Improved Constraints on the 21 cm EoR Power Spectrum and the X-Ray Heating of the IGM with HERA Phase I Observations

Abstract

We report the most sensitive upper limits to date on the 21 cm epoch of reionization power spectrum using 94 nights of observing with Phase I of the Hydrogen Epoch of Reionization Array (HERA). Using similar analysis techniques as in previously reported limits, we find at 95% confidence that Δ2(k = 0.34 h Mpc−1) ≤ 457 mK2 at z = 7.9 and that Δ2(k = 0.36 h Mpc−1) ≤ 3496 mK2 at z = 10.4, an improvement by a factor of 2.1 and 2.6, respectively. These limits are mostly consistent with thermal noise over a wide range of k after our data quality cuts, despite performing a relatively conservative analysis designed to minimize signal loss. Our results are validated with both statistical tests on the data and end-to-end pipeline simulations. We also report updated constraints on the astrophysics of reionization and the cosmic dawn. Using multiple independent modeling and inference techniques previously employed by HERA Collaboration, we find that the intergalactic medium must have been heated above the adiabatic cooling limit at least as early as z = 10.4, ruling out a broad set of so-called “cold reionization” scenarios. If this heating is due to high-mass X-ray binaries during the cosmic dawn, as is generally believed, our result’s 99% credible interval excludes the local relationship between soft X-ray luminosity and star formation and thus requires heating driven by evolved low-metallicity stars.

The Star Formation Efficiency during Reionization as Inferred from the Hubble Frontier Fields

“…The discrete points in the figure show that currently observable galaxies should easily yield WF coupling at z ∼ 7.9 and that coupling at z ∼ 10.4 requires only a modest extrapolation in redshift and luminosity beyond those of current UVLF measurements. This further validates the bounds in Abdurashidova et al (2023), although we caution that the average WF coupling considered here provides only a loose figure of merit.…”

“…The symbols marked "Observed," without extrapolation toward low halo mass, are relevant for the interpretation of current 21 cm fluctuation measurements. For example, the HERA collaboration recently used upper limits on the 21 cm fluctuation power spectrum at z = 7.9, and z = 10.4 to place a lower bound on the amount of X-ray heating at these redshifts (Abdurashidova et al 2023). That is, a cold IGM might lead to larger 21 cm fluctuations than observed.…”

21cm signal sensitivity to dark matter decay

“…HERA has completed deployment [29] and is currently analysing data from an extended observational campaign. An initial observational result performed with 71 antennas (out of the total 331) and only 94 nights of measurement has already provided the most constraining upper bounds on the 21 cm power spectrum at redshifts z = 8 and 10 [30]. Combined with complementary observations of galaxy UV luminosity functions and the timing of the EoR, these upper limits imply significant early IGM heating [30].…”

“…An initial observational result performed with 71 antennas (out of the total 331) and only 94 nights of measurement has already provided the most constraining upper bounds on the 21 cm power spectrum at redshifts z = 8 and 10 [30]. Combined with complementary observations of galaxy UV luminosity functions and the timing of the EoR, these upper limits imply significant early IGM heating [30]. If this heating was provided by high mass X-ray binary stars in CD galaxies, expected to dominate the X-ray background at high redshifts (e.g., [31]), they would need to be considerably brighter than those observed today (perhaps because they were born in extremely metal poor environments; see e.g., [32]).…”

“…Local, star-forming galaxies typically have L X ∼10 39.5 [62], though CD galaxies due to their lower metallicity ISM are expected to have higher values of L X (e.g., [63][64][65]). We set log motivated by recent analyses of the HERA observation [30,66]. The consequences of a larger value log 10 L II X = 41 are discussed in appendix C. For illustration, figure 1 shows the 21cm signal for three values of L II X considering a single population of galaxies.…”