21 cm observation of large-scale structures atz ~  1

Abstract

Context. Large-scale structures (LSS) in the universe can be traced using the neutral atomic hydrogen H I through its 21 cm emission. Such a 3D matter distribution map can be used to test the cosmological model and to constrain the dark energy properties or its equation of state. A novel approach, called intensity mapping, can be used to map the H I distribution, using radio interferometers with a large instantaneous field of view and waveband. Aims. We study the sensitivity of different radio interferometer configurations, or multi-beam instruments for observing LSS and baryon acoustic oscillations (BAO) in 21 cm, and we discuss the problem of foreground removal. Methods. For each configuration, we determined instrument response by computing the (u, v) or Fourier angular frequency plane coverage using visibilities. The (u, v) plane response determines the noise power spectrum, hence the instrument sensitivity for LSS P(k) measurement. We also describe a simple foreground subtraction method of separating LSS 21 cm signal from the foreground due to the galactic synchrotron and radio source emission. Results. We have computed the noise power spectrum for different instrument configurations, as well as the extracted LSS power spectrum, after separating the 21 cm-LSS signal from the foregrounds. We have also obtained the uncertainties on the dark energy parameters for an optimized 21 cm BAO survey. Conclusions. We show that a radio instrument with few hundred simultaneous beams and a collecting area of ∼10 000 m 2 will be able to detect BAO signal at redshift z ∼ 1 and will be competitive with optical surveys.

Blind foreground subtraction for intensity mapping experiments

“…After reionisation, most of the neutral hydrogen in the Universe is thought to reside in high density regions inside galaxies, in what are known as damped Lyman-α systems, where it is shielded from the ionising UV photons. Unfortunately, the HI emission from individual galaxies is usually too weak to be measured efficiently, and thus a technique known as intensity mapping has been advocated (Battye et al 2004;McQuinn et al 2006;Chang et al 2008;Mao et al 2008;Pritchard & Loeb 2008;Peterson et al 2009;Bagla et al 2010;Seo et al 2010;Lidz et al 2011;Ansari et al 2012;Battye et al 2013;Barkana & Loeb 2005;Masui et al 2013;Switzer et al 2013;Bull et al 2014), in which the combined emission from all sources in wide angular pixels is observed instead. While small angular scales are lost through this method, it is possible to retain the larger scales of most cosmological relevance, such as the baryon acoustic oscillation (BAO) scale.…”

Weak gravitational lensing with the Square Kilometre Array

“…HI intensity mapping is a technique that has been proposed for measuring the distribution of HI gas before and during reionization (see Pritchard et al in this volume) and measuring the BAO at redshifts of order unity (Chang et al 2008(Chang et al , 2010Seo et al 2010;Masui et al 2010;Ansari et al 2012;Battye et al 2013;Chen 2012;Pober et al 2013). In this technique, no attempt is made to detect individual objects.…”

21cm Cosmology

“…The result is a map of large-scale fluctuations in 21cm intensity, similar to a CMB map, except now the signal is also a function of redshift. Combined with the high frequency (and thus redshift) resolution of modern radio telescopes, this intensity mapping (IM) methodology makes it possible to efficiently survey extremely large volumes (Battye et al 2004;McQuinn et al 2006;Chang et al 2008;Mao et al 2008;Pritchard & Loeb 2008;Peterson et al 2009;Bagla et al 2010;Seo et al 2010;Lidz et al 2011;Ansari et al 2012;Battye et al 2013). By not requiring galaxy detections, the intensity mapping technique transfers the problem to one of foreground removal: how to develop cleaning methods to remove everything that is not the HI 21cm signal at a given frequency.…”