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We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Lyα line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ≈ 2. However, in contrast to the classical picture of gas cooling from the ∼ 10 6 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20, 000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Lyα line. While a galaxy's cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Lyα photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (∼ 100 kpc), luminous (L Lyα ∼ 10 44 erg s −1 ) "blobs" of Lyα emission found in Steidel et al.'s (1999) narrow band survey of a z = 3 proto-cluster. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Lyα blobs, and we present predictions for the contribution of cooling radiation to the Lyα luminosity function of galaxies as a function of redshift.

We forecast the sensitivity with which the Murchison Widefield Array (MWA) can measure the 21 cm power spectrum of cosmic hydrogen. The MWA is sensitive to roughly a decade in scale (wavenumbers of k $ 0:1Y1 h Mpc À1 ). This amounts primarily to constraints on two numbers: the amplitude and the slope of the 21 cm power spectrum on the scales probed. We find, however, that the redshift evolution in these quantities can yield important information about reionization. We examine a range of theoretical models, spanning uncertainties in the nature of the ionizing sources and the abundance of minihalos during reionization. Although the power spectrum differs substantially among these models, a generic prediction is that the amplitude of the 21 cm power spectrum on MWA scales (k $ 0:4 h Mpc À1 ) peaks near the epoch when the intergalactic medium (IGM) is %50% ionized. Moreover, the slope of the 21 cm power spectrum flattens as the ionization fraction increases and the sizes of the H ii regions grow. With regards to detection sensitivity, we show that the optimal MWA antenna configuration for power spectrum measurements would pack all 500 antenna tiles as closely as possible in a compact core. Detecting the characteristic redshift evolution of our models will help to confirm that observed 21 cm fluctuations originate from the IGM, and not from foregrounds, and will provide an indirect constraint on the evolution of the volume-filling factor of H ii regions during reionization. After two years of observations, the MWA can constrain the filling factor at an epoch when x i h i$ 0:5 to within roughly AE x i h i$ 0:1 at 2 confidence. Subject headingg s: cosmology: theory -intergalactic medium -large-scale structure of universe

We present the results of three-dimensional radiation hydrodynamics simulations of the formation and evolution of early H ii/He iii regions around the first stars. Cooling and recollapse of the gas in the relic H ii region is also followed in a full cosmological context, until second-generation stars are formed. We first carry out ray-tracing simulations of ionizing radiation transfer from the first star. Hydrodynamics is directly coupled with photoionization heating as well as radiative and chemical cooling. The photoionized hot gas is evacuated out of the host halo at a velocity of $30 km s À1. This radiative feedback effect quenches further star formation within the halo for over tens to a hundred million years. We show that the thermal and chemical evolution of the photoionized gas in the relic H ii region is remarkably different from that of a neutral primordial gas. Efficient molecular hydrogen production in the recombining gas enables it to cool to $100 K, where fractionation of HD/H 2 occurs. The gas further cools by HD line cooling down to a few tens of kelvins. Interestingly, at high redshifts (z > 10), the minimum gas temperature is limited by that of the cosmic microwave background with T CMB ¼ 2:728(1þ z). The gas cloud experiences runaway collapse when its mass is $40 M , which is significantly smaller than a typical clump mass of $200Y300 M for early primordial gas clouds. We argue that massive, rather than very massive, primordial stars may form in the relic H ii region. Such stars might be responsible for early metal enrichment of the interstellar medium from which recently discovered hyperYmetal-poor stars were born.

We use numerical simulations of structure formation in a cold dark matter cosmology to compare the angular momentum distributions of dark matter and nonradiative gas in a large sample of halos. We show that the two components have identical spin parameter distributions and that their angular momentum distributions within individual halos are very similar, all in excellent agreement with standard assumptions. Despite these similarities, however, we find that the angular momentum vectors of the gas and dark matter are poorly aligned, with a median misalignment angle of $30 , which might have important implications for spin correlation statistics used in weak lensing studies. We present distributions for the component of the angular momentum that is aligned with the total angular momentum of each halo and find that for between 5% and 50% of the mass, this component is negative. This disagrees with the generally adopted '' universal '' angular momentum distribution, for which the mass fraction with negative specific angular momentum is zero. We discuss the implications of our results for the formation of disk galaxies. Since galactic disks generally do not contain counterrotating stars or gas, disk formation cannot occur under detailed conservation of specific angular momentum. We suggest that the material with negative specific angular momentum combines with positive angular momentum material to build a bulge component, and we show that in such a scenario the remaining material can form a disk with a density distribution that is very close to exponential.