Naoya Fukuda

Three-dimensional Magnetohydrodynamic Simulations of Cold Fronts in Magnetically Turbulent ICM

Steep gradients of temperature and density, called cold fronts, are observed by Chandra in a leading edge of subclusters moving through the intracluster medium (ICM). The presence of cold fronts indicates that thermal conduction across the front is suppressed by magnetic fields. We carried out three-dimensional magnetohydrodynamic (MHD) simulations including anisotropic thermal conduction of a subcluster moving through a magnetically turbulent ICM. We found that turbulent magnetic fields are stretched and amplified by shear flows along the interface between the subcluster and the ambient ICM. Since magnetic fields reduce the efficiency of thermal conduction across the front, the cold front survives for at least 1 Gyr. We also found that a moving subcluster works as an amplifier of magnetic fields. Numerical results indicate that stretched turbulent magnetic fields accumulate behind the subcluster and are further amplified by vortex motions. The moving subcluster creates a long tail of ordered magnetic fields, in which the magnetic field strength attains a value of β = Pgas/Pmag 10.

Three-dimensional MHD simulations of X-ray emitting subcluster plasmas in cluster of galaxies

Recent high resolution observations by the Chandra X-ray satellite revealed various substructures in hot X-ray emitting plasmas in cluster of galaxies. For example, Chandra revealed the existence of sharp discontinuities in the surface brightness at the leading edge of subclusters in merging clusters (e.g., Abell 3667), where the temperature drops sharply across the fronts. These sharp edges are called cold fronts. We present results of three-dimensional (3D) magnetohydrodynamic simulations of the interaction between a dense subcluster plasma and ambient magnetized intracluster medium. Anisotropic heat conduction along magnetic field lines is included. At the initial state, magnetic fields are assumed to be uniform and transverse to the motion of the dense subcluster. Since magnetic fields ahead of the subcluster slip toward the third direction in the 3D case, the strength of magnetic fields in this region can be reduced compared to that in the 2D case. Nevertheless, a cold front can be maintained because the magnetic field lines wrapping around the forehead of the subcluster suppress the heat conduction across them. On the other hand, when the magnetic field is absent, a cold front cannot be maintained because isotropic heat conduction from the hot ambient plasma rapidly heats the cold subcluster plasma.

Sequential Star Formation in A Cometary Globule (BRC37) of IC1396

We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an H II region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slitless spectroscopy we detected ten Hα emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally toward an O9.5 star, but has evolved to align toward a younger O6 star when that formed. Near-IR and optical photometry suggests that four of these six stars are low-mass young stellar objects (YSOs) with masses of ~0.4 M ☉. Their estimated ages of ~1 Myr indicate that they were formed at the tip in advance of the formation of the IRAS source. Therefore, it is likely that sequential star formation has been taking place along the direction from the exciting stars toward the IRAS source, due to the UV impact of the exciting star(s). Interestingly, one faint, Hα emission star, which is the closest to the exciting star(s), seems to be a young brown dwarf that was formed by the UV impact in advance of the formation of other YSOs at the tip.

Magnetohydrodynamic Simulations of the Formation of Cold Fronts in Clusters of Galaxies Including Heat Conduction

Recent Chandra observations of clusters of galaxies revealed the existence of a sharp ridge in the X-ray surface brightness where the temperature drops across the front. This front is called the cold front. We present the results of two-dimensional magnetohydrodynamic simulations of the time evolution of a dense subcluster plasma moving in a cluster of galaxies. Anisotropic heat conduction along the magnetic field lines is included. In the models without magnetic fields, the numerical results indicate that the heat conduction from the hot ambient plasma heats the cold dense plasma of the subcluster and diffuses out the cold front. When magnetic fields exist in a cluster of galaxies, however, cold fronts can be maintained because the heat conduction across the magnetic field lines is suppressed. We found that even when the magnetic fields in a cluster of galaxies are disordered, heat conduction across the front is restricted because the magnetic field lines are stretched along the front. Numerical results reproduced the X-ray intensity distribution observed in the A3667 cluster of galaxies.

Near-Infrared Polarimetry of the Eagle Nebula (M 16)

We present a theoretical framework which establishes how the core radius of a star cluster varies with the mass of an assumed central black hole. Our result is that rc/rh ∝ (Mbh/M) 3/4 when the system is well relaxed. The theory compares favourably with a number of simulations of this problem, which extend to black hole masses of order 10% of the cluster mass. Though strictly limited as yet to clusters with stars of equal mass, our conclusion strengthens the view that clusters with large core radii are the most promising candidates in which to find a massive black hole.

MHD SIMULATIONS OF A MOVING SUBCLUMP WITH HEAT CONDUCTION

High resolution observations of cluster of galaxies by Chandra have revealed the existence of an X-ray emitting comet-like galaxy C153 in the core of cluster of galaxies A2125. The galaxy C153 moving fast in the cluster core has a distinct X-ray tail on one side, obviously due to ram pressure stripping, since the galaxy C153 crossed the central region of A2125. The X-ray emitting plasma in the tail is substantially cooler than the ambient plasma. We present results of two-dimensional magnetohydrodynamic simulations of the time evolution of a sub clump like C153 moving in magnetized intergalactic matter. Anisotropic heat conduction is included. We found that the magnetic fields are essential for the existence of the cool X-ray tail, because in non-magnetized plasma the cooler sub clump tail is heated up by isotropic heat conduction from the hot ambient plasma and does not form such a comet-like tail.

DECAY OF ALFVEN WAVES IN A FILAMENTARY CLOUD

The decay of Alfven waves in a filamentary molecular cloud is investigated through three-dimensional numerical simulations. We have considered a filamentary molecular cloud supported in part by the Alfven wave against the self-gravity. Our attention has been focused on the basic physical mechanism for the decay. The decay rate is obtained as a function of the wavelength and amplitude. It is found that when the wave is circularly polarized, the decay e-folding timescale is several times the fast wave crossing timescale for the filament and independent of the wavelength, whereas when the wave is linearly polarized, the amplitude of the wave decreases inversely proportional to time. It is also found that the decay of Alfven waves induces rotation and shear flow in the filamentary cloud. The propagation of two Alfven waves in the medium results in the excitation of daughter waves due to nonlinear coupling between mother waves. The wavenumber of the daughter waves is the sum or difference between those of the mother waves, and below a critical wavenumber of the daughter wave, the filamentary cloud fragments as a result of Jeans instability. The fragments collapse to form high-density rotating magnetized disks. In contrast, below a critical wavenumber of the mother wave, the cloud becomes a dense helical filament after the decay of the Alfven waves. The present models are compared with previous simulations and observations with regard to the rotation, fragmentation, and helical structure of filamentary clouds.

MHD Simulations of Clusters of Galaxies Including Radiative Cooling

We carried out 2D and 3D magnetohydrodynamic simulations including radiative cooling and anisotropic thermal conduction of a magnetically turbulent intracluster medium. In 3D simulations, we found that magnetic pressure increases by a factor of 2 at 1.4 Gyr because magnetic fields accumulate in the center along with the plasma contracting by the cooling instability. In 2D simulations including thermal conduction, we found that a low temperature region exists along the loop-shaped magnetic field lines. Thus, the restriction of thermal conduction across magnetic field lines enables the coexistence of hot and cool plasmas in cluster cores.

Three-Dimensional MHD Simulations of a Subcluster Plasma Moving in Turbulent ICM

We carried out 3D magnetohydrodynamic simulations of a subcluster moving in turbulent ICM by including anisotropic heat conduction. Since magnetic fields stretched along the subcluster surface suppress the heat conduction across the front, cold fronts are formed and sustained.