He II REIONIZATION AND ITS EFFECT ON THE INTERGALACTIC MEDIUM

Abstract

Observations of the intergalactic medium (IGM) suggest that quasars reionize He ii in the IGM at z ≈ 3. We have run a set of 190 and 430 comoving Mpc simulations of He ii being reionized by quasars to develop an understanding of the nature of He ii reionization and its potential impact on observables. We find that He ii reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ∼ 12,000 K and with large temperature fluctuations on ∼ 50 Mpc scales. Much of this heating occurs far from quasars by photons with long mean free path. We find a temperature-density equation of state of γ − 1 ≈ 0.3 during He ii reionization, but with a wide dispersion in this relation having σ T ∼ 10 4 K. He ii reionization by the observed population of quasars cannot produce an inverted relation (γ − 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He ii Lyα forest. We argue that the heat input from He ii reionization is unable to cause the observed depression at z ≈ 3.2 in the H i Lyα forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of He ii Lyman limit systems influence our predictions.

On the intergalactic temperature–density relation

“…1). This relation is established ∆z ∼ 1 − 2 after the end of cosmological reionization processes (Hui & Gnedin 1997;Trac et al 2008;Furlanetto & Oh 2009;McQuinn et al 2009). Motivated by this limiting behavior, a power-law parameterization for the thermal state is adopted in almost all analyses of the Lyα forest (e.g., McDonald et al 2005;Viel et al 2006;Lidz et al 2010).…”

“…Equation (27) encompasses the one solved in Theuns et al (1998), being more general because we have not imposed dT /dt = 0. The approximation of dropping the d∆/dt terms is also made in the He ii reionization simulations of Compostella et al (2013), which may explain why their simulations resulted in somewhat smaller γ − 1 than the simulations of McQuinn et al (2009). baryon is 4000(2 + α bk ) −1 Z3 ∆ 0.6 K Gyr −1 , and it becomes 12, 000(2 + α bk ) −1 Z3 ∆ 0.6 K Gyr −1 after He ii reionization (Table 1).…”

The ESO UVES Advanced Data Products Quasar Sample – IV. On the deficiency of argon in DLA systems

“…Our new data sample includes a total of 8 Ar i measurements at z abs 3 (5 more than in Paper I), but only one of these lies at z abs > 4. Therefore, the redshift coverage of our sample is still marginal if one wants to follow the evolution of He ii reionisation, which apparently starts around z ≈ 6 (Bolton et al 2012) and is completed around z ≈ 3 (Dall' Aglio et al 2008;Syphers et al 2009;McQuinn et al 2009). At z 3 our statistics is relatively large and suggests that hard photons are able to ionise argon, with the exception of the most self-shielded cases mentioned above.…”

“…In the upper range of the DLA redshift interval the cosmic reionisation of He ii is expected to be completed (Furlanetto & Oh 2008;McQuinn et al 2009). When cosmic He iii regions become completely overlapped, the photons with hν > 54.4 eV should travel undisturbed through the IGM, boosting the efficiency of argon ionisation processes.…”

“…According to our current understanding of the reionisation history of the Universe (Fan et al 2006;McQuinn et al 2009;Jarosik et al 2011;Haardt & Madau 2012), hydrogen was reionised by the ultraviolet radiation of the first stars and quasars, starting at z 10; with the increasing overlap of IGM H ii regions, hydrogen reionisation was completed around z 6 , leaving the IGM completely transparent to photons with hν > 13.6 eV. During the early stages of H i reionisation, the ionising radiation field was sufficiently hard to ionise He i (IP = 24.6 eV), but not He ii (IP = 54.4 eV).…”