Continuous statistics of the Lyα forest at 0 < z < 1.6

The mean flux, flux distribution and autocorrelation from HST FOS spectra

David Kirkman, David Tytler, Dan Lubin, Jane Camilla Charlton

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

We measure the amount of absorption in the Lya forest at 0 < z < 1.6 in Hubble Space Telescope Faint Object Spectrograph spectra of 74 quasi-stellar objects (QSOs). Starting with a 334 QSO sample compiled by Bechtold et al, we selected 74 QSOs that have the highest signal-to-noise ratio and complete coverage of rest-frame wavelengths 1070-1170 Å. We measure the absorption from the flux in each pixel in units of the unabsorbed continuum level. We mask out regions of spectra that contain metal lines, or strong Lyα lines that are accompanied by other Lyman series line or metals at the same redshift, leaving Lyα absorption from the low-density intergalactic medium (IGM). At 0 < z < 1.6 we find that 79 per cent of the absorption is from the low-density IGM, 12 per cent from metals and 9 per cent from the strong HI lines, nearly identical to the percentages (78, 15 and 7) that we measured independently at z = 2 from spectra taken with the Kast Spectrograph on the Lick 3-m. At z = 1 the low-density IGM absorbs 0.037 ± 0.004 of the flux. The error includes some but not all of the uncertainty in the continuum level. The remaining part gives relative errors of approximately 0.21 when we report the mean absorption in eight independent redshift intervals, and 0.047 when we average over all redshifts. We find 1.46 times more absorption from the low-density IGM than comes from Lya lines that Bechtold et al. listed in the same spectra. The amount of absorption increases with z and can be fit by a power law (1 + z)α, with α = 1.01. If the absorption comes entirely from lines with fixed rest equivalent width, this result implies the number density of lines evolves like (1 + z)0.01, or no change in the number of lines per unit redshift, consistent with the Janknecht et al. results on the distribution of lines. When we include similar measurements from higher redshifts, we need more degrees of freedom to fit the amount of absorption at 0 < z < 3.2. A power law with a break in slope, changing from index 1.5 at low z to 3.0 above z ∼ 1.1 is a better but only marginally acceptable fit. We also calculate two other continuous statistics, the flux probability distribution function and the flux autocorrelation function that is non-zero out to ν ∼ 500 km s-1 at 0.5 < z < 1.5.

Original languageEnglish (US)
Pages (from-to)1227-1237
Number of pages11
JournalMonthly Notices of the Royal Astronomical Society
Volume376
Issue number3
DOIs
StatePublished - Apr 1 2007

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autocorrelation
statistics
intergalactic media
spectrographs
metal
power law
metals
continuums
distribution
faint objects
probability distribution functions
Hubble Space Telescope
signal-to-noise ratio
pixel
signal to noise ratios
masks
degrees of freedom
pixels
slopes
wavelength

All Science Journal Classification (ASJC) codes

  • Space and Planetary Science

Cite this

@article{8232344d8ced488997bd7a10fe582ba9,
title = "Continuous statistics of the Lyα forest at 0 < z < 1.6: The mean flux, flux distribution and autocorrelation from HST FOS spectra",
abstract = "We measure the amount of absorption in the Lya forest at 0 < z < 1.6 in Hubble Space Telescope Faint Object Spectrograph spectra of 74 quasi-stellar objects (QSOs). Starting with a 334 QSO sample compiled by Bechtold et al, we selected 74 QSOs that have the highest signal-to-noise ratio and complete coverage of rest-frame wavelengths 1070-1170 {\AA}. We measure the absorption from the flux in each pixel in units of the unabsorbed continuum level. We mask out regions of spectra that contain metal lines, or strong Lyα lines that are accompanied by other Lyman series line or metals at the same redshift, leaving Lyα absorption from the low-density intergalactic medium (IGM). At 0 < z < 1.6 we find that 79 per cent of the absorption is from the low-density IGM, 12 per cent from metals and 9 per cent from the strong HI lines, nearly identical to the percentages (78, 15 and 7) that we measured independently at z = 2 from spectra taken with the Kast Spectrograph on the Lick 3-m. At z = 1 the low-density IGM absorbs 0.037 ± 0.004 of the flux. The error includes some but not all of the uncertainty in the continuum level. The remaining part gives relative errors of approximately 0.21 when we report the mean absorption in eight independent redshift intervals, and 0.047 when we average over all redshifts. We find 1.46 times more absorption from the low-density IGM than comes from Lya lines that Bechtold et al. listed in the same spectra. The amount of absorption increases with z and can be fit by a power law (1 + z)α, with α = 1.01. If the absorption comes entirely from lines with fixed rest equivalent width, this result implies the number density of lines evolves like (1 + z)0.01, or no change in the number of lines per unit redshift, consistent with the Janknecht et al. results on the distribution of lines. When we include similar measurements from higher redshifts, we need more degrees of freedom to fit the amount of absorption at 0 < z < 3.2. A power law with a break in slope, changing from index 1.5 at low z to 3.0 above z ∼ 1.1 is a better but only marginally acceptable fit. We also calculate two other continuous statistics, the flux probability distribution function and the flux autocorrelation function that is non-zero out to ν ∼ 500 km s-1 at 0.5 < z < 1.5.",
author = "David Kirkman and David Tytler and Dan Lubin and Charlton, {Jane Camilla}",
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Continuous statistics of the Lyα forest at 0 < z < 1.6 : The mean flux, flux distribution and autocorrelation from HST FOS spectra. / Kirkman, David; Tytler, David; Lubin, Dan; Charlton, Jane Camilla.

