Avery Meiksin

21 Centimeter 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 that 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. We find that bright quasars with only a small fraction of the observed comoving density at z ~ 4 will suffice to preheat the entire universe at z 6. We also show that, in a cold dark matter dominated cosmology, the thermal bremsstrahlung radiation associated with collapsing galactic mass halos (1010-1011 M?) may warm the IGM to ~100 K by z ~ 7. Alternatively, the equivalent of ~10% of the star formation rate density in the local universe, whether in isolated pregalactic stars, dwarf, or normal galaxies, would be capable of heating the entire IGM to a temperature above that of the CBR by Ly? scattering in a small fraction of the Hubble time at z ~ 6. In the presence of a sufficiently strong ambient flux of Ly? 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-1. Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with ?T/T 10-3 on scales ? ~ 1?. This scale is set either by the thermalization distance from a QSO within which Ly? pumping determines the spin temperature of the IGM or by the quasar lifetime. 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. A next generation facility like the Square Kilometer Array Interferometer could effectively open much of the universe to a direct study of the reheating epoch and possibly probe the transition from a neutral universe to one that is fully ionized.

Spectral Analysis of the Lyα Forest in a Cold Dark Matter Cosmology

We simulate the Lyα forest in a standard cold dark matter universe using a two-level hierarchical grid code to evolve the dark and baryonic matter components self-consistently. We solve the time-dependent ionization equations for hydrogen and helium, adopting Haardt & Madaus recent estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lyα forest by constructing synthetic spectra and analyzing them using an automated procedure to identify, deblend, and fit Voigt line profiles to the absorption features. The H I column density and Doppler parameter distributions that we obtain agree closely with those measured by the Keck Observatorys High Resolution Eschelle Spectrograph (HIRES) and earlier high spectral resolution observations over the column density range 1012 cm-2 3.5). We also compare with measured values of the intergalactic He II opacity. Our results require an He II ionizing background lower than the Haardt & Madau estimate by a factor of 4, corresponding to a soft intrinsic QSO spectrum, with αQ ≈ 1.8-2.

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.

Lyman-Alpha Forest Correlations Using Neighbor Counts

We present a novel technique for calculating the two-point autocorrelation function of the Lyα 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 ~2 from the traditional pair-counting method. We apply the technique to the Lyα forest line lists determined from the spectra of seven z > 2 QSOs observed at high resolution (Δv < 25 km s-1). 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-1, 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 anticlustering may require inhomogeneity in the UV background and possibly in the process of reionization itself.

The lyman alpha forest in hierarchical cosmologies

The comparison of quasar absorption spectra with numerically simulated spectra from hierarchical cosmological models of structure formation promises to be a valuable tool to discriminate among these models. We present simulation results for the column density, Doppler b parameter, and optical depth probability distributions for five popular cosmological models.

The HeII Proximity Effect

A major uncertainty in the interpretation of the recent HST detection of He ii Lyα resonant absorption in Q0302-003 (Jakobsen et al. 1994), is the contribution to the opacity arising from line-blanketing by the Lyα forest. It is shown how moderate resolution UV spectra may be used to distinguish between the case of total opacity dominated by line-blanketing from that including a substantial component from a diffuse medium. The method permits a direct measurement of the total baryonic density of the IGM.