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On Estimating the QSO Transmission Power Spectrum

Hui, Lam and Burles, Scott and Seljak, Uroš and Rutledge, Robert E. and Magnier, Eugene and Tytler, David (2001) On Estimating the QSO Transmission Power Spectrum. Astrophysical Journal, 552 (1). pp. 15-35. ISSN 0004-637X.

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The Lyα forest has become an important tool for measuring the mass power spectrum at high redshifts (z = 2-4). A crucial intermediate step is the measurement of the transmission power spectrum. We present new methods to minimize the systematic and random errors for such a measurement and discuss their implications for observing strategies. Sources of systematic errors explored include metal line contamination and continuum fitting. We advocate the technique of trend removal in place of traditional continuum fitting: here a spectrum is normalized by its (smoothly varying) mean rather than its continuum; this method is easily automated and removes biases introduced by continuum fitting. Moreover, trend removal can be easily applied to spectra where continuum fitting is difficult, such as when the resolution or signal-to-noise ratio (S/N) is low, or for spectra at high redshifts. We further show that a measurement of the continuum power spectrum (plus a related quantity) using trend removal, from either low-redshift quasar spectra or the red side of Lyα, can be used to constrain the amount of spurious large-scale power introduced by the uncertain continuum and in principle allows the removal of such contamination and thereby expanding scales probed to larger ones. We also derive expressions for the shot noise bias and variance of the power spectrum estimate, taking into account the non-Poissonian nature of the shot noise and the non-Gaussianity of the cosmic fluctuations. An appropriate minimum variance weighting of the data is given. Finally, we give practical suggestions on observing strategy: the desired resolution and S/N for different purposes and instruments, as well as how to distribute one's finite observing time among quasar targets. Also discussed is the quasar spectroscopic study of the Sloan Digital Sky Survey (SDSS), which has the potential to measure the power spectrum accurate to better than 1% per mode (Δk ~ 10^(-4) s km^(-1)). The techniques presented here will be useful for tackling the anticipated issues of shot noise and continuum contamination.

Item Type:Article
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URLURL TypeDescription
Magnier, Eugene0000-0002-7965-2815
Additional Information:© 2001 The American Astronomical Society. Received 2000 May 3; accepted 2001 January 12. We thank for useful discussions Len Cowie, David Kirkman, Patrick Petitjean, Michael Rauch, Wal Sargent, Don Schneider, David Weinberg, Don York, and the participants of the 1999 Haifa workshop on the intergalactic medium and large-scale structure. We also thank Nick Gnedin for supplying a simulation. Special thanks are due to Matias Zaldarriaga for pointing out the importance of aliasing and for many interesting discussions. This work was supported in part by the DOE and NASA grant NAG 5-7092 at Fermilab, as well as by NSF grant PHY-9513835. L. H. thanks the IAS for the Taplin Fellowship.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Department of Energy (DOE)UNSPECIFIED
NASANAG 5-7092
Institute for Advanced StudyUNSPECIFIED
Subject Keywords:cosmology: observations; intergalactic medium; large-scale structure of universe; methods: data analysis; quasars: absorption lines
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Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2001-01
Issue or Number:1
Record Number:CaltechAUTHORS:20150225-162314297
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:55225
Deposited By: Deborah Miles
Deposited On:26 Feb 2015 03:59
Last Modified:03 Oct 2019 08:04

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