arXiv subjects

Avery Meiksin

Publications and source records attributed to Avery Meiksin.

79 records · Page 5Linked to original sources

arXiv1996

21-cm Tomography of the Intergalactic Medium at High Redshift

We investigate the 21-cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z>5). In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas which has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR. We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars. In the presence of a sufficiently strong ambient flux of Lyman-alpha photons, the hyperfine transition in the warmed H I will be excited. A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky. This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology. We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their ``spheres of influence'' over which they warm the IGM. As a result of the redshifted 21-cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km/s. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with (delta T/T)=1E-3 on degree-scales. Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5<z<10.

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arXiv1996

Higher Order Statistics from the EDSGC Survey I: Counts in Cells

Counts in cells are used to analyse the higher order properties of the statistics of the EDSGC survey. The probability distribution is obtained from an equal area projection source catalog with infinite oversampling over the range $0.015^\circ-2^\circ$. The factorial moments of the resulting distribution and the $s_N$'s characterizing the non-Gaussian nature of the distribution are extracted. These results are compared to previous results from the APM survey, and to theoretical results from perturbation theory. The deprojected 3D values corresponding to the $s_N$'s are also determined. We find that the 3D values match the scaling relation for strongly nonlinear clustering found in N-body simulations remarkably well, suggesting that the galaxies are reliable tracers of the underlying mass distribution.

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arXiv1996

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.

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arXiv1996

Cosmological Filaments and Minivoids: The Origin of Intergalactic Absorption

Soon after the first QSO was identified, Gunn & Peterson searched for the expected characteristic absorption trough on the blueward side of Lya in the spectrum of the QSO due to an Intergalactic Medium (IGM). They failed to find it, placing a constraint on the density of neutral hydrogen in the IGM that was less than 1 part in 10^5 of that residing in galaxies, and concluded that the IGM must be highly ionized. Soon afterwards absorption by Lya was detected in the IGM; not by a diffuse component, but by a clumpy component of intergalactic gas clouds, the Lya forest. The goal then became to search for `excess' absorption beyond that expected from the forest. No clear absorption by a diffuse HI component, however, was ever detected. The results of recent numerical hydrodynamics computations of the formation of the Lya forest appear to indicate that the division between a clumpy component and a diffuse one may be inappropriate. We find that in a CDM-dominated cosmology, no substantial diffuse medium should be expected. Instead, the medium condenses into a network of complex structures that reveal themselves as discrete absorption systems in the spectra of QSOs. The lowest column density lines arise from the fine structure in minivoids -- small regions with densities below the cosmic mean. These results suggest that the long sought for diffuse HI Gunn-Peterson effect may not exist.

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arXiv1995

Lyman Alpha Forest Clustering Using Cell Counts Statistics

We present a novel technique for calculating \Lya forest correlations based on cell counts. It is applied to the line lists from 7 QSOs observed at high resolution ($Δv<25\kms$). Two spectra (Q0055$-$259 and Q0014$+$813) appear to be sufficiently free of biases to obtain meaningful estimates. We find for both positive correlations, with a maximum of $0.5-1$, on the scale of $0.5-3 h^{-1}$ Mpc (comoving), or $100-600\kms$, which is consistent with the primordial power spectrum inferred from optically-selected galaxy surveys. Strong evidence for anti-correlation on the scale of $3-6 h^{-1}$ Mpc is also found. If this first detection of anti-correlations is physical, additional factors to gravity may play a role in determining the clustering of the \Lya forest on comoving scales $> 3h^{-1}\Mpc$.}

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arXiv1995

LYMAN-ALPHA FOREST CORRELATIONS USING NEIGHBOR COUNTS

We present a novel technique for calculating the two-point autocorrelation function of the Lyman-alpha forest based on the relation between the two-point correlation function and the Neighbor Probability Distribution Functions. The technique appears to reduce the scatter in estimates of the correlation function by a factor of about two from the traditional pair-counting method. We apply the technique to the Lyman-alpha forest line lists determined from the spectra of seven z > 2 QSOs observed at high resolution ( < 25 km/s ). Of these, only two spectra, those of Q0055-259 and Q0014+813, appear to be sufficiently free of systematics to obtain meaningful estimates of the correlation function. We find positive correlations, with a maximum amplitude of 0.5 - 1, on the scale of 0.5 - 3 h^{-1} Mpc (comoving), or 100 - 600 km/s, in the forests of both QSOs. The technique also finds strong evidence for anticorrelation on the scale of 3 - 6 h^{-1} Mpc. The strength of the positive correlations is comparable to that predicted from the primordial power spectrum inferred from optically-selected galaxy redshift surveys. If the anticorrelations are physical, it becomes unclear whether the detected clustering is consistent with current models of structure formation with a homogeneous photoionizing UV metagalactic background. Instead, the anti-clustering may require inhomogeneity in the UV background, and possibly in the process of reionization itself.

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arXiv1994

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.

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