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Register a new userCII and HI 21-cm line intensity mapping from the EoR: Impact of the light-cone effect on auto and cross-power spectra
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Added by
Chandra Shekhar Murmu
Publication date
2021
fields
Physics
and research's language is
English
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CII line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the CII LIM signal fluctuations from the EoR, enabling us to estimate the CII power-spectrum and CII$times$21-cm cross-power spectrum. Observed LIM signal will have its time evolution embedded in it along the Line of Sight (LoS) due to the finite travel time of the signal from its origin to the observer. We have investigated this so-called light-cone effect on the observed statistics of our semi-numerically simulated CII signal from the EoR. Using a suit of simulated CII and neutral hydrogen 21-cm maps and corresponding light-cone boxes, we have shown that the light-cone effect can impact the CII power spectrum by more than 15% at large scales ($ksim 0.1, text{Mpc}^{-1}$, at $z=6.8$). We have also observed that the impact of light-cone effect on the CII power spectrum drops with decreasing redshift within the redshift range considered here ($7.2 lesssim z lesssim 6$). The CII$times$21-cm cross-power spectrum is also affected by light-cone, and in our models where reionization ends before $z=6$, we find that the maximum impact on cross-power can reach up to 20%. At $z=6.4$, we find comparatively pronounced variation in the light-cone effect with reionization history on the cross power. Faster reionization histories have a more drastic light-cone effect on cross-power. We conclude that we need to incorporate the light-cone in order to properly model the signal, constrain the EoR-related astrophysical parameters and reionization history using the CII$times$21-cm cross-power spectrum.
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Observations of redshifted 21-cm radiation from neutral hydrogen during the epoch of reionization (EoR) are considered to constitute the most promising tool to probe that epoch. One of the major goals of the first generation of low frequency radio telescopes is to measure the 3D 21-cm power spectrum. However, the 21-cm signal could evolve substantially along the line of sight (LOS) direction of an observed 3D volume, since the received signal from different planes transverse to the LOS originated from different look-back times and could therefore be statistically different. Using numerical simulations we investigate this so-called light cone effect on the spherically averaged 3D 21-cm power spectrum. For this version of the power spectrum, we find that the effect mostly `averages out and observe a smaller change in the power spectrum compared to the amount of evolution in the mean 21-cm signal and its rms variations along the LOS direction. Nevertheless, changes up to 50% at large scales are possible. In general the power is enhanced/suppressed at large/small scales when the effect is included. The cross-over mode below/above which the power is enhanced/suppressed moves toward larger scales as reionization proceeds. When considering the 3D power spectrum we find it to be anisotropic at the late stages of reionization and on large scales. The effect is dominated by the evolution of the ionized fraction of hydrogen during reionization and including peculiar velocities hardly changes these conclusions. We present simple analytical models which explain qualitatively all the features we see in the simulations.
The amplitude of redshifted 21 cm fluctuations during the Epoch of Reionization (EoR) is expected to show a distinctive rise and fall behavior with decreasing redshift as reionization proceeds. On large scales (k <~ 0.1 Mpc^{-1}) this can mostly be characterized by evolution in the product of the mean 21 cm brightness temperature and a bias factor, b_21(z). This quantity evolves in a distinctive way that can help in determining the average ionization history of the intergalactic medium (IGM) from upcoming 21 cm fluctuation data sets. Here we consider extracting <T_21> b_21(z) using a combination of future redshifted 21 cm and [CII] line-intensity mapping data sets. Our method exploits the dependence of the 21 cm-[CII]-[CII] cross-bispectrum on the shape of triangle configurations in Fourier space. This allows one to determine <T_21> b_21(z) yet, importantly, is less sensitive to foreground contamination than the 21 cm auto-spectrum, and so can provide a valuable cross-check. We compare the results of simulated bispectra with second-order perturbation theory: on large scales the perturbative estimate of <T_21> b_21(z) matches the true value to within 10% for <x_i> <~ 0.8. We consider the 21 cm auto-bispectrum and show that this statistic may also be used to extract the 21 cm bias factor. Finally, we discuss the survey requirements for measuring the cross-bispectrum. Although we focus on the 21 cm-[CII]-[CII] bispectrum during reionization, our method may be of broader interest and can be applied to any two fields throughout cosmic history.
We quantify the prospects for using emission lines from rotational transitions of the CO molecule to perform an `intensity mapping observation at high redshift during the Epoch of Reionization (EoR). The aim of CO intensity mapping is to observe the combined CO emission from many unresolved galaxies, to measure the spatial fluctuations in this emission, and use this as a tracer of large scale structure at very early times in the history of our Universe. This measurement would help determine the properties of molecular clouds -- the sites of star formation -- in the very galaxies that reionize the Universe. We further consider the possibility of cross-correlating CO intensity maps with future observations of the redshifted 21 cm line. The cross spectrum is less sensitive to foreground contamination than the auto power spectra, and can therefore help confirm the high redshift origin of each signal. Furthermore, the cross spectrum measurement would help extract key information about the EoR, especially regarding the size distribution of ionized regions. We discuss uncertainties in predicting the CO signal at high redshift, and discuss strategies for improving these predictions. Under favorable assumptions, and feasible specifications for a CO survey mapping the CO(2-1) and CO(1-0) lines, the power spectrum of CO emission fluctuations and its cross spectrum with future 21 cm measurements from the MWA are detectable at high significance.
Measurements of the HI 21-cm power spectra from the reionization epoch will be influenced by the evolution of the signal along the line-of-sight direction of any observed volume. We use numerical as well as semi-numerical simulations of reionization in a cubic volume of 607 Mpc across to study this so-called light cone effect on the HI 21-cm power spectrum. We find that the light cone effect has the largest impact at two different stages of reionization: one when reionization is $sim 20%$ and other when it is $sim 80%$ completed. We find a factor of $sim 4$ amplification of the power spectrum at the largest scale available in our simulations. We do not find any significant anisotropy in the 21-cm power spectrum due to the light cone effect. We argue that for the power spectrum to become anisotropic, the light cone effect would have to make the ionized bubbles significantly elongated or compressed along the line-of-sight, which would require extreme reionization scenarios. We also calculate the two-point correlation functions parallel and perpendicular to the line-of-sight and find them to differ. Finally, we calculate an optimum frequency bandwidth below which the light cone effect can be neglected when extracting power spectra from observations. We find that if one is willing to accept a $10 %$ error due to the light cone effect, the optimum frequency bandwidth for $k= 0.056 , rm{Mpc}^{-1}$ is $sim 7.5$ MHz. For $k = 0.15$ and $0.41 , rm{Mpc}^{-1}$ the optimum bandwidth is $sim 11$ and $sim 16$ MHz respectively.
The 21-cm and Lyman Alpha lines are the dominant line-emission spectral features at opposite ends of the spectrum of hydrogen. Each line can be used to create three dimensional intensity maps of large scale structure. The sky brightness at low redshift due to Lyman Alpha emission is estimated to be 0.4 Jy/Steradian, which is brighter than the zodiacal light foreground.
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