Journal
JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS
Volume -, Issue 1, Pages -Publisher
IOP Publishing Ltd
DOI: 10.1088/1475-7516/2018/01/003
Keywords
Lyman alpha forest; power spectrum; baryon acoustic oscillations; redshift surveys
Funding
- STFC Ernest Rutherford Fellowship [ST/N003853/1]
- UCL Cosmoparticle Initiative
- Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]
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We derive and numerically implement an algorithm for estimating the 3D power spectrum of the Lyman-alpha alpha (Ly alpha) forest flux fluctuations. The algorithm exploits the unique geometry of Ly alpha forest data to efficiently measure the cross-spectrum between lines of sight as a function of parallel wavenumber, transverse separation and redshift. We start by approximating the global covariance matrix as block-diagonal, where only pixels from the same spectrum are correlated. We then compute the eigenvectors of the derivative of the signal covariance with respect to cross-spectrum parameters, and project the inverse-covariance-weighted spectra onto them. This acts much like a radial Fourier transform over redshift windows. The resulting cross-spectrum inference is then converted into our final product, an approximation of the likelihood for the 3D power spectrum expressed as second order Taylor expansion around a fiducial model. We demonstrate the accuracy and scalability of the algorithm and comment on possible extensions. Our algorithm will allow efficient analysis of the upcoming Dark Energy Spectroscopic Instrument dataset.
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