Partially Cooled Shocks:
Detectable Precursors in the Warm/Hot Intergalactic Medium
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(Gnat O.,  2010,  ApJ submitted)

The submitted manuscript is available here.
Electronic Table 2 (post-shock columns) is available here.
Electronic Table 4 (precursor columns) is available here.
 
      It is a remarkable fact that in the present day universe more than half of the baryonic matter is missing. Hydrodynamical simulations of structure formation suggest that these baryons may reside in a "warm-hot intergalactic medium" with temperatures in the range 105-107 K. The WHIM is produced by the shock waves that occur as gas falls from the diffuse intergalactic medium into the dense regions where galaxies form. Recent observations have confirmed the existence of warm/hot gas both in the local Universe and in more distant environments. However, it remains to be determined what fraction of the missing baryonic matter they harbor, and whether they confirm the theoretical predictions regarding the warm-hot intergalactic medium.

In this paper I present computations of the integrated column densities produced in the post-shock cooling layers and in the radiative precursors of partially-cooled fast shocks as a function of the shock age. The results are applicable to the shock-heated WHIM. My computations indicate that readily observable amounts of intermediate and high ions, such as C IV, N V, and O VI are created in the precursors of young shocks, for which the shocked gas remains hot and difficult to observe. I suggest that such precursors may provide a way to identify and estimate the ”missing” baryonic mass associated with the shocks. The absorption-line signatures predicted here may be used to construct ion-ratio diagrams, which will serve as diagnostics for the photoionized gas in the precursors.

In my numerical models, the time-evolution of the shock structure, self-radiation, and associated metal-ion column densities are computed by a series of quasi-static models, each appropriate for a different shock age. The shock code used in this work calculates the nonequilibrium ionization and cooling, follows the radiative transfer of the shock self-radiation through the post-shock cooling layers, takes into account the resulting photoionization and heating rates, follows the dynamics of the cooling gas, and self-consistently computes the photoionization states in the precursor gas. I present a complete set of the age-dependent post-shock and precursor columns for all ionization states of the
elements H, He, C, N, O, Ne, Mg, Si, S, and Fe, as functions of the shock velocity, gas metallicity, and magnetic field. I present my numerical results in convenient online tables.


This plot demonstrates the evolution of the post-shock and precursor column densities as a function of shock age. The post-shock cooling layers of young shocks do not produce easily observable ions (and hence the missing baryons problem). However, their radiative precursors may be readily observable even is the shocked gas is not.
Postshock and precursor column densities (cm-2) as a function of shock age (n0 x t) for C IV, N V, O VI, and Ne VIII in a T=5x106 K, Z=1 solar shock in the strong-B limit.

To infer that a certain intergalactic absorber is, in fact, related to a young fast radiative shock, diagnostic ratio diagrams for the (postshock and) precursor gas may be constructed to compare with observations:


Column density ratios NC IV/NO VI vs. NN V/NO VI in strong-B shocks with T=5x106 K and Z=1 solar. Shock age is indicated by color along the trajectories, from young (blue) to old (red). Age (cm-3 s) vs. color legend is on the right. The left panel is for the shocked gas, and the right panel is for the radiative precursor. The trajectories are only shown for ages at which the O VI column is greater than 1011 cm-2.