Harvard Astronomy 201b

The Void IGM at z < 6: Key Properties, How We Know

In Uncategorized on April 28, 2011 at 4:14 am

As Nathan described, the IGM transitioned from an HI dominated phase to an HII dominated phase at z \approx 6. 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\alpha 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 N_{HI} column density distribution, N_{HI} \approx 10^{13} \textrm{cm}^{-2} (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:

T_{HI} \sim 10^{4}, \ b_{T} \sim 10 \textrm{km/s} \gtrsim H \times N_{HI}/\overline{n}_{HI}

Where the last expression is a back of the envelope estimate of the redshift broadening (H being the Hubble parameter, \overline{n}_{HI} being a guess of the density based on the average value that accounts for all the baryons in \Omega_b).

Ionization Fraction

With a temperature estimate in hand, the ionization fraction can be calculated from radiative transfer, balancing ionization rate and collisional recombination rate (dependent on T through the radiative recombination coefficient \alpha(T)).

\dot{n}_{HII} = n_{HI}\int_{\nu_{0}}^{\infty}\frac{4\pi J_{\nu}\sigma_{H}(\nu)d\nu}{h\nu}, \ \sigma_{H} \sim \sigma_{0}(\nu/\nu_0)^{-3}

\dot{n}_{HI} = n_{HII}^2\alpha(T)

A crude estimate of the first integral can be obtained by knowing approximately the UV ionizing background, yielding t_{ion} \sim 5 \times 10^{12} \ s (see for example some simple calculations and general IGM discussion by Piero Madau). Again using the \overline{n}_{HI} consistent with \Omega_b and using \alpha(10^4 \textrm{K}), balancing the rates of ionization/recombination yields n_{HI}/n_{HII} \sim 10^{-4}.

More on line-of-sight observations

Virtually all information about the IGM at z < 6 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 \lambda=2795\textrm{\AA}, 2802\textrm{\AA}, and CIV (\lambda=1548\textrm{\AA},1551\textrm{\AA}). 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 (\textrm{cm}^{-2})
narrow Ly\alpha HI < 10^{17}
Lyman limit HI > 10^{17}, < 10^{20}
damped Ly\alpha HI > 10^{20}
metal CIV, MgII, etc. > 10^{17}, < 10^{21}

Figure 1: Example of a quasar spectrum, showing some common absorption features. The source is at redshift ~2. Figure from Schneider "Extragalactic Astronomy & Cosmology".

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 e^{+}/e^{-} 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 B_{IGM} are \sim 10^{-15}-10^{-17} Gauss…not too stringent of a lower bound!

The Warm Hot Intergalactic Medium

Simulations suggest that the z<6 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 T \sim 10^6 \ K conditions in filaments overdense relative to the cosmic average baryon density by a factor \delta \sim 5-50 (Fang et al. 2002, see Fig. 2).

Figure 2: Detection of Lyman-alpha analogue absorption from seven times ionized oxygen in the WHIM, from Fang et al. 2002.


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