As Nathan described, the IGM transitioned from an HI dominated phase to an HII dominated phase at . Here I will go into a bit more detail about the ionization, temperature, density and magnetic field of the void ISM post-reionization, and how these properties are determined. The term “void” IGM is meant to exclude particularities of the intracluster medium, which is beyond the scope of this brief posting.
The temperature of the IGM is dictated by a balance of adiabatic cooling from the Hubble expansion and photoheating by the UV photons that keep the IGM ionized. One simple way to obtain the IGM temperature is to measure Doppler broadening of narrow Ly absorption lines along the quasar sight-lines. This is most appropriately done with a large sample of absorption lines (along various sight-lines) at the lowest end of the absorber column density distribution, (see e.g. Ricotti et al. 2000). These considerations minimize potential for inflated linewidth estimates due to redshift broadening arising from the physical extent of the HI overdensity system. The result obtained is:
Where the last expression is a back of the envelope estimate of the redshift broadening ( being the Hubble parameter, being a guess of the density based on the average value that accounts for all the baryons in ).
With a temperature estimate in hand, the ionization fraction can be calculated from radiative transfer, balancing ionization rate and collisional recombination rate (dependent on through the radiative recombination coefficient ).
A crude estimate of the first integral can be obtained by knowing approximately the UV ionizing background, yielding s (see for example some simple calculations and general IGM discussion by Piero Madau). Again using the consistent with and using , balancing the rates of ionization/recombination yields .
More on line-of-sight observations
Virtually all information about the IGM at is gleaned from line-of-sight observations, most frequently in the optical. Conclusive detection of metals generally requires identification of multiple lines, so that the intervening systems’ redshift can be determined. Doublets are particularly useful in this context (especially MgII , , and CIV (,). Various common absorber classifications are listed in Table 1 below. See Fig. 1 below for a sample quasar spectrum showing many of the features listed in Table 1 (spectrum from Schneider “Extragalactic Astronomy and Cosmology”).
|system classification||absorber||corresponding HI column density ()|
|metal||CIV, MgII, etc.||,|
IGM Magnetic Fields
Very little is currently known about magnetic fields in the IGM. Recently, lower bounds on the magnitude of the IGM magnetic field have been derived using line-of-sight blazar gamma-ray data (e.g. Tavecchio et al. 2010). The premise of these B-field limits is that conversion of gamma-rays emitted by the blazar to / pairs on their way to Earth is sensitive to the IGM B-field and can be observed in the high-energy blazar spectra. The present lower limits on are Gauss…not too stringent of a lower bound!
The Warm Hot Intergalactic Medium
Simulations suggest that the IGM has a large-scale filamentary structure (the “cosmic web“), and quasar line of sight data have provided evidence for this WHIM. By analyzing quasar spectra in the X-ray with Chandra, Prof. Julia Lee and collaborators identified OVIII absorption thought to trace hot K conditions in filaments overdense relative to the cosmic average baryon density by a factor (Fang et al. 2002, see Fig. 2).
- Fang et al. 2002
- Piero Madau’s “The Intergalactic Medium” introduction
- Ricotti et al. 2000
- Schneider “Extragalactic Astronomy and Cosmology”
- Tavecchio et al. 2010