We have conducted a deep multicolor imaging survey of 0.2 deg 2 centered on the Hubble Deep Field North (HDF-N). We shall refer to this region as the Hawaii HDF-N. Deep data were collected in U, B, V, R, I, and z 0 bands over the central 0.2 deg 2 and in HK 0 over a smaller region covering the Chandra Deep Field North. The data were reduced to have accurate relative photometry and astrometry across the entire field to facilitate photometric redshifts and spectroscopic follow-up. We have compiled a catalog of 48,858 objects in the central 0.2 deg 2 detected at 5 significance in a 3 00 aperture in either R or z 0 band. Number counts and color-magnitude diagrams are presented and shown to be consistent with previous observations. Using color selection we have measured the density of objects at 3 < z < 7. Our multicolor data indicates that samples selected at z > 5.5 using the Lyman break technique suffer from more contamination by low-redshift objects than suggested by previous studies.

Aims. This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C 196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project. Methods. We have used the high band antennas (HBA) of LOFAR to image this region and rotation measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth. Results. The brightness temperature of the detected Galactic emission is 5−15 K in polarized intensity and covers the range from -3 to +8 rad m −2 in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m −2 running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350 MHz. There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto-ionic medium. Conclusions. The mean line-of-sight magnetic field component, B , is determined to be 0.3 ± 0.1 µG and its spatial variation across the 3C 196 field is 0.1 µG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (n e B > 6.2 cm −3 µG) compared to its surroundings.

The redshifted ultraviolet light from early stars at z $ 10 contributes to the cosmic near-infrared background. We present detailed calculations of its spectrum with various assumptions about metallicity and the mass spectrum of early stars. We show that if the near-infrared background has a stellar origin, metal-free stars are not the only explanation of the excess near-infrared background; stars with metals (e.g., Z ¼ 1/50 Z ) can produce the same amount of background intensity as the metal-free stars. We quantitatively show that the predicted average intensity at 1-2 m is essentially determined by the efficiency of nuclear burning in stars, which is not very sensitive to metallicity. We predict I / à ' 4 8 nW m À2 sr À1 , where à is the mean star formation rate at z ¼ 7 15 (in units of M yr À1 Mpc À3 ) for stars more massive than 5 M . On the other hand, since we have very little knowledge about the form of the mass spectrum of early stars, the uncertainty in the average intensity due to the mass spectrum could be large. An accurate determination of the near-infrared background allows us to probe the formation history of early stars, which is difficult to constrain by other means. While the star formation rate at z ¼ 7 15 inferred from the current data is significantly higher than the local rate at z < 5, it does not rule out the stellar origin of the cosmic near-infrared background. In addition, we show that a reasonable initial mass function, coupled with this star formation rate, does not overproduce metals in the universe in most cases and may produce as little as less than 1% of the metals observed in the universe today.

The near-infrared background (NIRB) is one of a few methods that can be used to observe the redshifted light from early stars at a redshift of 6 and above, and thus it is imperative to understand the significance of any detection or nondetection of the NIRB. Fluctuations of the NIRB can provide information on the first structures, such as halos and their surrounding ionized regions in the intergalactic medium (IGM). We combine, for the first time, N-body simulations, radiative transfer code, and analytic calculations of luminosity of early structures to predict the angular power spectrum (C l ) of fluctuations in the NIRB. We study in detail the effects of various assumptions about the stellar mass, the initial mass spectrum of stars, the metallicity, the star formation efficiency (f * ), the escape fraction of ionizing photons (f esc ), and the star formation timescale (t SF ), on the amplitude as well as the shape of C l . The power spectrum of NIRB fluctuations is maximized when f * is the largest (as C l ∝ f 2 * ) and f esc is the smallest (as more nebular emission is produced within halos). A significant uncertainty in the predicted amplitude of C l exists due to our lack of knowledge of t SF of these early populations of galaxies, which is equivalent to our lack of knowledge of the mass-to-light ratio of these sources. We do not see a turnover in the NIRB angular power spectrum of the halo contribution, which was claimed to exist in the literature, and explain this as the effect of high levels of nonlinear bias that was ignored in the previous calculations. This is partly due to our choice of the minimum mass of halos contributing to NIRB (∼2 × 10 9 M ), and a smaller minimum mass, which has a smaller nonlinear bias, may still exhibit a turnover. Therefore, our results suggest that both the amplitude and shape of the NIRB power spectrum provide important information regarding the nature of sources contributing to the cosmic reionization. The angular power spectrum of the IGM, in most cases, is much smaller than the halo angular power spectrum, except when f esc is close to unity, t SF is longer, or the minimum redshift at which the star formation is occurring is high. In addition, low levels of the observed mean background intensity tend to rule out high values of f * 0.2.

