The Low-Redshift Intergalactic Medium

J. Michael Shull , Steven V. Penton , John T. Stocke, PASA, 16 (1), 95
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The Low-Redshift Intergalactic Medium

J. Michael Shull 1,2
Steven V. Penton 1
John T. Stocke 1

1 Center for Astrophysics and Space Astronomy, Dept. of Astrophysical and Planetary Sciences, CB-389, University of Colorado, Boulder, CO 80309 (USA)

2 also at JILA, University of Colorado and NIST, Boulder, CO 80309, mshull@casa.colorado.edu

Abstract:

The low-redshift Ly$\alpha $ forest of absorption lines provides a probe of large-scale baryonic structures in the intergalactic medium, some of which may be remnants of physical conditions set up during the epoch of galaxy formation. We discuss our recent Hubble Space Telescope (HST) observations and interpretation of low-z Ly$\alpha $ clouds toward nearby Seyferts and QSOs, including their frequency, space density, estimated mass, association with galaxies, and contribution to $\Omega_b$. Our HST/GHRS detections of $\sim70$ Ly$\alpha $ absorbers with N

$_{\rm HI} \geq 10^{12.6}$ cm-2 along 11 sightlines covering pathlength

$\Delta(cz) = 114,000$ km s-1 show

$f(>N_{\rm HI}) \propto N_{\rm HI}^{-0.63 \pm 0.04}$ and a line frequency

$d {\cal N}/dz = 200 \pm 40$ for

NHI > 1012.6 cm-2 (one every 1500 km s-1 of redshift). A group of strong absorbers toward PKS 2155-304 may be associated with gas

(400-800)h75-1 kpc from 4 large galaxies, with low metallicity ($\leq0.003$ solar) and D/H

$\leq 2 \times 10^{-4}$. At low-z, we derive a metagalactic ionizing radiation field from AGN of

J0 = 1.3+0.8-0.5 x 10-23 ergs cm-2 s-1 Hz-1 sr-1 and a Ly$\alpha $-forest baryon density

$\Omega_b = (0.008 \pm 0.004) h_{75}^{-1} [J_{-23} N_{14} b_{100}]^{1/2}$ for clouds of characteristic size

$b = (100~{\rm kpc})b_{100}$.





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