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Register a new userMeasurement of the baryonic acoustic oscillation scale in 21 cm intensity fluctuations during the reionisation era
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It has recently been suggested that the power spectrum of redshifted 21cm fluctuations could be used to measure the scale of baryonic acoustic oscillations (BAOs) during the reionisation era. The resulting measurements are potentially as precise as those offered by the next generation of galaxy redshift surveys at lower redshift. However unlike galaxy redshift surveys, which in the linear regime are subject to a scale independent galaxy bias, the growth of ionised regions during reionisation is thought to introduce a strongly scale dependent relationship between the 21cm and mass power spectra. We use a semi-numerical model for reionisation to assess the impact of ionised regions on the precision and accuracy with which the BAO scale could be measured using redshifted 21cm observations. For a model in which reionisation is completed at z~6, we find that the constraints on the BAO scale are not systematically biased at z > 6.5. In this scenario, and assuming the sensitivity attainable with a low-frequency array comprising 10 times the collecting area of the Murchison Widefield Array, the BAO scale could be measured to within 1.5 per cent in the range 6.5 < z < 7.5.
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We investigate the impact of neutral hydrogen (HI) in galaxies on the statistics of 21-cm fluctuations using analytic and semi-numerical modelling. Following the reionisation of hydrogen the HI content of the Universe is dominated by damped absorption systems (DLAs), with a cosmic density in HI that is observed to be constant at a level equal to ~2% of the cosmic baryon density from z~1 to z~5. We show that extrapolation of this constant fraction into the reionisation epoch results in a reduction of 10-20% in the amplitude of 21-cm fluctuations over a range of spatial scales. The assumption of a different percentage during the reionisation era results in a proportional change in the 21-cm fluctuation amplitude. We find that consideration of HI in galaxies/DLAs reduces the prominence of the HII region induced shoulder in the 21-cm power spectrum (PS), and hence modifies the scale dependence of 21-cm fluctuations. We also estimate the 21cm-galaxy cross PS, and show that the cross PS changes sign on scales corresponding to the HII regions. From consideration of the sensitivity for forthcoming low-frequency arrays we find that the effects of HI in galaxies/DLAs on the statistics of 21-cm fluctuations will be significant with respect to the precision of a PS or cross PS measurement. In addition, since overdense regions are reionised first we demonstrate that the cross-correlation between galaxies and 21-cm emission changes sign at the end of the reionisation era, providing an alternative avenue to pinpoint the end of reionisation. The sum of our analysis indicates that the HI content of the galaxies that reionise the universe will need to be considered in detailed modelling of the 21-cm intensity PS in order to correctly interpret measurements from forthcoming low-frequency arrays.
We assess the effect of a population of high-redshift quasars on the 21-cm power spectrum during the epoch of reionisation. Our approach is to implement a semi-numerical scheme to calculate the three-dimensional structure of ionised regions surrounding massive halos at high redshift. We include the ionising influence of luminous quasars by populating a simulated overdensity field with quasars using a Monte Carlo Markov Chain algorithm. We find that quasars modify both the amplitude and shape of the power spectrum at a level which is of the same order as the fractional contribution to reionisation. The modification is found both at constant redshift and at constant global neutral fraction, and arises because ionising photons produced by quasars are biased relative to the density field at a level that is higher than steller ionising photons. Our results imply that quasar ionisation will need to be included in detailed modelling of observed 21-cm power spectra.
We incorporate a contribution to reionization from X-rays within analytic and semi-numerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionization. We explore the impact that X-ray ionizations have on the power spectrum (PS) of 21-cm fluctuations by varying both the average X-ray MFP and the fractional contribution of X-rays to reionization. In general, prior to the epoch when the intergalactic medium is dominated by ionized regions (H {sevensize II} regions), X-ray-induced ionization enhances fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray heating does not strongly supress galaxy formation. Conversely, at later times when H2 regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionization. Our modelling also shows that the modification of the 21-cm signal due to the presence of X-rays is sensitive to the relative scales of the X-ray MFP, and the characteristic size of H2 regions. We therefore find that X-rays imprint an epoch and scale-dependent signature on the 21-cm PS, whose prominence depends on fractional X-ray contribution. The degree of X-ray heating of the IGM also determines the extent to which these features can be discerned. We show that the MWA will have sufficient sensitivity to detect this modification of the PS, so long as the X-ray photon MFP falls within the range of scales over which the array is most sensitive ($sim0.1$ Mpc$^{-1}$). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required.
The cosmic microwave background (CMB) polarisation and the 21 cm line fluctuations are powerful probes of cosmological reionisation. We study how the cross-correlation between the CMB polarisation (E-modes) and the 21 cm line fluctuations can be used to gain further understanding of the reionisation history, within the framework of inhomogeneous reionisation. Since the E-mode polarisation reflects the amplitude of the quadrupole component of the CMB temperature fluctuations, the angular power spectrum of the cross-correlation exhibits oscillations at all multipoles. The first peak of the power spectrum appears at the scale corresponding to the quadrupole at the redshift that is probed by the 21 cm line fluctuations. The peak reaches its maximum value in redshift when the average ionisation fraction of the universe is about half. On the other hand, on small scales, there is a damping that depends on the duration of reionisation. Thus, the cross-correlation between the CMB polarisation and the 21 cm line fluctuations has the potential to constrain accurately the epoch and the duration of reionisation.
Using the 21 cm line, observed all-sky and across the redshift range from 0 to 5, the large scale structure of the Universe can be mapped in three dimensions. This can be accomplished by studying specific intensity with resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in order to address the question: What is the nature of Dark Energy? In addition, the study of Large Scale Structure across this range addresses the questions: How does Gravity effect very large objects? and What is the composition our Universe? The same data set can be used to search for and catalog time variable and transient radio sources.
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