0657-56 (the Bullet cluster), using a series of N-body and hydrodynamic simulations. We find that the electron temperature at the shock layer associated with the X-ray sub peak (bullet) is quite different depending on the thermal relaxation model between electrons and ions; ~25 keV for the Coulomb thermal relaxation model and ~45 keV for the instantaneous thermal relaxation model in the simulations which reproduce the observed X-ray morphology. Furthermore, both of Fe xxv and Fe xxvi are overabundant compared with the ionization equilibrium state around the shock layer, and thus, the intensity ratio between Fe xxv and Fe xxvi K alpha lines are significantly altered from that in the ionization equilibrium state. We also carry out the simulations with various sets of merger parameters, and discuss a possible range of the non-equilibrium effects in this system. Our results could be tested with future X-ray observations such as Astro-H with better sensitivity in high energy band.

astro-ph.CO↗

arXiv2011

Faraday Rotation Measure due to the Intergalactic Magnetic Field II: the Cosmological Contribution

We investigate the Faraday rotation measure (RM) due to the intergalactic magnetic field (IGMF) through the cosmic web up to cosmological distances, using a model IGMF based on turbulence dynamo in the large-scale structure of the universe. By stacking the IGMF and gas density data up to redshift $z=5$ and taking account of the redshift distribution of polarized background radio sources against which the RM is measured, we simulate the sky map of the RM. The contribution from galaxy clusters is subtracted from the map, based on several different criteria of X-ray brightness and temperature. Our findings are as follows. The distribution of RM for radio sources of different redshifts shows that the root-mean-square (rms) value increases with redshift and saturates for $z \ga 1$. The saturated value is RM$_{\rm rms} \approx$ several ${\rm rad m^{-2}}$. The probability distribution function of $|{\rm RM}|$ follows the lognormal distribution. The power spectrum has a broad plateau over the angular scale of $\sim 1 - 0.1^\circ$ with a peak around $\sim 0.15^\circ$. The second-order structure function has a flat profile in the angular separation of $\ga 0.2^\circ$. Our results could provide useful insights for surveys to explore the IGMF with the Square Kilometer Array (SKA) and upcoming SKA pathfinders.

astro-ph.CO↗

arXiv2010

Faraday Rotation Measure due to the Intergalactic Magnetic Field

Studying the nature and origin of the intergalactic magnetic field (IGMF) is an outstanding problem of cosmology. Measuring Faraday rotation would be a promising method to explore the IGMF in the large-scale structure (LSS) of the universe. We investigated the Faraday rotation measure (RM) due to the IGMF in filaments of galaxies using simulations for cosmological structure formation. We employed a model IGMF based on turbulence dynamo in the LSS of the universe; it has an average strength of $< B > \sim 10$ nG and a coherence length of several $\times\ 100\ h^{-1}$ kpc in filaments. With the coherence length smaller than path length, the inducement of RM would be a random walk process, and we found that the resultant RM is dominantly contributed by the density peak along line of sight. The rms of RM through filaments at the present universe was predicted to be $\sim 1\ {\rm rad\ m^{-2}}$. In addition, we predicted that the probability distribution function of $|{\rm RM}|$ through filaments follows the log-normal distribution, and the power spectrum of RM in the local universe peaks at a scale of $\sim 1\ h^{-1}$ Mpc. Our prediction of RM could be tested with future instruments.

astro-ph.CO↗

arXiv2010

Hydrodynamic Simulations of Merging Galaxy Clusters: Non-Equilibrium Ionization State and Two-Temperature Structure

