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Register a new userAntisymmetric Cross-correlation between H I and CO Line Intensity Maps as a New Probe of Cosmic Reionization
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Intensity mapping of the H I 21 cm line and the CO 2.61 mm line from the epoch of reionization has emerged as powerful, complementary, probes of the high-redshift Universe. However, both maps and their cross-correlation are dominated by foregrounds. We propose a new analysis by which the signal is unbiased by foregrounds, i.e. it can be measured without foreground mitigation. We construct the antisymmetric part of two-point cross-correlation between intensity maps of the H I 21 cm line and the CO 2.61 mm line, arising because the statistical fluctuations of two fields have different evolution in time. We show that the sign of this new signal can distinguish model-independently whether inside-out reionization happens during some interval of time. More importantly, within the framework of the excursion set model of reionization, we demonstrate that the slope of the dipole of H I-CO cross-power spectrum at large scales is linear to the rate of change of global neutral fraction of hydrogen in a manner independent of reionization parameters, until the slope levels out near the end of reionization, but this trend might possibly depend on the framework of reionization modelling. The H I-CO dipole may be a smoking-gun probe for the speed of reionization, or standard speedometer for cosmic reionization. Observations of this new signal will unveil the global reionization history from the midpoint to near the completion of reionization.
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The cross-correlation between fluctuations in the electron scattering optical depth $tau_{rm es}$ as probed by future Cosmic Microwave Background (CMB) experiments, and fluctuations in the 21cm differential brightness temperature $Delta T_{rm 21cm}$ as probed by ground-based radio interferometers, will trace the reionization history of the Universe. In particular, the $tau_{rm es}-$21cm cross-correlation should yield a determination of the characteristic bubble size distribution and ionization fraction as a function of redshift. When assuming that the cross-correlation signal is limited by instrumental noise rather than by foregrounds, we estimate its potential detectability by upcoming experiments. Specifically, the combination of HERA and Simons Observatory, CMB-S4 and PICO should yield a signal-to-noise ratio around 3 - 6, while and the exploitation of the SKA should increase it to 10-20. Finally, we have discussed how such levels of detectability can be affected when (simply modeled) 21cm foregrounds are present. For the most promising PICO$times$SKA configuration, an efficiency of foreground removal to a level of $7times 10^{-4}$ is needed to achieve a $5sigma$ detection of the cross-correlation signal; in addition, safe avoidance of foreground contamination in the line-of-sight Fourier modes above $0.03 ,h,rm Mpc^{-1}$ would guarantee a detection significance around $3sigma$.
Current data indicate that the reionization of the Universe was complete by redshift z~6-7, and while the sources responsible for this process have yet to be identified, star-forming galaxies are often considered the most likely candidates. However, the contribution from z>6 galaxies to cosmic reionization critically depends on the fraction of ionizing (Lyman continuum, LyC) photons escaping from these objects and into the intergalactic medium. At z<4, the escaping LyC flux can be measured directly, but the opacity of the neutral intergalactic medium precludes such measurements at higher redshifts. In a recent paper, we argue that since the LyC escape fraction regulates the contribution of nebular emission to the rest-frame optical/UV spectra of galaxies, the James Webb Space Telescope should be able to indirectly assess the LyC escape fraction for galaxies at z~6-9. JWST can, on the other hand, not constrain the fraction of LyC photons directly absorbed by dust, and this is where SPICA comes in. The dust continuum emission from gravitationally lensed LyC-leakers at z=6 may in principle be detectable with SPICA, thereby constraining the level of LyC extinction in these objects.
Current and future generations of intensity mapping surveys promise dramatic improvements in our understanding of galaxy evolution and large-scale structure. An intensity map provides a census of the cumulative emission from all galaxies in a given region and redshift, including faint objects that are undetectable individually. Furthermore, cross-correlations between line intensity maps and galaxy redshift surveys are sensitive to the line intensity and clustering bias without the limitation of cosmic variance. Using the Fisher information matrix, we derive simple expressions describing sensitivities to the intensity and bias obtainable for cross-correlation surveys, focusing on cosmic variance evasion. Based on these expressions, we conclude that the optimal sensitivity is obtained by matching the survey depth, defined by the ratio of the clustering power spectrum to noise in a given mode, between the two surveys. We find that mid- to far-infrared space telescopes could benefit from this technique by cross-correlating with coming galaxy redshift surveys such as those planned for the Nancy Grace Roman Space Telescope, allowing for sensitivities beyond the cosmic variance limit. Our techniques can therefore be applied to survey design and requirements development to maximize the sensitivities of future intensity mapping experiments to tracers of galaxy evolution and large-scale structure cosmology.
Intensity mapping of the HI 21 cm line and the CO 2.61 mm line from the epoch of reionization has emerged as powerful, complementary, probes of the high-redshift Universe. However, both maps and their cross-correlation are dominated by foregrounds. We propose a new analysis by which the signal is unbiased by foregrounds, i.e. it can be measured without foreground mitigation. We construct the antisymmetric part of the HI-CO cross-correlation, arising because the statistical fluctuations of two fields have different evolution in time. We show that the sign of this new signal can distinguish model-independently whether inside-out reionization happens during some interval of time.
Following the first two annual intensity mapping workshops at Stanford in March 2016 and Johns Hopkins in June 2017, we report on the recent advances in theory, instrumentation and observation that were presented in these meetings and some of the opportunities and challenges that were identified looking forward. With preliminary detections of CO, [CII], Lya and low-redshift 21cm, and a host of experiments set to go online in the next few years, the field is rapidly progressing on all fronts, with great anticipation for a flood of new exciting results. This current snapshot provides an efficient reference for experts in related fields and a useful resource for nonspecialists. We begin by introducing the concept of line-intensity mapping and then discuss the broad array of science goals that will be enabled, ranging from the history of star formation, reionization and galaxy evolution to measuring baryon acoustic oscillations at high redshift and constraining theories of dark matter, modified gravity and dark energy. After reviewing the first detections reported to date, we survey the experimental landscape, presenting the parameters and capabilities of relevant instruments such as COMAP, mmIMe, AIM-CO, CCAT-p, TIME, CONCERTO, CHIME, HIRAX, HERA, STARFIRE, MeerKAT/SKA and SPHEREx. Finally, we describe recent theoretical advances: different approaches to modeling line luminosity functions, several techniques to separate the desired signal from foregrounds, statistical methods to analyze the data, and frameworks to generate realistic intensity map simulations.
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