Two-Phase Cooling Flows with Magnetic Reconnection

Motivated by the observations of high Faraday rotation measures measured in cooling flow clusters we propose a model relevant to plasmas with comparable thermal and magnetic pressures. Magnetic field reconnection may play a major role in changing the topology of the magnetic field in the central cooling flow regions. The effect of the topology change is that cool flux loops can reconnect to hot flux loops that are connected to the overall thermal reservoir of the cluster. There can be a rapid recycling of mass between hot and cold phases on a time scale of 3 x 10^8-10^9 yr which may reduce the inferred inflow and mass condensation rates by at least an order of magnitude. A central multiphase medium is a direct consequence of such a model. Throughout the cooling flow the filling factor of the hot loops (T > 2 x 10^7 K) is of order unity. The filling factor of the cool loops (T < 2 x 10^7 K) is 0.1-1% with a corresponding mass fraction of cold phase of 1-10%. A crucial parameter is the coherence length of the field relative to the cooling radius and the distribution of field energy with scale. When the cooling radius is greater than the field coherence length then cooling flows proceed as usual. When the coherence length is greater than the central cooling radius, however, the thermal energy of the reservoir can be tapped and the mass condensation rates may be very significantly reduced. Three additional conditions must be satisfied: I. Cold loops must be able to fall at least as far as the mean distance between hot loops in a cooling time; II. Loops must enter an evaporative phase on reconnecting; and III. A sufficient number of hot loops penetrate the cold phase region to power the radiative losses.Comment: 16 pages, uses AAS macro aasm

Can quasars photoionize the intergalactic medium at high redshift?

The reionization of the intergalactic medium (IGM) by quasar sources at high redshift are discussed. The integrated UV background from observed QSO's, taking into account the hydrogen opacity associated with intervening Ly-alpha clouds and Lyman limit systems are computed. It is noted that the published data appear to indicate a significant underdensity of absorption systems in the Ly-alpha forest with column densities N(sub HI) greater than 10(exp 15) cm(sup -2). This deficit results in a reduction of the opacity of the universe by a factor of 1.5-3 at z = 3-5 relative to previous estimates. The QSO contribution to the metagalactic flux at the Lyman edge may be as large as J(sub 912)(z) is approximately 6((1 + z)/4.5)(sup 0.5) x 10(exp -22) erg cm(sup -2) s(sup -1) Hz(sup -1) sr(sup -1) for q(sup o) = O, and slightly lower for q(sub o) = 1/2. For a density of the diffuse component of the IGM of omega(sub D)(h(sub 50)(sup 2)) less than 0.025, QSO's could photoionize a smooth IGM sufficiently to satisfy the constraints imposed by the Gunn-Peterson effect. The epoch of reionization could be as recent as z is approximately greater than 5. As a result, neutral patches of IGM would be detectable in the spectra of high redshift quasars. The patches would appear as absorption line systems with typical column densities of 10(exp 19) - 10(exp 20) cm(sup -2), and velocity widths of 100 - 1000 km s(sup -1)

The Structure and Evolution of Lyman-alpha Forest Clouds in the Minihalo Model

Results are presented for one-dimensional numerical hydrodynamics computations of the structure and evolution of Lya forest clouds gravitationally confined by dark matter minihalos. The clouds are developed from linear perturbations at high redshift and exposed to either a QSO- or galaxy-dominated metagalactic radiation field at moderate redshifts. While the emphasis is on spherical systems, slab symmetry is also considered. Three zones may be identified in a collapsed cloud: (1) a quasi-hydrostatic core in thermal equilibrium, (2) a nonhydrostatic intermediate zone out of thermal equilibrium, and (3) a cosmological accretion layer joining onto the Hubble expansion. Most of the measured Lya forest column densities arise in the intermediate zone. The development of the core would result in a flattening in the column density distribution near an HI column density of 10**15-10**16 cm**-2. The cloud diameters corresponding to an HI column density of 10**14 cm**-2 lie in the range 10-60 kpc, while systems with column densities exceeding 10**15 cm**-2 have diameters smaller than 10 kpc. Systems with circular velocities exceeding 50 km/s result in clouds which contract until they become Jeans unstable and collapse. The critical column density for collapse is 10**17-10**18 cm**-2. A mild correlation of Doppler parameter with neutral hydrogen column density is found in several models for systems with column densities less than 10**13.5 cm**-2, with Doppler parameters occurring as low as b > 20 km/s for 10**13 cm**-2 column density systems. No lines with b < 15 km/s are found.Comment: 28 pages, uuencoded compressed postscrip

The possible detection of high redshift Type II QSOs in deep fields

The colours of high redshift Type II QSOs are synthesized from observations of moderate redshift systems. It is shown that Type II QSOs are comparable to starbursts in their success at matching the colours of z_850-dropouts and i_775-drops in the Hubble Ultra Deep Field, and more naturally account for the bluest objects detected. Type II QSOs may also account for some of the i_775-drops detected in the GOODS fields. It is shown that by combining imaging data from the Hubble Space Telescope and the James Webb Space Telescope, it will be possible to clearly separate Type II QSOs from Type I QSOs and starbursts based on their colours. Similarly, it is shown that the UKIDSS ZYJ filters may be used to discriminate high redshift Type II QSOs from other objects. If Type II QSOs are prevalent at high redshifts, then AGN may be major contributors to the re-ionization of the Intergalactic Medium.Comment: 11 pages, 10 figures; to be published in MNRAS; new predictions for corrected UKIDSS UDS limit

Radiative transfer through the Intergalactic Medium

We use a probabilistic method to compute the propagation of an ionization front corresponding to the re-ionization of the intergalactic medium in a LCDM cosmology, including both hydrogen and helium. The effects of radiative transfer substantially boost the temperature of the ionized gas over the case of uniform re-ionization. The resulting temperature-density relation of the ionized gas is both non-monotonic and multiple-valued, reflecting the non-local character of radiative transfer and suggesting that a single polytropic relation between local gas density and temperatue is a poor description of the thermodynamic state of baryons in the post-reionization universe.Comment: 5 pages, 7 figures, letter accepted for publication in MNRA

The influence of metagalactic ultra-violet background fluctuations on the high-redshift Lyα forest

Under the assumption that galaxies and Quasi-Stellar Objects (QSOs) dominate the metagalactic ultra-violet (UV) background, it is shown that at high redshifts fluctuations in the UV background are dominated by QSO shot noise and have an auto-correlation length of a few to several comoving Mpcs, depending on the bright end of the QSO luminosity function. The correlations create long range spatial coherence in the neutral hydrogen fraction. Using a semi-analytic model, it is demonstrated that the coherence may account for the broad distribution in effective optical depths measured in the Ly$\alpha$ forest spectra of background QSOs, for line-of-sight segments of comoving length $50h^{-1}$ Mpc at redshifts $5<z<6$. Capturing the fluctuations in a numerical simulation requires a comoving box size of $\sim1h^{-1}$ Gpc, although a box half this size may be adequate if sufficient random realizations of the QSO population are performed.Comment: 11 pages, 9 figures. Accepted for publication in MNRA