Absorption signatures of warm-hot gas at low redshift: Broad Lyman-Alpha Absorbers

Details Absorption signatures of warm-hot gas at low redshift: Broad Lyman-Alpha Absorbers Absorption signatures of warm-hot gas at low redshift: Broad Lyman-Alpha Absorbers

2012-01-26

arxiv.org/abs/1201.5641v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

25 pages (20 pages main text), 19 figures; submitted to MNRAS; comments are very welcome

Scientific paper

We investigate the physical state of HI absorbing gas at low redshift (z=0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS project, focusing in particular on broad (b_HI > 40 km/s) Lyman-Alpha absorbers (BLAs), which are believed to originate in shock-heated gas in the Warm-Hot Intergalactic Medium (WHIM). Our fiducial model, which includes radiative cooling by heavy elements and feedback by supernovae and active galactic nuclei, predicts that by z=0.25 nearly 60 per cent of the gas mass ends up at densities and temperatures characteristic of the WHIM and we find that half of this fraction is due to outflows. The standard HI observables (distribution of HI column densities N_HI, distribution of Doppler parameters b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our simulations are in remarkably good agreement with observations. BLAs arise in gas that is hotter, more highly ionised and more enriched than the gas giving rise to typical Lyman-Alpha forest absorbers. Although the majority of the BLAs arise in warm-hot (log(T/K) ~ 5) gas at low (log Delta < 1.5) densities, their line width correlates only weakly with the gas temperature, and is thus a poor indicator of the thermal state of the gas. Detectable BLAs account for only a small fraction of the true baryon content of the WHIM at low redshift. In order to detect the bulk of the mass in this gas phase, a sensitivity at least one order of magnitude better than achieved by current UV spectrographs is required. We argue that BLAs mostly trace gas that has been shock-heated and enriched by outflows and that they therefore provide an important window on a poorly understood feedback process.

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