Interpreting LOFAR 21-cm signal upper limits at z~9.1 in the context of high-z galaxy and reionisation observations

TL;DR

This paper uses LOFAR 21-cm signal upper limits at z~9.1 to constrain high-redshift galaxy properties and reionisation, demonstrating the effectiveness of Bayesian forward modelling in combining 21-cm data with other observations.

Contribution

It introduces a Bayesian framework using 21CMMC and 21cmFAST to jointly analyze 21-cm upper limits and other high-z observations, constraining galaxy and IGM properties.

Findings

Models exceeding LOFAR limits are excluded by other probes.

Limits on IGM spin temperature are set at 2.6 K for certain neutral fractions.

Current data already constrain some astrophysical parameters.

Abstract

Using the latest upper limits on the 21-cm power spectrum at $z\approx9.1$ from the Low Frequency Array (LOFAR), we explore regions of parameter space which are inconsistent with the data. We use 21CMMC, a Monte Carlo Markov Chain sampler of 21cmFAST which directly forward models the 3D cosmic 21-cm signal in a fully Bayesian framework. We use the astrophysical parameterisation from 21cmFAST, which includes mass-dependent star formation rates and ionising escape fractions as well as soft-band X-ray luminosities to place limits on the properties of the high-$z$ galaxies. Further, we connect the disfavoured regions of parameter space with existing observational constraints on the Epoch of Reionisation such as ultra-violet (UV) luminosity functions, background UV photoionisation rate, intergalactic medium (IGM) neutral fraction and the electron scattering optical depth. We find that all models exceeding the 21-cm signal limits set by LOFAR at $z\approx9.1$ are excluded at $\gtrsim2\sigma$ by other probes. Finally, we place limits on the IGM spin temperature from LOFAR, disfavouring at 95 per cent confidence spin temperatures below $\sim2.6$ K across an IGM neutral fraction range of $0.15 \lesssim \bar{x}_{H{\scriptscriptstyle I}} \lesssim 0.6$. Note, these limits are only obtained from 141 hrs of data in a single redshift bin. With tighter upper limits, across multiple redshift bins expected in the near future from LOFAR, more viable models will be ruled out. Our approach demonstrates the potential of forward modelling tools such as 21CMMC in combining 21-cm observations with other high-$z$ probes to constrain the astrophysics of galaxies.