Work

Relativistic Ionization Fronts

Year: 2006

Type: article

Abstract: We derive the equations for the propagation of relativistic ionization fronts (I-fronts) in both static and moving gases, including the cosmologically expanding intergalactic medium (IGM). For the supersonic R-type phase that occurs right after a source turns on, relativistic corrections can be significant up until the light crossing time of the equilibrium Strömgren sphere. When q , the ratio of this light-crossing time to the recombination time, exceeds unity, the time for the expanding I-front to reach the Strömgren radius is delayed by a factor of q . For a static medium, we obtain exact analytical solutions and apply them to the illustrative problems of an O star in a molecular cloud and a starburst in a high-redshift cosmological halo. Relativistic corrections can be important both at early times when the H II regions are small and at later times if a density gradient causes the I-front to accelerate. An analytical solution is also derived for a steady source in a cosmologically expanding IGM. Here relativistic corrections are significant for short-lived, highly luminous sources such as QSOs at the end of reionization ( z ≃ 6) but negligible for weaker or higher redshift sources. Finally, we numerically solve the equations for relativistic, cosmological I-fronts in the presence of evolving small-scale structure, for a large-galaxy starburst and a luminous QSO. For such strong and short-lived sources at z ~ 7, relativistic corrections are quite significant, and small-scale structure can decrease the size of the H II region by up to an additional ~25%. However, most of the IGM was ionized by smaller, higher redshift sources. Thus, the effect of relativistic corrections on global reionization is small and can usually be neglected.

Source: The Astrophysical Journal

DOI: 10.1086/506242

Authors: Paul R. Shapiro, Ilian T. Iliev, Marcelo A. Alvarez, Evan Scannapieco

Institutions: The University of Texas at Austin, University of Toronto, Canadian Institute for Theoretical Astrophysics, University of California, Santa Barbara, Kavli Institute for Theoretical Physics

Language: English

Topic: Galaxies: Formation, Evolution, Phenomena

Subfield: Astronomy and Astrophysics

Field: Physics and Astronomy

Domain: Physical Sciences

Open Access status: bronze

Funders: National Science Foundation, U.S. Department of Energy, National Aeronautics and Space Administration

Awards: Institute for Theoretical Physics, REIONIZATION OF THE UNIVERSE (ATP - 2000)