Partially Cooled Shocks: Detectable Precursors in the Warm/Hot Intergalactic Medium

TL;DR

This paper models the ionization and cooling processes in partially cooled shocks within the warm/hot intergalactic medium, predicting observable ion signatures that can help locate missing baryons.

Contribution

It introduces a comprehensive numerical framework for simulating shock evolution, ionization states, and precursor signatures in the intergalactic medium, aiding in baryon detection.

Findings

Observable ion signatures in shock precursors suggest a method to detect missing baryons.

Age-dependent ion column densities are provided for various elements and shock conditions.

Predicted absorption-line diagnostics can distinguish photoionized precursor gas.

Abstract

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 warm/hot intergalactic medium (WHIM) which is expected to be a major baryonic reservoir, and contain a large fraction of the so-called "missing baryons". My computations indicate that readily observable amounts of intermediate and high ions, such as CIV, NV, and OVI 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.