The Low-Redshift Intergalactic Medium

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-baryon. Our HST/GHRS detections of 70 Ly-alpha absorbers with N_HI > 10^12.6 cm-2 along 11 sightlines covering pathlength Delta(cz) = 114,000 km/s show f(>N_HI) ~ N_HI^{-0.63 +- 0.04} and a line frequency dN/dz = 200 +- 40 for N_HI > 10^12.6 cm-2 (one every 1500 km/s of redshift). A group of strong absorbers toward PKS 2155-304 may be associated with gas (400-800) h_75^-1 kpc from 4 large galaxies, with low metallicity (< 0.003 solar) and D/H < 2 x 10^-4. At low-z, we derive a metagalactic ionizing radiation field from AGN of J_0 = 1.3^{+0.8 -0.5} x 10^-23 ergs/cm2/s/Hz/sr and a Ly-alpha-forest baryon density Omega-baryon = (0.008 +- 0.004) h_75^-1 [J_-23 N_14 b_100]^{1/2} For clouds of characteristic size b = (100 kpc)b_100.Comment: 5 figure

Constraint on intergalactic dust from thermal history of intergalactic medium

This Letter investigates the amount of dust in the intergalactic medium (IGM). The dust photoelectric heating can be the most efficient heating mechanism in the IGM where the density is very small and there are a lot of hard ultraviolet photons. Comparing the observational thermal history of IGM with a theoretical one taking into account the dust photoelectric heating, we can put an upper limit on the dust-to-gas ratio, ${\cal D}$, in the IGM. Since the rate of the dust photoelectric heating depends on the size of dust, we find the following results: If the grain size is \ga 100 \AA, ${\cal D}$ at $z \sim 3$ is \la 1/100 Galactic value corresponding to \Omega_{\rm dust}^{\rm IGM}\la 10^{-5}. On the other hand, if the grain size is as small as $\sim 10$ \AA, ${\cal D}$ is \la 1/1000 Galactic value corresponding to \Omega_{\rm dust}^{\rm IGM}\la 10^{-6}.Comment: 5 pages, 2 figures; accepted for publication in MNRAS pink page

Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars

Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound $B\ge 3\times 10^{-16}$~gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera-electron volt gamma-ray in intergalactic medium. The bound improves as $\lambda_B^{-1/2}$ if magnetic field correlation length, $\lambda_B$, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.Comment: 14 pages, 3 figure

Radiative transfer through the Intergalactic Medium

We use a probabilistic method to compute the propagation of an ionization front corresponding to the re-ionization of the intergalactic medium in a LCDM cosmology, including both hydrogen and helium. The effects of radiative transfer substantially boost the temperature of the ionized gas over the case of uniform re-ionization. The resulting temperature-density relation of the ionized gas is both non-monotonic and multiple-valued, reflecting the non-local character of radiative transfer and suggesting that a single polytropic relation between local gas density and temperatue is a poor description of the thermodynamic state of baryons in the post-reionization universe.Comment: 5 pages, 7 figures, letter accepted for publication in MNRA

Lower Metal Enrichment of Virialized Gas in Minihalos

We differentiate between the metal enrichment of the gas in virialized minihalos and that of the intergalactic medium at high redshift, pertinent to cosmological reionization, with the initial expectation that gas in the high density regions within formed dark matter halos may be more robust thus resistant to mixing with lower density intergalactic medium. Using detailed hydrodynamic simulations of gas clouds in minihalos subject to destructive processes associated with the encompassing intergalactic shocks carrying metal-enriched gas, we find, as an example, that, for realistic shocks of velocities of 10-100km/s, more than (90%,65%) of the high density gas with rho>500 rhob inside a minihalo virialized at z=10 of mass (10^7,10^6)Msun remains at a metallicity lower than 3% of that of the intergalactic medium by redshift z=6. It may be expected that the high density gas in minihalos becomes fuel for subsequent star formation, when they are incorporated into larger halos where efficient atomic cooling can induce gas condensation hence star formation. Since minihalos virialize at high redshift when the universe is not expected to have been significantly reionized, the implication is that gas in virialized minihalos may provide an abundant reservoir of primordial gas to possibly allow for the formation of Population-III metal-free stars to extend to much lower redshift than otherwise expected based on the enrichment of intergalactic medium.Comment: 22 pages, 7 figures, submitted to ApJ, comments welcom