In: Monthly Notices of the Royal Astronomical Society, Vol. 376, No. 3, 01.04.2007, p. 1227-1237.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Continuous statistics of the Lyα forest at 0 < z < 1.6

T2 - The mean flux, flux distribution and autocorrelation from HST FOS spectra

AU - Kirkman, David

AU - Tytler, David

AU - Lubin, Dan

AU - Charlton, Jane Camilla

PY - 2007/4/1

Y1 - 2007/4/1

N2 - We measure the amount of absorption in the Lya forest at 0 < z < 1.6 in Hubble Space Telescope Faint Object Spectrograph spectra of 74 quasi-stellar objects (QSOs). Starting with a 334 QSO sample compiled by Bechtold et al, we selected 74 QSOs that have the highest signal-to-noise ratio and complete coverage of rest-frame wavelengths 1070-1170 Å. We measure the absorption from the flux in each pixel in units of the unabsorbed continuum level. We mask out regions of spectra that contain metal lines, or strong Lyα lines that are accompanied by other Lyman series line or metals at the same redshift, leaving Lyα absorption from the low-density intergalactic medium (IGM). At 0 < z < 1.6 we find that 79 per cent of the absorption is from the low-density IGM, 12 per cent from metals and 9 per cent from the strong HI lines, nearly identical to the percentages (78, 15 and 7) that we measured independently at z = 2 from spectra taken with the Kast Spectrograph on the Lick 3-m. At z = 1 the low-density IGM absorbs 0.037 ± 0.004 of the flux. The error includes some but not all of the uncertainty in the continuum level. The remaining part gives relative errors of approximately 0.21 when we report the mean absorption in eight independent redshift intervals, and 0.047 when we average over all redshifts. We find 1.46 times more absorption from the low-density IGM than comes from Lya lines that Bechtold et al. listed in the same spectra. The amount of absorption increases with z and can be fit by a power law (1 + z)α, with α = 1.01. If the absorption comes entirely from lines with fixed rest equivalent width, this result implies the number density of lines evolves like (1 + z)0.01, or no change in the number of lines per unit redshift, consistent with the Janknecht et al. results on the distribution of lines. When we include similar measurements from higher redshifts, we need more degrees of freedom to fit the amount of absorption at 0 < z < 3.2. A power law with a break in slope, changing from index 1.5 at low z to 3.0 above z ∼ 1.1 is a better but only marginally acceptable fit. We also calculate two other continuous statistics, the flux probability distribution function and the flux autocorrelation function that is non-zero out to ν ∼ 500 km s-1 at 0.5 < z < 1.5.

AB - We measure the amount of absorption in the Lya forest at 0 < z < 1.6 in Hubble Space Telescope Faint Object Spectrograph spectra of 74 quasi-stellar objects (QSOs). Starting with a 334 QSO sample compiled by Bechtold et al, we selected 74 QSOs that have the highest signal-to-noise ratio and complete coverage of rest-frame wavelengths 1070-1170 Å. We measure the absorption from the flux in each pixel in units of the unabsorbed continuum level. We mask out regions of spectra that contain metal lines, or strong Lyα lines that are accompanied by other Lyman series line or metals at the same redshift, leaving Lyα absorption from the low-density intergalactic medium (IGM). At 0 < z < 1.6 we find that 79 per cent of the absorption is from the low-density IGM, 12 per cent from metals and 9 per cent from the strong HI lines, nearly identical to the percentages (78, 15 and 7) that we measured independently at z = 2 from spectra taken with the Kast Spectrograph on the Lick 3-m. At z = 1 the low-density IGM absorbs 0.037 ± 0.004 of the flux. The error includes some but not all of the uncertainty in the continuum level. The remaining part gives relative errors of approximately 0.21 when we report the mean absorption in eight independent redshift intervals, and 0.047 when we average over all redshifts. We find 1.46 times more absorption from the low-density IGM than comes from Lya lines that Bechtold et al. listed in the same spectra. The amount of absorption increases with z and can be fit by a power law (1 + z)α, with α = 1.01. If the absorption comes entirely from lines with fixed rest equivalent width, this result implies the number density of lines evolves like (1 + z)0.01, or no change in the number of lines per unit redshift, consistent with the Janknecht et al. results on the distribution of lines. When we include similar measurements from higher redshifts, we need more degrees of freedom to fit the amount of absorption at 0 < z < 3.2. A power law with a break in slope, changing from index 1.5 at low z to 3.0 above z ∼ 1.1 is a better but only marginally acceptable fit. We also calculate two other continuous statistics, the flux probability distribution function and the flux autocorrelation function that is non-zero out to ν ∼ 500 km s-1 at 0.5 < z < 1.5.

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U2 - 10.1111/j.1365-2966.2007.11502.x

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