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On the Physical Origin of OVI Absorption-Line Systems | | T.M. Heckman
; C.A. Norman
; D.K. Strickland
; & K. R. Sembach
; | | Date: |
31 May 2002 | | Journal: | Astrophys.J. 577 (2002) 691-700 | | Subject: | astro-ph | | Abstract: | We present a unified analysis of the O{sc vi} absorption-lines seen in the disk and halo of the Milky Way, high velocity clouds, the Magellanic Clouds, starburst galaxies, and the intergalactic medium. We show that these disparate systems define a simple relationship between the O{sc vi} column density and absorption-line width that is independent of the Oxygen abundance over the range O/H $sim$ 10% to twice solar. We show that this relation is exactly that predicted theoretically as a radiatively cooling flow of hot gas passes through the coronal temperature regime - independent of its density or metallicity (for O/H $gtrsim$ 0.1 solar). Since most of the intregalactic O{sc vi} clouds obey this relation, we infer that they can not have metallicities less than a few percent solar. In order to be able to cool radiatively in less than a Hubble time, the intergalactic clouds must be smaller than $sim$1 Mpc in size. We show that the cooling column densities for the O{sc iv}, O{sc v}, Ne{sc v}, and Ne{sc vi} ions are comparable to those seen in O{sc vi}. This is also true for the Li-like ions Ne{sc viii}, Mg{sc x}, and Si{sc xii} (if the gas is cooling from $T gtrsim 10^6$ K). All these ions have strong resonance lines in the extreme-ultraviolet spectral range, and would be accessible to $FUSE$ at $z gtrsim$ 0.2 to 0.8. We also show that the Li-like ions can be used to probe radiatively cooling gas at temperatures an order-of-magnitude higher than where their ionic fraction peaks. We calculate that the H-like (He-like) O, Ne, Mg, Si, and S ions have cooling columns of $sim10^{17}$ cm$^{-2}$. The O{sc vii}, O{sc viii}, and Ne{sc ix} X-ray absorption-lines towards PKS 2155-304 may arise in radiatively cooling gas in the Galactic disk or halo. | | Source: | arXiv, astro-ph/0205556 | | Services: | Forum | Review | PDF | Favorites | |
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