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Vincent Desjacques Adi Nusser Ravi K. Sheth 《Monthly notices of the Royal Astronomical Society》2007,374(1):206-219
We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lyman α (Lyα) forest from a sample of 3492 quasars included in the Sloan Digital Sky Survey data release 3 (SDSS DR3). Our intention is to investigate the sensitivity of the Lyα flux PDF as measured from low-resolution and low signal-to-noise ratio data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. The quasar continuum is described by the superposition of a power law and emission lines. We perform a power-law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of αν = 0.59 ± 0.36 in the redshift range 2.5 < z < 3.5 . We take into account that the variation in the continuum indices increases the mean flux by 3 and 7 per cent at z = 3 and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lyα flux PDF and power spectrum of high-resolution data. Using our power-law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of ∼10–15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than ≲10 per cent. 相似文献
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Natural Hazards - Oceanic eddies span over a wide range of sizes and affect the thermodynamic properties of water column. By modifying the thermal structure of the upper ocean, these eddies... 相似文献
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Paswan Ranjit K. Roy M. P. Shankar Ravi Singh P. K. 《Geotechnical and Geological Engineering》2021,39(5):3469-3485
Geotechnical and Geological Engineering - The presence of structural inhomogeneity in a rock mass in the form of joints, fractures, bedding planes is a very common phenomenon. These discontinuities... 相似文献
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We show how the excursion set moving barrier model for halo abundances may be generalized to the local non-Gaussian f nl model. Our estimate assumes that the distribution of step sizes depends on f nl , but that they are otherwise uncorrelated. Our analysis is consistent with previous results for the case of a constant barrier, and highlights some implicit assumptions. It also clarifies the basis of an approximate analytic solution to the moving barrier problem in the Gaussian case, and shows how it might be improved. 相似文献