A Survey for Missing Baryons in Highly Ionized Intergalactic Gas at Low Redshift

Mathematics – Logic

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Hst Proposal Id #9184 Quasar Absorption Lines And Igm

Scientific paper

Hydrodynamic cosmological simulations predict that at z 0, 30-50 of the baryons are in shock-heated intergalactic gas at 10^5 - 10^7 K. Gas with T <=q 10^6 K may be detected via O VI absorption lines in the spectrum of a background QSO. With STIS, we have recently discovered a surprising number of low redshift O VI absorption line systems in the directions of two QSOs, and the remarkably high number per unit redshift suggests that these absorbers are indeed a major baryon reservoir. However, the sample is very small. To substantially improve the sample size, we propose to observe six additional low-z QSOs with the STIS FUV E140M echelle mode {7 km s^-1 resolution}. Combined with archival data, this will increase the sample redshift path by a factor of 7 compared to the published data. With the echelle data, we will {1} measure the number of O VI absorbers per unit redshift {dN/dz} and their minimum cosmological mass density with a limiting equivalent width of W_Lambda 50 mAngstrom , {2} examine whether the O VI absorption arises in photoionized, collisionally ionized, or multiphase gas, and {3} study the dependence of the O VI system properties on environment. In addition to testing this prediction regarding the location of the missing baryons, the data will have applications to many other topics such as low-z LyAlpha absorbers and the physical properties and abundances of gas in the Milky Way halo.

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