Reaching small scales with low frequency imaging: applications to the Dark Ages

TL;DR

This paper explores the potential of low-frequency radio imaging, specifically with LOFAR, to probe small-scale structures in the early Universe during the Dark Ages, highlighting current limitations in sensitivity.

Contribution

It assesses the feasibility of measuring the HI matter power spectrum at high redshifts with LOFAR, identifying the scale of $k$ that can be reached and the challenges involved.

Findings

LOFAR can achieve high resolution at 50 MHz.

Current sensitivity is about five orders of magnitude below the needed level.

Measuring small-scale structures during the Dark Ages remains challenging with existing technology.

Abstract

The initial conditions for the density perturbations in the early Universe, which dictate the large scale structure and distribution of galaxies we see today, are set during inflation. Measurements of primordial non-Gaussianity are crucial for distinguishing between different inflationary models. Current measurements of the matter power spectrum from the CMB only constrain this on scales up to $k\sim 0.1$ Mpc$^{-1}$. Reaching smaller angular scales (higher values of $k$) can provide new constraints on non-Gaussianity. A powerful way to do this is by measuring the $\textrm{H}\textrm{I}$ matter power spectrum at $z\gtrsim30$. In this paper, we investigate what values of $k$ can be reached for the LOw Frequency ARray (LOFAR), which can achieve $\lesssim$1$''$ $\,$resolution at $\sim$50 MHz. Combining this with a technique to isolate the spectrally smooth foregrounds to a wedge in $k_{\parallel}$ - $k_{\perp}$ space, we demonstrate what values of $k$ we can feasibly reach within observational constraints. We find that LOFAR is $\sim$5 orders of magnitude away from the desired sensitivity, for 10 years of integration time.