Detection of extended He ii reionization in the temperature evolution of the intergalactic medium★

Abstract

We present new measurements of the temperature of the intergalactic medium (IGM) derived from the Lyα forest over 2.0 ≤ z ≤ 4.8. The small-scale structure in the forest of 61 highresolution quasi-stellar object spectra is quantified using a new statistic, the curvature, and the conversion to temperature calibrated using a suite of hydrodynamic simulations. At each redshift, we focus on obtaining the temperature at an optimal overdensity probed by the Lyα forest, T (¯ ), where the temperature is nearly a one-to-one function of the curvature regardless of the slope of the temperature-density relation. The median 2σ statistical uncertainty in these measurements is 8 per cent, though there may be comparable systematic errors due to the unknown amount of Jeans smoothing in the IGM. We use our T (¯ ) results to infer the temperature at the mean density, T 0 . Even for a maximally steep temperature-density relation, T 0 must increase from ∼8000 K at z 4.4 to 12 000 K at z 2.8. This increase is not consistent with the monotonic decline in T 0 expected in the absence of He II reionization. We therefore interpret the observed rise in temperature as evidence of He II reionization beginning at z 4.4. The evolution of T 0 is consistent with an end to He II reionization at z ∼ 3, as suggested by opacity measurements of the He II Lyα forest, although the redshift at which T 0 peaks will depend somewhat on the evolution of the temperature-density relation. These new temperature measurements suggest that the heat input due to the reionization of He II dominates the thermal balance of the IGM over an extended period with z 1.

On the intergalactic temperature–density relation

“…Compton cooling and recombination cooling are smaller but never negligible, with Compton cooling being quite important for shaping T0. Figure 3 is the same as Figure 2 except that it considers the case starting with T0 = 15, 000 K and γ − 1 = 0 at z = 3.5, values more consistent with the IGM after He ii was reionized (McQuinn et al 2009;Becker et al 2011), and it takes the applicable rates for after He ii reionization. In this case, Compton cooling is less important and neglecting nonadiabatic cooling processes is a much better approximation.…”

“…history (Becker et al 2011). The trajectory of a fluid element is clockwise in this plot, with nH → ∞ as z → z + coll .…”

Measuring the equation of state of the high-z intergalactic medium using curvature statistics

“…It is clear from the top panel that the curvature distribution is skewed, the mean value is always higher than the median. The mean curvature was shown to follow a tight relationship with the gas temperature at a characteristic overdensity (Becker et al 2011;Boera et al 2014). In the present work, we explore the possibility of using the inferred characteristic overdensities at different percentiles of the curvature distribution to simultaneously constrain T0 and γ.…”

“…A significant contribution to the rise in T (∆) comes from the increase of∆ itself with decreasing redshifts. However, in order to obtain T0 from T (∆), one has to assume the value of γ (assumed to be in the range 1.3 -1.5 in Becker et al 2011). Thus, any uncertainty in γ will propagate into the uncertainty in the derived value of T0.…”

“…Recently, the temperature of the IGM over redshifts 4.5 to ∼ 2.8 has been measured to a high precision ( 10%) by Becker et al (2011) (and extended upto z ∼ 1.5 by Boera et al 2014) using the curvature statistic, which also avoids the fitting of individual spectral lines and can be used to detect the additional heating effect in high-redshift quasar near-zones (Padmanabhan, Choudhury & Srianand 2014, hereafter Paper I). The curvature is normally sensitive to both T0 and γ, however, it is found that at a characteristic overdensity∆, it becomes more sensitive to T (∆) and fairly independent of γ.…”