42 research outputs found
A Search for Oxygen in the Low-Density Lyman-alpha Forest Using the Sloan Digital Sky Survey
We use 2167 Sloan Digital Sky Survey (SDSS) quasar spectra to search for
low-density oxygen in the Intergalactic Medium (IGM). Oxygen absorption is
detected on a pixel-by-pixel basis by its correlation with Lyman-alpha forest
absorption. We have developed a novel Locally Calibrated Pixel (LCP) search
method that uses adjacent regions of the spectrum to calibrate interlopers and
spectral artifacts, which would otherwise limit the measurement of OVI
absorption. Despite the challenges presented by searching for weak OVI within
the Lyman-alpha forest in spectra of moderate resolution and signal-to-noise,
we find a highly significant detection of absorption by oxygen at 2.7 < z < 3.2
(the null hypothesis has a chi^2=80 for 9 data points).
We interpret our results using synthetic spectra generated from a lognormal
density field assuming a mixed quasar-galaxy photoionizing background (Haardt &
Madau 2001) and that it dominates the ionization fraction of detected OVI. The
LCP search data can be fit by a constant metallicity model with [O/H] =
-2.15_(-0.09)^(+0.07), but also by models in which low-density regions are
unenriched and higher density regions have a higher metallicity. The
density-dependent enrichment model by Aguirre et al. (2008) is also an
acceptable fit. All our successful models have similar mass-weighted oxygen
abundance, corresponding to [_MW] = -2.45+-0.06. This result can be used
to find the cosmic oxygen density in the Lyman-alpha forest, Omega_(Oxy, IGM) =
1.4(+-0.2)x10^(-6) = 3x10^(-4) Omega_b. This is the tightest constraint on the
mass-weighted mean oxygen abundance and the cosmic oxygen density in the
Lyman-alpha forest to date and indicates that it contains approximately 16% of
metals produced by star formation (Bouch\'e et al. 2008) up to z = 3.Comment: 12 pages, 9 figures. Accepted by ApJ (minor changes
The End of the Rainbow: What Can We Say About the Extragalactic Sub-Megahertz Radio Sky?
The Galactic disc is opaque to radio waves from extragalactic sources with
frequencies nu less than ~3 MHz. However, radio waves with kHz, Hz, and even
lower frequencies may propagate through the intergalactic medium (IGM). I argue
that the presence of these waves can be inferred by using the Universe as our
detector. I discuss possible sub-MHz sources and set new non-trivial upper
limits on the energy density of sub-MHz radio waves in galaxy clusters and the
average cosmic background. Limits based on five effects are considered: (1)
changes in the expansion of the Universe from the radiation energy density (2)
heating of the IGM by free-free absorption; (3) radiation pressure squeezing of
IGM clouds by external radio waves; (4) synchrotron heating of electrons in
clusters; and (5) Inverse Compton upscattering of sub-MHz radio photons. Any
sub-MHz background must have an energy density much smaller than the CMB at
frequencies below 1 MHz. The free-free absorption bounds from the Lyman-alpha
forest are potentially the strongest, but are highly dependent on the
properties of sub-MHz radio scattering in the IGM. I estimate an upper limit of
6 * 10^4 L_sun Mpc^-3 for the emissivity within Lyman-alpha forest clouds in
the frequency range 5 - 200 Hz. The sub-MHz energy density in the Coma cluster
is constrained to be less than ~10^-15 erg cm^-3. At present, none of the
limits is strong enough to rule out a maximal T_b = 10^12 K sub-MHz synchrotron
background, but other sources may be constrained with a better knowledge of
sub-MHz radio propagation in the IGM.Comment: Accepted by MNRAS, 4 figures, 20 pages
Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland
Recent Icelandic rifting events have illuminated the roles of centralized crustal magma reservoirs and lateral magma transport1,2,3,4, important characteristics of mid-ocean ridge magmatism1,5. A consequence of such shallow crustal processing of magmas4,5 is the overprinting of signatures that trace the origin, evolution and transport of melts in the uppermost mantle and lowermost crust6,7. Here we present unique insights into processes occurring in this zone from integrated petrologic and geochemical studies of the 2021 Fagradalsfjall eruption on the Reykjanes Peninsula in Iceland. Geochemical analyses of basalts erupted during the first 50 days of the eruption, combined with associated gas emissions, reveal direct sourcing from a near-Moho magma storage zone. Geochemical proxies, which signify different mantle compositions and melting conditions, changed at a rate unparalleled for individual basaltic eruptions globally. Initially, the erupted lava was dominated by melts sourced from the shallowest mantle but over the following three weeks became increasingly dominated by magmas generated at a greater depth. This exceptionally rapid trend in erupted compositions provides an unprecedented temporal record of magma mixing that filters the mantle signal, consistent with processing in near-Moho melt lenses containing 107–108 m3 of basaltic magma. Exposing previously inaccessible parts of this key magma processing zone to near-real-time investigations provides new insights into the timescales and operational mode of basaltic magma systems
Iron Behaving Badly: Inappropriate Iron Chelation as a Major Contributor to the Aetiology of Vascular and Other Progressive Inflammatory and Degenerative Diseases
The production of peroxide and superoxide is an inevitable consequence of
aerobic metabolism, and while these particular "reactive oxygen species" (ROSs)
can exhibit a number of biological effects, they are not of themselves
excessively reactive and thus they are not especially damaging at physiological
concentrations. However, their reactions with poorly liganded iron species can
lead to the catalytic production of the very reactive and dangerous hydroxyl
radical, which is exceptionally damaging, and a major cause of chronic
inflammation. We review the considerable and wide-ranging evidence for the
involvement of this combination of (su)peroxide and poorly liganded iron in a
large number of physiological and indeed pathological processes and
inflammatory disorders, especially those involving the progressive degradation
of cellular and organismal performance. These diseases share a great many
similarities and thus might be considered to have a common cause (i.e.
iron-catalysed free radical and especially hydroxyl radical generation). The
studies reviewed include those focused on a series of cardiovascular, metabolic
and neurological diseases, where iron can be found at the sites of plaques and
lesions, as well as studies showing the significance of iron to aging and
longevity. The effective chelation of iron by natural or synthetic ligands is
thus of major physiological (and potentially therapeutic) importance. As
systems properties, we need to recognise that physiological observables have
multiple molecular causes, and studying them in isolation leads to inconsistent
patterns of apparent causality when it is the simultaneous combination of
multiple factors that is responsible. This explains, for instance, the
decidedly mixed effects of antioxidants that have been observed, etc...Comment: 159 pages, including 9 Figs and 2184 reference