[Paper Review] Joint analysis constraints on the physics of the first galaxies with low frequency radio astronomy data

Harry T. J. Bevins, Stefan Heimersheim+6 more|arXiv (Cornell University)|Jan 9, 2023

Radio Astronomy Observations and Technology4 citations

TL;DR

This paper introduces a novel machine learning-based joint analysis of sky-averaged 21-cm data from SARAS3 (z ≈ 15–25) and 21-cm power spectrum upper limits from HERA (z ≈ 8 and 10), demonstrating that combining these datasets tightens constraints on early galaxy astrophysics. The joint analysis reduces the consistent parameter space to 64.9% of the explored theoretical range, disfavoring models with highly efficient radio emission (>32× present-day levels) and weak X-ray emission (<33× present-day levels) at 68% confidence.

ABSTRACT

The first billion years of cosmic history remains largely unobserved. We demonstrate, using a novel machine learning technique, how combining upper limits on the spatial fluctuations in the 21-cm signal with observations of the sky-averaged 21-cm signal from neutral hydrogen can improve our understanding of this epoch. By jointly analysing data from SARAS3 (redshift $z\approx15-25$) and limits from HERA ($z\approx8$ and $10$), we show that such a synergetic analysis provides tighter constraints on the astrophysics of galaxies 200 million years after the Big Bang than can be achieved with the individual data sets. Although our constraints are weak, this is the first time data from a sky-averaged 21-cm experiment and power spectrum experiment have been analysed together. In synergy, the two experiments leave only $64.9^{+0.3}_{-0.1}$% of the explored broad theoretical parameter space to be consistent with the joint data set, in comparison to $92.3^{+0.3}_{-0.1}$% for SARAS3 and $79.0^{+0.5}_{-0.2}$% for HERA alone. We use the joint analysis to constrain star formation efficiency, minimum halo mass for star formation, X-ray luminosity of early emitters and the radio luminosity of early galaxies. The joint analysis disfavours at 68% confidence a combination of galaxies with X-ray emission that is $\lesssim 33$ and radio emission that is $\gtrsim 32$ times as efficient as present day galaxies. We disfavour at $95$% confidence scenarios in which power spectra are $\geq126$ mK$^{2}$ at $z=25$ and the sky-averaged signals are $\leq-277$ mK.

Motivation & Objective

• To improve constraints on the astrophysics of the first galaxies during Cosmic Dawn and the Epoch of Reionization using multi-instrument 21-cm data.
• To develop and apply a novel machine learning technique for joint analysis of sky-averaged and power spectrum 21-cm signals.
• To quantify the synergy between global signal and fluctuation data in reducing uncertainty in early galaxy models.
• To test the consistency of theoretical models with current observational limits from SARAS3 and HERA.
• To establish a methodology for future multi-wavelength synergy in low-frequency radio cosmology.

Proposed method

• The authors use a semi-analytic emulator to compute the 21-cm global signal and power spectrum across a broad theoretical parameter space of early galaxy properties.
• They apply a Bayesian inference framework using the nested sampling algorithm Polychord to explore the posterior distribution of model parameters.
• The analysis combines SARAS3’s non-detection of the sky-averaged 21-cm signal with HERA’s upper limits on 21-cm power spectrum fluctuations.
• A machine learning-based emulator (globalemu) is used to efficiently compute the 21-cm signal across the parameter space, reducing computational cost.
• The joint likelihood is constructed by combining data from two distinct experiments with different redshift sensitivities (SARAS3 at z ≈ 15–25, HERA at z ≈ 8 and 10).
• The method interpolates between redshift ranges, enabling constraints across a wider redshift baseline than individual experiments.

Experimental results

Research questions

• RQ1 How does combining sky-averaged 21-cm signal data with 21-cm power spectrum data improve constraints on early galaxy astrophysics?
• RQ2 What fraction of the theoretical parameter space remains consistent with the joint data from SARAS3 and HERA?
• RQ3 Which combinations of radio and X-ray luminosities for early galaxies are disfavored by the joint data at 68% and 95% confidence?
• RQ4 Can a joint analysis reduce model degeneracies that limit individual experiments?
• RQ5 How does the synergy between SARAS3 and HERA compare to the constraints from MWA and LOFAR, which probe similar redshifts?

Key findings

• The joint analysis constrains only 64.9% of the explored theoretical parameter space to be consistent with the data, significantly tighter than the 92.3% from SARAS3 alone and 78.7% from HERA alone.
• At 68% confidence, the joint data disfavor models with radio emission efficiency more than 32 times that of present-day galaxies and X-ray emission less than 33 times that of present-day galaxies.
• At 95% confidence, the data disfavor scenarios with a 21-cm power spectrum ≥126 mK² at z=25 and a sky-averaged signal ≤−277 mK.
• The synergy between SARAS3 and HERA provides stronger constraints than either experiment alone, demonstrating the value of multi-instrument analysis.
• Adding MWA and LOFAR data to the analysis leads to negligible improvement due to weaker current limits compared to HERA.
• The study establishes a new methodology for joint analysis of global and fluctuation 21-cm data, which will be essential for upcoming experiments like REACH, PRIZM, and the SKA.