Jaehong Park

An Infrared Survey of Brightest Cluster Galaxies. II. Why are Some Brightest Cluster Galaxies Forming Stars

Quillen et al. presented an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission located in the cores of X-ray-luminous clusters. They found that at least half of these sources have signs of excess IR emission. Here we discuss the nature of the IR emission and its implications for cool core clusters. The strength of the mid-IR excess emission correlates with the luminosity of the optical emission lines. Excluding the four systems dominated by an AGN, the excess mid-IR emission in the remaining brightest cluster galaxies is likely related to star formation. The mass of molecular gas (estimated from CO observations) is correlated with the IR luminosity as found for normal star-forming galaxies. The gas depletion timescale is about 1 Gyr. The physical extent of the IR excess is consistent with that of the optical emission-line nebulae. This supports the hypothesis that star formation occurs in molecular gas associated with the emission-line nebulae and with evidence that the emission-line nebulae are mainly powered by ongoing star formation. We find a correlation between mass deposition rates () estimated from the X-ray emission and the star formation rates estimated from the IR luminosity. The star formation rates are 1/10 to 1/100 of the mass deposition rates, suggesting that the reheating of the intracluster medium is generally very effective in reducing the amount of mass cooling from the hot phase but not eliminating it completely.

An Infrared Survey of Brightest Cluster Galaxies. I.

We report on an imaging survey with the Spitzer Space Telescope of 62 brightest cluster galaxies with optical line emission. These galaxies are located in the cores of X-ray luminous clusters selected from the ROSAT All-Sky Survey. We find that about half of these sources have a sign of excess infrared emission; 22 objects out of 62 are detected at 70 μm, 18 have 8/5.8 μm flux ratios above 1.0 and 28 have 24/8 μm flux ratios above 1.0. Altogether 35 of 62 objects in our survey exhibit at least one of these signs of infrared excess. Four galaxies with infrared excesses have a 4.5/3.6 μm flux ratio indicating the presence of hot dust, and/or an unresolved nucleus at 8 μm. Three of these have high measured [O III](5007 A)/Hβ flux ratios suggesting that these four, Abell 1068, Abell 2146, Zwicky 2089, and R0821+07, host dusty active galactic nuclei (AGNs). Nine objects (including the four hosting dusty AGNs) have infrared luminosities greater than 1011 L☉ and so can be classified as luminous infrared galaxies (LIRGs). Excluding the four systems hosting dusty AGNs, the excess mid-infrared emission in the remaining brightest cluster galaxies is likely related to star formation.

Evolution of a relativistic electron beam–plasma return current system

Evolution of a relativistic electron beam-plasma return current system has been studied using particle-in-cell simulations. The mode number-resolved linear growth rates of the oblique instabilities that the system suffers generally agree with the existing theory [A. Bret et al., Phys. Rev. E 72, 016403 (2005)]. The comparison of in- and out-of-plane simulations shows that two-stream type of instabilities dominates the early stage of energy transfer from the beam drift energy to the beam and plasma thermal energy. The end stage of the nonlinear evolution is dominated Weibel/filament type of instabilities, resulting a beam with a moderately increased angular spread, reduced drift energy, and no reduction in the initial cross section.

The minimum gap‐opening planet mass in an irradiated circumstellar accretion disc

We consider the minimum mass planet, as a function of radius, that is capable of opening a gap in an �-accretion disk. We estimate that a half Jupiter mass planet can open a gap in a disk with accretion rate u M . 10 −8 M⊙/yr for viscosity parameter � = 0.01, and Solar mass and luminosity. The minimum mass is approximately proportional to u M 0.48 � 0.8 M 0.42 ∗ L −0.08 ∗ . This estimate can be used to rule out the presence of massive planets in gapless accretion disks. We identify two radii at which an inwardly migrating planet may become able to open a gap and so slow its migration; the radius at which the heating from viscous dissipation is similar to that from stellar radiation in a flared disk, and the radius at which the disk has optical depth order 1 in a self-shadowed disk. If a gap opening planet cuts off disk accretion allowing the formation of a central hole or clearing in the disk then we would estimate that the clearing radius would approximately be proportional to the stellar mass.

Particle-in-cell Simulations of Particle Energization via Shock Drift Acceleration from Low Mach Number Quasi-perpendicular Shocks in Solar Flares

Low Mach number, high beta fast mode shocks can occur in the magnetic reconnection outflows of solar flares. These shocks, which occur above flare loop tops, may provide the electron energization responsible for some of the observed hard X-rays and contemporaneous radio emission. Here we present new two-dimensional particle-in-cell simulations of low Mach number/high beta quasi-perpendicular shocks. The simulations show that electrons above a certain energy threshold experience shock-drift-acceleration. The transition energy between the thermal and non-thermal spectrum and the spectral index from the simulations are consistent with some of the X-ray spectra from RHESSI in the energy regime of E 40 ~ 100xa0keV. Plasma instabilities associated with the shock structure such as the modified-two-stream and the electron whistler instabilities are identified using numerical solutions of the kinetic dispersion relations. We also show that the results from PIC simulations with reduced ion/electron mass ratio can be scaled to those with the realistic mass ratio.

Particle-in-cell simulations of particle energization from low Mach number fast mode shocks

Astrophysical shocks are often studied in the high Mach number limit but weakly compressive fast shocks can occur in magnetic reconnection outflows and are considered to be a site of particle energization in solar flares. Here we study the microphysics of such perpendicular, low Mach number collisionless shocks using two-dimensional particle-in-cell (PIC) simulations with a reduced ion/electron mass ratio and employ a moving wall boundary method for initial generation of the shock. This moving wall method allows for more control of the shock speed, smaller simulation box sizes, and longer simulation times than the commonly used fixed wall, reflection method of shock formation. Our results, which are independent of the shock formation method, reveal the prevalence shock drift acceleration (SDA) of both electron and ions in a purely perpendicular shock with Alfven Mach number

Energy transfer and magnetic field generation via ion-beam driven instabilities in an electron-ion plasma

M_A=6.8

Why are some brightest cluster galaxies forming stars

and ratio of thermal to magnetic pressure

Spitzer observations of star formation in brightest cluster galaxies

beta=8

Comparison of 2-D PIC simulations with 2-D quasilinear calculations of ion-election energy transfer from instabilities driven by counter-streaming ion beams

. We determine the respective minimum energies required for electrons and ions to incur SDA. We derive a theoretical electron distribution via SDA that compares to the simulation results. We also show that a modified two-stream instability due to the incoming and reflecting ions in the shock transition region acts as the mechanism to generate collisionless plasma turbulence that sustains the shock.