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Register a new userQuantiles as Robust Probes of Non-Gaussianity in 21-cm Images
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Alon Banet
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The early epoch in which the first stars and galaxies formed is among the most exciting unexplored eras of the Universe. A major research effort focuses on probing this era with the 21-cm spectral line of hydrogen. While most research focused on statistics like the 21-cm power spectrum or the sky-averaged global signal, there are other ways to analyze tomographic 21-cm maps, which may lead to novel insights. We suggest statistics based on quantiles as a method to probe non-Gaussianities of the 21-cm signal. We show that they can be used in particular to probe the variance, skewness, and kurtosis of the temperature distribution, but are more flexible and robust than these standard statistics. We test these statistics on a range of possible astrophysical models, including different galactic halo masses, star-formation efficiencies, and spectra of the X-ray heating sources, plus an exotic model with an excess early radio background. Simulating data with angular resolution and thermal noise as expected for the Square Kilometre Array (SKA), we conclude that these statistics can be measured out to redshifts above 20 and offer a promising statistical method for probing early cosmic history.
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We investigate future constraints on primordial local-type non-Gaussianity from 21 cm angular power spectrum from minihalos. We particularly focus on the trispectrum of primordial curvature perturbations which are characterized by the non-linearity parameters $tau_{rm NL}$ and $g_{rm NL}$. We show that future measurements of minihalo 21 cm angular power spectrum can probe these non-linearity parameters with an unprecedented precision of $tau_{rm NL}sim30$ and $g_{rm NL}sim2times10^3$ for Square Kilometre Array (SKA) and $tau_{rm NL}sim0.6$ and $g_{rm NL}sim8times10^2$ for Fast Fourier Transform Telescope (FFTT). These levels of sensitivity would give significant implications for models of the inflationary Universe and the origin of cosmic density fluctuations.
We forecast ability of dedicated 21 cm intensity mapping experiments to constraint primordial non-Gaussianity using power spectrum and bispectrum. We model the signal including the non-linear biasing expansion using a generalized halo model approach. We consider the importance of foreground filtering scale and of the foreground wedge. We find that the current generation intensity mapping experiments like CHIME do not posses sufficient sensitivity to be competitive with the existing limits. On the other hand, upcoming experiments like HIRAX can improve the current constraints and the proposed PUMA experiment can substantially improve them, reaching sensitivities below $sigma (f_{rm NL})<5$ for equilateral and orthogonal configurations and $sigma( f_{rm NL}) < 1$ for the local shape if good foreground control is achieved.
Measuring the small primordial nonGaussianity (PNG) predicted by cosmic inflation theories may help diagnose them. The detectability of PNG by its imprint on the 21cm power spectrum from the epoch of reionization is reassessed here in terms of $f_{NL}$, the local nonlinearity parameter. We find that an optimum, multi-frequency observation by SKA can achieve $Delta f_{NL} sim 3$ (comparable to recent Planck CMB limits), while a cosmic-variance-limited array of this size like Omniscope can even detect $Delta f_{NL} sim 0.2$. This substantially revises the methods and results of previous work.
We introduce the numbers of hot and cold spots, $n_h$ and $n_c$, of excursion sets of the CMB temperature anisotropy maps as statistical observables that can discriminate different non-Gaussian models. We numerically compute them from simulations of non-Gaussian CMB temperature fluctuation maps. The first kind of non-Gaussian model we study is the local type primordial non-Gaussianity. The second kind of models have some specific form of the probability distribution function from which the temperature fluctuation value at each pixel is drawn, obtained using HEALPIX. We find the characteristic non-Gaussian deviation shapes of $n_h$ and $n_c$, which is distinct for each of the models under consideration. We further demonstrate that $n_h$ and $n_c$ carry additional information compared to the genus, which is just their linear combination, making them valuable additions to the Minkowski Functionals in constraining non-Gaussianity.
The 21-cm anisotropies from the neutral hydrogen distribution prior to the era of reionization is a sensitive probe of primordial non-Gaussianity. Unlike the case with cosmic microwave background, 21-cm anisotropies provide multi-redshift information with frequency selection and is not damped at arcminute angular scales. We discuss the angular trispectrum of the 21-cm background anisotropies and discuss how the trispectrum signal generated by the primordial non-Gaussianity can be measured with the three-to-one correlator and the corresponding angular power spectrum. We also discuss the separation of primordial non-Gaussian information in the trispectrum with that generated by the subsequent non-linear gravitational evolution of the density field. While with the angular bispectrum of 21-cm anisotropies one can limit the second order corrections to the primordial fluctuations below f_NL< 1, using the trispectrum information we suggest that the third order coupling term, f_2 or g_NL, can be constrained to be arounde 10 with future 21-cm observations over the redshift interval of 50 to 100.
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