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Register a new userThe High-Redshift Neutral Hydrogen Signature of an Anisotropic Matter Power Spectrum
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An anisotropic power spectrum will have a clear signature in the 21cm radiation from high-redshift hydrogen. We calculate the expected power spectrum of the intensity fluctuations in neutral hydrogen from before the epoch of reionization, and predict the accuracy to which future experiments could constrain a quadrupole anisotropy in the power spectrum. We find that the Square Kilometer Array will have marginal detection abilities for this signal at z~17 if the process of reionization has not yet started; reionization could enhance the detectability substantially. Pushing to higher redshifts and higher sensitivity will allow highly precise (percent level) measurements of anisotropy.
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We study the effects of dark energy (DE) anisotropic stress on features of the matter power spectrum (PS). We employ the Parametrized Post-Friedmannian (PPF) formalism to emulate an effective DE, and model its anisotropic stress properties through a two-parameter equation that governs its overall amplitude ($g_0$) and transition scale ($c_g$). For the background cosmology, we consider different equations of state to model DE including a constant $w_0$ parameter, and models that provide thawing (CPL) and freezing (nCPL) behaviors. We first constrain these parameters by using the Pantheon, BAO, $H_0$ and CMB Planck data. Then, we analyze the role played by these parameters in the linear PS. In order for the anisotropic stress not to provoke deviations larger than $10%$ and $5%$ with respect to the $Lambda$CDM PS at $k sim 0.01 ,h/text{Mpc}$, the parameters have to be in the range $-0.30< g_0 < 0.32$, $0 leq c_g^2 < 0.01$ and $-0.15 < g_0 < 0.16$, $0 leq c_g^2 < 0.01$, respectively. Additionally, we compute the leading nonlinear corrections to the PS using standard perturbation theory in real and redshift space, showing that the differences with respect to the $Lambda$CDM are enhanced, especially for the quadrupole and hexadecapole RSD multipoles.
The most commonly used estimators of the anisotropic galaxy power spectrum employ Fast Fourier transforms, and rely on a specific choice of the line-of-sight that breaks the symmetry between the galaxy pair. This leads to wide-angle effects, including the presence of odd power spectrum multipoles like the dipole ($ell = 1$) and octopole ($ell = 3$). In Fourier-space these wide-angle effects also couple to the survey window function. We present a self-consistent framework extending the commonly used window function treatment to include the wide-angle effects. We show that our framework can successfully model the wide-angle effects in the BOSS DR12 dataset. We present estimators for the odd power spectrum multipoles and, detect these multipoles in BOSS DR12 with high significance. Understanding the impact of the wide-angle effects on the power spectrum multipoles is essential for many cosmological observables like primordial non-Gaussianity and the detection of General Relativistic effects and represents a potential systematic for measurements of Baryon Acoustic Oscillations and redshift-space distortions.
The spatial distribution of neutral hydrogen (HI) in the Universe contains a wealth of cosmological information. The 21 cm emission line can be used to map the HI up to very high redshift and therefore reveal us something about the evolution of the large scale structures in the Universe. However little is known about the abundance and clustering properties of the HI over cosmic time. Motivated by this, we build an analytic framework where the relevant parameters that govern how the HI is distributed among dark matter halos can be fixed using observations. At the same time we provide tools to study the column density distribution function of the HI absorbers together with their clustering properties. Our formalism is the first one able to account for all observations at a single redshift, $z = 2.3$. The linear bias of the HI and the mean number density of HI sources, two main ingredients in the calculation of the signal-to-noise ratio of a cosmological survey, are then discussed in detail, also extrapolating the results to low and high redshift. We find that HI bias is relatively higher than the value reported in similar studies, but the shot noise level is always sub dominant, making the HI Power Spectrum always a high signal-to-noise measurements up to $zsimeq5$ in the limit of no instrumental noise and foreground contamination.
Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions. This was estimated to imprint a large-scale fluctuation signal of about 2 mK in the 21-cm spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays and major enhancements of the suppression. A large velocity difference reduces the abundance of halos and requires the first stars to form in halos of about a million solar masses, substantially greater than previously expected. Here we report a simulation of the distribution of the first stars at z=20 (cosmic age of ~180 Myr), incorporating all these ingredients within a 400 Mpc box. We find that the 21-cm signature of these stars is an enhanced (10 mK) fluctuation signal on the 100-Mpc scale, characterized by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array or the Low Frequency Array but designed to operate in the range of 50-100 MHz.
Using a spectral stacking technique, we measure the neutral hydrogen (HI) properties of a sample of galaxies at $z < 0.11$ across 35 pointings of the Westerbork Synthesis Radio Telescope (WSRT). The radio data contains 1,895 galaxies with redshifts and positions known from the Sloan Digital Sky Survey (SDSS). We carefully quantified the effects of sample bias, aperture used to extract spectra, sidelobes and weighting technique and use our data to provide a new estimate for the cosmic HI mass density. We find a cosmic HI mass density of $Omega_{rm HI} = (4.02 pm 0.26)times 10^{-4} h_{70}^{-1}$ at $langle zrangle = 0.066$, consistent with measurements from blind HI surveys and other HI stacking experiments at low redshifts. The combination of the small interferometer beam size and the large survey volume makes our result highly robust against systematic effects due to confusion at small scales and cosmic variance at large scales. Splitting into three sub-samples with $langle zrangle$ = 0.038, 0.067 and 0.093 shows no significant evolution of the HI gas content at low redshift.
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