PAPER-64 CONSTRAINTS ON REIONIZATION. II. THE TEMPERATURE OF THEz= 8.4 INTERGALACTIC MEDIUM

Abstract

We present constraints on both the kinetic temperature of the intergalactic medium (IGM) at z = 8.4, and on models for heating the IGM at high-redshift with X-ray emission from the first collapsed objects. These constraints are derived using a semi-analytic method to explore the new measurements of the 21 cm power spectrum from the Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER), which were presented in a companion paper, Ali et al. (2015). Twenty-one cm power spectra with amplitudes of hundreds of mK 2 can be generically produced if the kinetic temperature of the IGM is significantly below the temperature of the Cosmic Microwave Background (CMB); as such, the new results from PAPER place lower limits on the IGM temperature at z = 8.4. Allowing for the unknown ionization state of the IGM, our measurements find the IGM temperature to be above ≈ 5 K for neutral fractions between 10% and 85%, above ≈ 7 K for neutral fractions between 15% and 80%, or above ≈ 10 K for neutral fractions between 30% and 70%. We also calculate the heating of the IGM that would be provided by the observed high redshift galaxy population, and find that for most models, these galaxies are sufficient to bring the IGM temperature above our lower limits. However, there are significant ranges of parameter space that could produce a signal ruled out by the PAPER measurements; models with a steep drop-off in the star formation rate density at high redshifts or with relatively low values for the X-ray to star formation rate efficiency of high redshift galaxies are generally disfavored. The PAPER measurements are consistent with (but do not constrain) a hydrogen spin temperature above the CMB temperature, a situation which we find to be generally predicted if galaxies fainter than the current detection limits of optical/NIR surveys are included in calculations of X-ray heating.

PAPER-64 CONSTRAINTS ON REIONIZATION: THE 21 cm POWER SPECTRUM ATz= 8.4

“…The result is an upper limit on Δ 2 (k) of (22.4 mK) 2 in the range k h 0.15 0.5 Mpc 1 < < -at z = 8.4. This result places constraints on the spin temperature of the IGM, and as is shown in a companion paper, Pober et al (2015), this supports and extends previous evidence against extremely cold reionization scenarios. In Section 2 we describe the observations used in this analysis.…”

“…Assuming the midpoint of reionization lies close to z = 8.4 (a reasonable assumption given that Planck Collaboration et al 2015 suggests a midpoint of z = 8.8), both the bounds found in this paper and Pober et al (2015) show evidence for heating that places constraints on the possible values for the star formation rate/X-ray luminosity correlation given certain models of the star formation rate density redshift evolution. We refer the reader to Pober et al (2015) for a detailed examination of these results.…”

“…They compare the limits they find to the amount of heating possible given the currently observed star formation rates in high-redshift galaxy populations (Bouwens et al 2014;McLeod et al 2015) and assumptions about the relationship between star formation rates and X-ray luminosities (Furlanetto et al 2006;Pritchard & Loeb 2008;Fialkov et al 2014). Assuming the midpoint of reionization lies close to z = 8.4 (a reasonable assumption given that Planck Collaboration et al 2015 suggests a midpoint of z = 8.8), both the bounds found in this paper and Pober et al (2015) show evidence for heating that places constraints on the possible values for the star formation rate/X-ray luminosity correlation given certain models of the star formation rate density redshift evolution. We refer the reader to Pober et al (2015) for a detailed examination of these results.…”

“…This significantly improves over the previous best upper limit of (41 mK) 2 at z = 7.7 reported in P14. As we show below and in greater detail in Pober et al (2015), this limit begins to have implications for the heating of the IGM prior to the completion of reionization.…”

“…Pober et al (2015) improves on these results by using a simulation-based framework, rather than relying on coarse parametrizations of the power spectrum shape. They compare the limits they find to the amount of heating possible given the currently observed star formation rates in high-redshift galaxy populations (Bouwens et al 2014;McLeod et al 2015) and assumptions about the relationship between star formation rates and X-ray luminosities (Furlanetto et al 2006;Pritchard & Loeb 2008;Fialkov et al 2014).…”