We investigate a non-equilibrium ionization state and an electron-ion two-temperature structure of the intracluster medium (ICM) in merging galaxy clusters using a series of N-body and hydrodynamic simulations. Mergers with various sets of mass ratios and impact parameters are systematically investigated, and it is found that, in most cases, ICM significantly departs from the ionization equilibrium state at the shock layers with a Mach number of ~1.5-2.0 in the outskirts of the clusters, and the shock layers with a Mach number of ~2-4 in front of the ICM cores. Accordingly, the intensity ratio between Fe xxv and Fe xxvi K alpha line emissions is significantly altered from that in the ionization equilibrium state. If the effect of the two-temperature structure of ICM is incorporated, the electron temperature is ~10-20 % and ~30-50 % lower than the mean temperature of ICM at the shock layers in the outskirts and in front of the ICM cores, respectively, and the deviation from the ionization equilibrium state becomes larger. We also address the dependence of the intensity ratio on the viewing angle with respect to the merging plane.

astro-ph.CO↗

arXiv2008

Non-Equilibrium Ionization State and Two-Temperature Structure in the Linked Region of Abell 399/401

We investigate a non-equilibrium ionization state and two-temperature structure of the intracluster medium in the linked region of Abell 399/401, using a series of N-body + SPH simulations, and find that there exist significant shock layers at the edge of the linked region, and that the ionization state of iron departs from the ionization equilibrium state at the shock layers and around the center of the linked region. As for the two-temperature structure, an obvious difference of temperature between electrons and ions is found in the edge of the linked regions. K alpha line emissions of Fe xxiv and Fe xxv are not severely affected by the deviation from the ionization equilibrium state around the center of the linked region, suggesting that the detection of relatively high metallicity in this area cannot be ascribed to the non-equilibrium ionization state of the intracluster medium. On the other hand, the K alpha emissions are significantly deviated from the equilibrium values at the shock layers, and the intensity ratio of K alpha lines between Fe xxiv-xxv and Fe xxvi is found to be significantly altered from that in the ionization equilibrium state.

astro-ph↗

arXiv2006

Core Structure of Intracluster Gas: Effects of Radiative Cooling on Core Sizes

We investigate the core structure of radiatively cooling intracluster gas, using a hydrodynamics code. We calculate evolution of model clusters of the initial core radii 160--300 kpc until the initial central cooling time, and analyze the resultant clusters using the double beta-model as done by observational studies. It is found that the core-size distribution thus obtained shows two peaks \sim 30--100 kpc and \sim 100--300 kpc and marginally can reproduce the observed distribution which exhibits two distinct peaks around \sim 50 kpc and \sim 200 kpc. This result may suggest radiative-cooling origin for small cores, while cooling is yet insignificant in the clusters of large cores. It should be noted that the small core peak is reproduced by clusters that are still keeping quasi-hydrostatic balance before the initial central cooling time has elapsed.

astro-ph↗

arXiv2005

Core Structure of Intracluster Gas: Isothermal Hydrostatic Equilibrium

We investigate core structures of X-ray emitting intracluster gas based on the so-called beta-model, which is an isothermal hydrostatic model often used in observational studies. We reconsider the beta-model and find that the virial temperature T_vir of a cluster may be represented better by beta T_X than T_X, where beta is the parameter obtained from the X-ray surface brightness and T_X is the emission-weighted mean temperature of the gas. We investigate 121 clusters observed by ROSAT and ASCA and find that the luminosity-temperature relation L_X - beta T_X is less steep than L_X - T_X. We classify the clusters into two core-size groups in order to investigate their properties in detail. While in the larger core group the core radius is marginally proportional to the virial radius, no significant relation is found for the smaller core group. This may suggest that the smaller cores reflect the presence of cD galaxies, effect of radiative cooling or asymmetry in the surface brightness. We examine such possibilities, and find that the clusters of smaller cores have shorter cooling time than the Hubble time, while no significant correlation is found with cD or asymmetry. We carry out hydrodynamical calculations to simulate the beta-model, intending to see the behavior of the isothermal gas under the gravitational potential including the dark matter and galaxies with or without a central cD galaxy. Calculations show r_c \propto r_vir and T_vir \simeq beta T consistently with our consideration to the beta-model. Also is found from calculations that the presence of a large cD galaxy may form a gas core \sim 40 kpc, which seems too small to account for the range of the core sizes, 40--80 kpc, of the smaller core group.

astro-ph↗