LOFAR is the LOw‐Frequency Radio interferometer ARray located at midlatitude (52°53′N). Here we present results on ionospheric structures derived from 29 LOFAR nighttime observations during the winters of 2012/2013 and 2013/2014. We show that LOFAR is able to determine differential ionospheric total electron content values with an accuracy better than 0.001 total electron content unit = 1016m−2 over distances ranging between 1 and 100 km. For all observations the power law behavior of the phase structure function is confirmed over a long range of baseline lengths, between 1 and 80 km, with a slope that is, in general, larger than the 5/3 expected for pure Kolmogorov turbulence. The measured average slope is 1.89 with a one standard deviation spread of 0.1. The diffractive scale, i.e., the length scale where the phase variance is 1rad2, is shown to be an easily obtained single number that represents the ionospheric quality of a radio interferometric observation. A small diffractive scale is equivalent to high phase variability over the field of view as well as a short time coherence of the signal, which limits calibration and imaging quality. For the studied observations the diffractive scales at 150 MHz vary between 3.5 and 30 km. A diffractive scale above 5 km, pertinent to about 90% of the observations, is considered sufficient for the high dynamic range imaging needed for the LOFAR epoch of reionization project. For most nights the ionospheric irregularities were anisotropic, with the structures being aligned with the Earth magnetic field in about 60% of the observations.

The escape fraction, f esc , of ionizing photons from early galaxies is a crucial parameter for determining whether the observed galaxies at z ≥ 6 are able to reionize the high-redshift intergalactic medium. Previous attempts to measure f esc have found a wide range of values, varying from less than 0.01 to nearly 1. Rather than finding a single value of f esc , we clarify through modeling how internal properties of galaxies affect f esc through the density and distribution of neutral hydrogen within the galaxy, along with the rate of ionizing photons production. We find that the escape fraction depends sensitively on the covering factor of clumps, along with the density of the clumped and interclump medium. One must therefore be cautious when dealing with an inhomogeneous medium. Fewer, high-density clumps lead to a greater escape fraction than more numerous low-density clumps. When more ionizing photons are produced in a starburst, f esc increases, as photons escape more readily from the gas layers. Large variations in the predicted escape fraction, caused by differences in the hydrogen distribution, may explain the large observed differences in f esc among galaxies. Values of f esc must also be consistent with the reionization history. High-mass galaxies alone are unable to reionize the universe, because f esc > 1 would be required. Small galaxies are needed to achieve reionization, with greater mean escape fraction in the past.

Detection of the 21-cm signal coming from the epoch of reionization (EoR) is challenging especially because, even after removing the foregrounds, the residual Stokes I maps contain leakage from polarized emission that can mimic the signal. Here, we discuss the instrumental polarization of LOFAR and present realistic simulations of the leakages between Stokes parameters. From the LOFAR observations of polarized emission in the 3C196 field, we have quantified the level of polarization leakage caused by the nominal model beam of LOFAR, and compared it with the EoR signal using power spectrum analysis. We found that at 134-166 MHz, within the central 4• of the field the (Q, U ) → I leakage power is lower than the EoR signal at k < 0.3 Mpc −1 . The leakage was found to be localized around a Faraday depth of 0, and the rms of the leakage as a fraction of the rms of the polarized emission was shown to vary between 0.2-0.3%, both of which could be utilized in the removal of leakage. Moreover, we could define an 'EoR window' in terms of the polarization leakage in the cylindrical power spectrum above the PSF-induced wedge and below k ∼ 0.5 Mpc −1 , and the window extended up to k ∼ 1 Mpc −1 at all k ⊥ when 70% of the leakage had been removed. These LOFAR results show that even a modest polarimetric calibration over a field of view of 4• in the future arrays like SKA will ensure that the polarization leakage remains well below the expected EoR signal at the scales of 0.02-1 Mpc −1 .

Several experiments are underway to detect the cosmic redshifted 21-cm signal from neutral hydrogen from the Epoch of Reionization (EoR). Due to their very low signalto-noise ratio, these observations aim for a statistical detection of the signal by measuring its power spectrum. We investigate the extraction of the variance of the signal as a first step towards detecting and constraining the global history of the EoR. Signal variance is the integral of the signal's power spectrum, and it is expected to be measured with a high significance. We demonstrate this through results from a simulation and parameter estimation pipeline developed for the Low Frequency Array (LOFAR)-EoR experiment. We show that LOFAR should be able to detect the EoR in 600 hours of integration using the variance statistic. Additionally, the redshift (z r ) and duration (∆z) of reionization can be constrained assuming a parametrization. We use an EoR simulation of z r = 7.68 and ∆z = 0.43 to test the pipeline. We are able to detect the simulated signal with a significance of 4 standard deviations and extract the EoR parameters as z r = 7.72 +0.37 −0.18 and ∆z = 0.53 +0.12 −0.23 in 600 hours, assuming that systematic errors can be adequately controlled. We further show that the significance of detection and constraints on EoR parameters can be improved by measuring the cross-variance of the signal by cross-correlating consecutive redshift bins.